Electroph ysiological actions of mexiletine on isolated rabbit atria and canine ventricular muscle and Purkinje fibres I W A O Y A M A G U C H I , B R A M A H N . S I N G H , A N D W I L L I A M J. M A N D E L From the Division oJ’ Curdiology, Ci&rs-Sirrui
Medical Ciwter,
inid
the Depurtmiwt o/’ MediciiriJ, UCLA
School of Medicine, Los Aiigeliv, Califbrtiia S U M M A R Y The electrophysiological effects of varying concentrations of mexiletine on isolated rabbit
sinus node and atrial preparations. Purkinje fibres, Purkinje-fibre-ventricular muscle junction and distal conduction system (‘gate’) were studied in oxygenated Tyrode solution at 35°C using standard microelectrode techniques. ‘Therapeutic’ concentrations of mexiletine (0.5 t o 2.0 pg. cm- 9 had little effect on the atrial action potentials or on sinus node automaticity but sino-atrial conduction was delayed. In Purkinje fibres, these concentrations of the drug depressed the dV/dt,,,, of phase 0, conduction velocity and membrane responsiveness accompanied by significant shortening of the action potential duration and the effective refractory period, the change in the former always being greater than that in the latter. Mexiletine decreased antegrade as well as retrograde conduction at the Purkinje-fibre-ventricular muscle junction, while producing a uniform shortening of the action potential duration in the distal conduction system without a preferential effect at the ‘gate’. The mechanisms of antiarrhythmic actions of mexiletine are thus complex but resemble those of lignocaine in superfused isolated cardiac muscle.
Mexiletine [KO I 1 73, 1-(2’, 6’-dimethyl-phenoxy) (1977) showed that concentrations of mexiletine 2-amio-propane] is a potent antiarrhythmic agent which were clinically meaningful reduced the which has recently undergone extensive experimental maximal rate of depolarisation of the transmem(Allen et al., 1972; Singh and Vaughan Williams, brane action potential in isolated rabbit atria and 1972; Hoffbrand et a/., 1977) and clinical (Campbell ventricular muscle strips without a change in the et al., 1973; Talbot et a/., 1973; Talbot, 1975; resting membrane potential or the action potential Campbell et a/., 1975; Hoffbrand et al., 1977) duration. However, there is little information on the evaluation. In contrast to lignocaine with which it effects of the drug in myocardial fibres from other shares close structural similarities as well as that in parts of the heart. The present study was therefore its antiarrhythmic spectrum of actions, mexiletine undertaken to characterise the antiarrhythmic has the advantage of being active following oral ad- actions of mexiletine in greater detail by determining ministration with a plasma half-life in excess of 10 to the electrophysiological effects of differing concen12 h (Campbell et a/., 1973; Talbot et a/., 1973; trations of the drug in isolated preparations of the Willox and Singh, 1976). The compound, therefore, sino-atrial node, atria, Purkinje fibres and Purkinjehas the pharmacokinetic characteristics of an agent fibre-ventricular muscle junctions. suitable for long-term prophylactic suppression of ventricular arrhythmias (Campbell et a/., 1975). Materials and methods Only limited data are however available with regard to the fundamental mechanism of action of the drug S I N 0 - A T R I A L N 0 D E P K E P A H A T I O N in cardiac muscle. Singh and Vaughan Williams This was the rabbit right atrial preparation dissected and isolated in the manner described by Paes de Address all correspondence end reprint requests l o : I’uhliCarvalho and co-workers (1959). Studies were percation Office, Department or Cardiology, Cedars-Sinai formed to define the effects of the increasing molar Medical Center, 8700 Beverly Boulevard, Los Angeles, concentrations of mexilctine on sinus node function. Calif. 90048. 288
28')
Mi~sileiitri~ oil isoluietl curtliuc t ~ r r t s c l ~ ~
Rabbits weighing between 1.5 and 3.0 kg were stunned with a blow on the head. The hearts were quickly excised and dissected in cold, modified Tyrode solution. The preparation was pinned in a wax-lined lucite bath gassed with 95 %8 0, and 5 % CO,. The composition of the Tyrode's solution was as follows: NaCI, 137; KCI, 4.5; NaH,PO,, 1.8; CaCI,, 2.7; dextrose, 5.5; NaHCO,, 12; in tripledistilled, deionised water. The tissue was superfused at a constant rate of 8 cm3.rnin-' with a temperature of 35.0 1 0.2 C. The pH of the bath was 7.40 i 0.2. Microelectrodes were prepared using standard techniques and filled with 3 mol.litre KCI. The electrodes had tip resistances between I5 to 30 MR. Microelectrodes were impaled in the sinus node and the adjacent crista terminalis. The cell considered to be representative of sinus node was chosen if it demonstrated earliest depolarisation, and slowest rate of phase 4 depolarisation with the longest conduction time to the crista terminalis. The two microelectrodes were coupled to a high input impedance neutralising amplifier ( N F I ; Bioelectric) and displayed on a dual beam oscilloscope (RM 565, Tektronics). All events were photographed (Grass C4 camera) so that permanent records could be analysed. Recordings were obtained during spontaneous sinus rhythm and following atrial pacing. The atrial pacing was accomplished by the use of closelycoupled bipolar electrodes positioned immediately adjacent to the microelectrode at the crista terminalis. In addition, directly measured sino-atrial conduction time (antegrade and retrograde) as well as estimated conduction times were obtained by the extra-stimulus method of Strauss and co-workers (1973). Mexiletine, as a dilution of the crystalline drug, was then superfused by the method of cumulative additions to achieve the final bath concentrations 1 x I W ; and 1 x 10 of: 1 x 10 ';I x 10 s; 5 x mol.litre I . The characteristics of the sinus node action potential were analysed with respect to changes in spontaneous cycle length; the slope of phase 4, the maximum resting potential, action potential duration and estimated threshold potential.
Mongrel dogs of either sex weighing 10 to 20 kg were anaesthetised with intravenous pentobarbital sodium 30 mg.kg'. The heart was rapidly excised and dissected in oxygenated, cooled Tyrode solution. Preparations containing Purkinje fibres and ventricular muscle were dissected from either ventricle and placed in a wax-lined tissue bath which was perfused at a constant rate of 8 cm3.rnin-' with Tyrode solution, gassed with 95 "/, 0, and 5 % CO,. The temperature of the perfusate was maintained at 37 ! 0.5 C (mean 1 SEM). The techniques for the stimulation of the preparation and for the recording of the transmembrane potentials were as those described previously (Bigger and Mandel, 1970). The 'mapping' of the entire right bundle branch was accomplished by utilizing previously described techniques (Wittig ef u/., 1973). Microelectrodes were impaled in both ventricular muscle and Purkinje fibres with electrodes having the characteristics described above. The following action potential characteristics were evaluated: action potential durations at 50, 75, and 95 "/, of full repolarisation, action potential amplitude and its rate of rise, resting membrane voltage, conduction velocity, effective refractory period and membrane responsiveness. I n addition, antegrade and retrograde conduction characteristics across the Purkinje fibre-ventricular muscle junction were measured. All measurements pertaining to the transmembrane potential were obtained at a cycle length of 800 ms (75 per min) unless otherwise specified. The techniques used and definitions of effective refractory period and membrane responsiveness were as previously reported (Bigger et a/., 1968; Bigger and Mandel, 1970). All studies were performed under control conditions and following superfusion with solutions containing increasing concentrations of mexiletine as outlined above. All data were analysed for significance using Student's r test for unpaired values, using P=0.05 as the limit of statistical significance.
ATRIAL TISSUE
STUDIES WITH SINUS NODE A N D ATRIUM
Rabbit right or left atrial tissue was studied utilising methods similar to those described above. These preparations excluded pacemaker tissue and were studied at a basic driving rate of 800 ms (75 per min). The following action potential characteristics were measured : dV /dt,,,,, resting membrane potential, overshoot, amplitude, action potential duration at 50,75, and 90% of full recovery, and the effective refractory period.
The mean data from five experiments in which the effects of varying molar concentrations of mexiletine on various electrophysiological parameters of the sino-atrial node function were determined, are presented in Table I . Records from a typical experiment are reproduced in Fig. 1 . A concentrationdependent prolongation of sino-atrial conduction time and a lengthening of the APD,, were the most significant effects; there were trivial effects on the
PURKINJE FIBRE A N D P U R K I N J E FIBREVENTRICULAR MUSCLE PREPARATIONS
Results
Table I
Effects of varying concentrations of niexiletine on the electroph?isioloRicaI properties of rabbit sino-urrial node
SN-AT CT (ms) CL (ms) Phase 4 M R P (mVJ APD 90 (nis) TP (mV)
36.7 f I .7 540.0 f25.7 5.0 f 0 . 8 71.6 f 3 . 3
38.3f9.3 540.0 f26.3 2.9f0.4 72.1 f2.7
37.0f5.0 546.0 432.1 3.4fl.2 76.6f6.0
41.0f4.9' 555.0 f27.3 3.4f0.9 78.4f5.5
48.3f3.3+ 560.0 f25.3 3.4f0.4 62.9f7.6
63.3f6.0: 589.0 f23.8'' 3.4f0.9 61.0f7.6
123.3f1.3 53.3 f 2 . 3
117.7f1.7' 45.0f4.4
131.7f1.7' 53.3f2.3
133.3f8.8' 54.6*2.9
133.3f13.0' 44.1 f8.3
145.0f10.4' 49.1 f 2 . 8
Abbreviations: C= Control; SN-AT CT= sino atrial conduction time; CL=cycle length; M R P = membrane resting potential; APD,,,= action potential duration (at 90% repolarisation time) and TP= threshold potential. The data represent mean values f S E M from five experiments. The significant of differences from control: *P< 0.05; tP< 0.01 ; l P < 0.0005.
spontaneous cycle length except at the highest concentration mol*litre-l). There were no significant effects either on the slope of phase 4 or the estimated threshold potential. I n Table 2, the data relative to the effects of mexiletine on the various parameters of atrial transmembrane potentials are presented. Concentrations
A . Control
?Oms
,
B. 1 x 1 0 - ~ ~ r' 1 .
Fig. I Efiects of varying concentrations of inexiletine on the transnienibrane potentials from a cell in the sinoatrial node and that bani crista terminalis. The horizontal trace in each panel indicates zero poteiitiuls. Calibration: horizontal for tinie and vertical for transnienibrane voltage. Note the minimal change in the spontaneous cycle length even ajier 10 -'to 10 mol. litrt-I. Mexiletine did, however, reduce the amplitude of the action potential fiotii the crista.
of mexiletine
-
10 mol-litre had no significant effects. Higher concentrations (10 to 10 mol. litre-') produced striking reductions in dV/dt,, overshoot potential, action potential amplitude with a significant lengthening of the APD,, and the ERP; the resting membrane potential was reduced but not significantly. A. Control 100 control
B. 1
m~.rl
H
Fig. 2 Effects of increasing concentrutions 0f inexiletine on the transnienibrane potential recorded /rorii a canine Purkinje fibre. The upper trace in each punel represents zero potential and the lower, the differential s.igiial of the upstroke velocity of phase 0 (dV/df,,,axor Vmox).A concentration-dependent depression of dV/d~,,,,~und the abbreviation of the action potetitiul duration are rlenionstrutecl.
Table 2
E8ecis of wrying conceniraiions of nwxileiine on the iransniemhrane uciioir poioiiiu1.s in rahhii uiria
dV idtinax ( V d ) RMP (mv)
Control
10 7
10
194.8 f3.9
194.4f7.2 NS 81.2f4.3 NS 20.2f1.9 NS 101.4f5.2 NS 36.6f4.7 NS 57.2f7.4
187.2f14.7 NS 84.2f2.9
162.0f13.8
*
64.0f22.3
NS 82.6k1.9
NS
NS
17.8f1.8 NS 102.0f4.6
78.2f2.6 NS 14.0f2.7
NS
*
72.4f7.8 NS 6.0f3.3
101.2f5.1
92.2f3.2
78.4f6.3
84.4 f3.7
OSP
20.4 f 2 . 4
(mV) AMP (mV) APD
104.8 f3.4 37.0 f 4 . 3
184.2fll.2
18.6f3.8
10
4
+
*
NS
t
35.2f7.5 57.8f8.5 NS 199.0 f 19. I
43.4f8.5 NS 69.2f10.4 NS 191.4 f 18.6
192.6f22.5
208.2f23.7
233.3f31.7
174.2 f 18.3
193.4 f23.6
191.4f18.6
197.8f18.6
NS
10 *
fl
40.8f11.7 NS 63.0fl3.5 NS I74 4 f 12.5
164.2f18.7
5R.6 f5.0
x I I)
NS 39.4f4.8 NS 59.4f6.9 NS 179.2 f I 1.5
50 (ms) APD 75 (ms) APD 90 (ms) ERP (ms)
.5
h
NS
t
8
z
Abbreviations: dV /dtmax= first derivative of phase 0 of the action potential; R M P - - resting membrane potential; OSP=overshoot potential; A P D = action potential duration at 50. 75. 907; repolarisation times and ERP- effective refractory period. 3 ) : * P < 0.05; t P < 0.01 The data represent mean values ( f S E M ) from five experiments (except where 3 is indicated; here n : $P< 0.005. P U R K 1N J E FI B R E P R E P A K A T I 0 NS
The effects of increasing concentrations of mexiletine on the various parameters of the Purkinje fibre action potential were determined in 15 preparations. The mean data are summarised in Table 3 and records from a representative study are reproduced in Fig. 2. The earliest change was found with dV /dt,,, which was depressed by 9 % ( P - 0.005) by 10 mol*litre Table 3
mexiletine. with a concentration-dependent reduction in dV /dt occurring thereafter. Action potential amplitude was also reduced but no fall in R M P occurred until concentrations of mexiletine reached 10 to 10 mol*litre I . A concentration-dependent decrease also occurred in A P D and in the E R P and these were particularly striking at higher concentrations of mexiletine used. Changes in conduction
Efecis of varying conceniruiions ofnrexileiinc on /hi, iran.snieiiihrane nclion poiiwials in canine Purkitije fihrcv
Concwirru/ion of inc~silc~rint~ (nrol.lirrc, Control
10 7
5 2 5 . I f 15.7
508.0 f 2 I . 2
10
5
'5
RMP (mV)
93.4 f I .4
94.5 f I .0
477.0 f26.0 P < 0.005 90.9 f I . I
OSP
30.0 f I .9
26.0 f 2 . 2
30.7 f 2 . 1
122.8 f2.7
120.5 f2.3
122.0 f 2 . 5
dV /dt
(mV) AMP (mV) A P D 50":, (ms) A P D 75y0 (ms) A P D 95",, fms) ERP (ms)
*cv
251.9 f 10.6
237.7 f 8 . 2
242.1 f9.0
x II)
380.6 f26.0 P< 0.0005 89.3 f I .8 26.8 412.3
288.5 f7.6
299.9 f9.4
351.1 flO.O
340.9 f8.0
347.5 f9.0
281.4 f 13.9
285.2 f 14.2
274.3 f 14.0
2.37 fO. 18
2.55 f0.23
2.49 f 0 . 2 5
~~~~~
Abbreviations: Meanings o f symbols as in Table 2. The data represent mean values f S E M from I5 experiments. 'n-
-
228.3 f 18.8
P< 0.0005
P < 0.0005
87.1 f3.9 P< 0.05 26.7 f 4 . 9
82.2 f4.3 P< 0.01 22.7 f 4 . 5
116.5 f 2 . 7
114.2 f3.4
104.9 f 3 . 7
P< 0.01
214.4 *9.1
203.3 f9.7 P< 0.005 265.9 f7.5 P < 0.005 309.4 f9.3
P< 0.0005 I I I .9 f9.2 P< 0.0005
272.5 f6.9 P< 0.005 3 17.9 f 7 . 3 P -0
300
*
l-
0
1
100 Fig. 3 Depression of conduction velocity by rirexiletitre in linear strands of Purkinje fibres. Ordinate: molar concentrations of niexiletine. Reduction in conduction velocity is concentration-depenciit with the threshold at I x I&' nrol~litrc',
. L
60
80
5
100
Fig. 5 Changes in itrenibratre responsiveness in a Purkinje fibre following increasing concentrations of nrexilerine. Ordinate: dVlrlt (Ves-1) of action potentials elicited bv stimulating the fibre at varying levels of inenrbrane potential during repolarisation before (control) and after mexiletine. Abscissa: Levels of membrane potential ( i n V ) during repolarisation of a driven action potential. Mexiletine shifred the membrane responsiveness curve ttownward~and to the left.
EFFECTS OF MEXILETINE ON PURKINJE FIBRE-VENTRICULAR MUSCLE JUNCTION
Fig. 4 The relationship between the changes in ERP on abscissa) and those in the APD ( A APD on ordinate) in a Purkinje fibre preparation following superfusion with varying concentrations of mexiletine. The shortening of the APD was greater at all concentrations of mexiletine.
(A ERP
These were studied in 12 preparations and results from a typical experiment with respect to the effects of different concentrations of mexiletine on antegrade and retrograde conduction at Purkinje fibreventricular muscle junction are shown in Fig. 5 . Significant reduction in both antegrade and retrograde refractoriness across the ventricular musclePurkinje fibre junction were apparent. However, substantial changes in the configuration of the refractory period curve were only apparent when the preparations were exposed to clearly toxic ( I x 10 mol*litre--')concentrations of mexiletine.
Mrxiletitie
011
A
isolateil carrliac tiiii,wli#
293 B
Ant e g rade
400 c
2 N -
s
Ret rogrcde
400 h
ffl
-E 300-
300
N
LL
a
I
I
€
li-
a
> 200
200 PF,-PF2
300 (ms)
20c -
300
200
400
VMI-VMz
400
(mS)
Fig. 6 A typical example of the effect of increasing concentration of mexiletine on the Purkinje fibre- papillary muscle junction. Panel A shows the effect of the drug on antegrade conclucrion. Note the prolongation of the effective refractory period at low doses with niarked alterations at higher dose level. Panel B shows inexiletine's effect on retrograde refractoriness. Note the prolongation of the refraciory period in a step-wise fashion related to the concentration of niexiletitie. The horizontal axis show the corcplin~interval at the stiniidation site and the vertical axis show the response at the sattipling sire
EFFECTS OF MEXILETINE O N THE ACTION POTENTIAL DURATION IN DIFFERENT PARTS O F T H E RIGHT BUNDLE BRANCH
These were studied utilising varying concentrations of mexiletine across the total length of the right bundle branch in five preparations. Action potential durations were measured at seven sites along the course of the right bundle branch stimulated at a cycle length of 800 ms. The mean data from these studies are summarised in Fig. 6. The shortening of the A P D that was observed was uniform along the preparation, there being no significant differential effect a t the 'gate' at both test concentrations (5x and I x mol*litre I ) used.
activity or by a combination of these mechanisms (Hauswirth and Singh, 1978). Our observations with mexiletine in isolated atria
"Oar 300 .
h
ffl
E
v
Discussion The fundamental mode of action of the available antiarrhythmic drugs have been interpreted for many years from an analysis of their effects on the various parameters of the action potential from tissues from different parts of the heart (Bigger and Mandel, 1970; Singh and Vaughan Williams, 1971; Rosen and Hoffman, 1973; Singh and Hauswirth, 1974). From such studies, it has been demonstrated that different antiarrhythmic agents exert their salutary effects essentially by depressing phases 0 and 4 of the cardiac action potential, by inhibiting sympathetic excitation, by affecting the repolarisation phase and by blocking the slow-channel
200
'
4' 'O0
P 1
2
k- RBB -14
3
4 5 6 7 False tendon-1Muscle
Fig. 7 Eflects of niexiletine on APD in differentparts of the ventricular conducting system. Ordinate: duration of APD to 90% of repolarisation in nisec. Abscissa: location of the recording sites between right bundle branch ( R B B ) and ventricular muscle. Nore lack of preferential shortening of the APD at the point where the APD is longest.
294
and canine Purkinje fibres extend previous observations (Singh and Vaughan Williams, 1972) which indicated that the drug acted predominantly by depressing the upstroke velocity of phase 0 of the action potential in rabbit atria and ventricular muscle. The present studies, utilising a wide range of concentrations, have demonstrated that concentrations of the drug corresponding to its therapeutic plasma levels to I x in man (0.5-2.0 pgacm-' or 2.21 x mol-litre-') exerted relatively trivial depressant effects on atrial intracellular potentials and on sinus node automaticity. This is consistent with the observation that the drug has variable effects on the spontaneous sinus rate at therapeutic plasma levels in man (Roos e/ a/., 1976). It is of interest, however, that sino-atrial conduction was depressed by high concentrations of mexiletine; it is thus conceivable that the depressant effect of the drug on sinus node automaticity and intra-atrial conduction may be more pronounced in diseased tissues. Indeed, sinus arrest and failure of sino-atrial conduction have been encountered in patients with the sick sinus syndrome even in the context of therapeutic serum levels of mexiletine (Roos et al.. 1976). The observed lack of significant effects of therapeutic concentrations of mexiletine in atrial tissues, however, suggests that the drug is unlikely to be of value in correcting supraventricular tachyarrhythmias but no systematic clinical studies have been reported. The lack of effect of mexiletine on normal sinoatrial automaticity emphasise the possibility that the fast sodium channel does not play a significant role in the genesis of the normal pacemaker potential. However, the fact that mexiletine does abort ectopic ventricular arrhythmias in varying clinical and experimental conditions (Hoffbrand e/ a/., 1977) not only suggests that abnormally generated phase 4 depolarisation is suppressed by the drug, but that the ionic mechanisms underlying such ectopic activity may differ from those in the normal pacemaker tissues. Clearly, the effects of mexiletine on automaticity produced by ischaemia, digitalis, o r other arrhythmogenic interventions will be of interest. Perhaps of the greatest clinical relevance are our studies which have examined the effects of mexiletine on Purkinje fibres, Purkinje fibre-ventricular muscle junction and on the so-called gating mechanism in the distal conduction tissues particularly in light of the observation that the drug is specially potent in controlling ventricular arrhythmias (Hoffbrand e/ a/., 1977). The effects here were striking with respect to the depression of V,, membrane responsiveness, and to a lesser extent, of directly measured conduction velocity. All the observed effects are accountable in terms of the drug's de-
Iwuo YuiiiuKuchi, Bruniuli N . Singh, untl Williuiii J. Muiiclc.l
pressant effect on the fast sodium channel consistent with its known local anaesthetic properties on nerve (Singh and Vaughan Williams, 1972), the reduction in the resting membrane potential occurring only in potentially toxic concentrations of the drug. O n the basis of these effects, one might have expected a lengthining of the E R P with a delayed return of excitability in the myocardium. I t was, therefore, somewhat surprising that the drug produced a concentration-dependent abbreviation of the ERP, the nature of which is not understood. However, the shortening of the ERP, was accompanied by a greater abbreviation of the APD. The precise mechanism of accelerated repolarisation induced by mexiletine is uncertain and will require voltage clamp studies (Hauswirth and Singh, 1978) for the elucidation of the nature of the changes in the underlying ionic conductances. Nevertheless, it is noteworthy that the ratio of E R P / A P D was always prolonged by mexiletine which, in association with depressed conduction velocity, may be significant in terminating re-entrant ventricular arrhythmias. The possibility must also be considered that mexiletine might exert a differential effect on the E R P in the normal and ischaemic tissues, shortening it in the former and lengthening it in the latter, a possibility supported by His bundle studies in man. For example, in patients with relatively normal conduction, McComish and co-workers (1977) found that the E R P was shortened by mexiletine, whereas in those with pre-existing but clinically occult conduction system disease, the drug produced a significant lengthening of the E R P in His-Purkinje system (Roos e / a/., 1976). The electrophysiological effects of mexiletine in ischaemic tissues will clearly be of much interest especially in view of the fact that many of the electrophysiological properties of mexiletine described here closely resemble those of lignocaine reported previously. Particularly significant in this context are the recent observations (Kuppersmith et a/., 1975; Lazzara et al., 1978) which have indicated that very much lower concentrations of lignocaine reduce the upstroke velocity of phase 0 in hypoxic and ischaemic cardiac cells than those which produce comparable effects in normal fibres. Thus, quantitative differences in the effects of mexiletine in normal and ischaemic tissues are also likely, a feature that is of potential significance in the overall action of the drug in the control of arrhythmias complicating myocardial ischaemia. In our studies, it is also noteworthy that the drug produced moderate decreases in retrograde as well as antegrade conduction at Purkinje fibre-ventricular muscle junction. Nevertheless, the basis for reentrant ventricular arrhythmias have been repeatedly demonstrated to be due to the presence of slowed
Mc.xilctini~on isolutcrl cirrcliuc muscli~
295
conduction, unidirectional block, reflection o r forteza for illustrations and to Mrs Bernadette summation. These basic reentrant mechanisms Fluharty for photography. Mexiletine was supplied would be expected to be more readily operative in as a gift for experimental studies by Dr W. Benson of areas where inherent electrophysiological differences Boehinger-lngleheim, Ltd., New York. exist in Purkinje-papillary muscle junction. Mexiletine’s effects at this junction may, in the setting of abnormalities such as those associated with ischaemia, be antiarrhythmic. However, our data have not proved conclusive in this regard. Further- References more, in our studies, the effects of the drug on the Allen, J. D., Kofi Ekue, J . M., Shanks, R. G., and Zaidi, APD were distributed uniformly in the major areas S. A. (1970). T h e effect on experimental cardiac arrhythof the right bundle branch when the preparation was mias of a new anticonvulsant agent. KO 1173. and its comparison with phenytoin and procainaniide. British stimulated at a frequency of 75 per min: no preJournal of Pharntacologv. 39, 183- 184. ferential change with respect to the shortening of the Bigger, J. T., Jr., Bassett, A. L., a n d Hoffman, B. (1968). APD at the level where the duration of the action Electrophysiological effects of diphenylhydantoin o n potential was maximal (‘gate’) was demonstrated. In canine Purkinje fibres. Circulation Research. 22, 2 13-224. contrast, Vaughan Williams (1977) who used a Bigger. J. T.. Jr., and Mandel, W. J. (1970). The electrophysiologic effects of lidocaine on canine Purkinje fibre lower stimulation frequency, found that mexiletine and ventricular muscle. Jocirnal of Clinicul Invvsfi#arion, like lidocaine (Wittig rt u/., 1973) shortened the 49,63-77. APD throughout the conduction pathway and in the Campbell, N. P. S., Chaturvedi. N. C.. Kelly, J. G., Strong. J. E.. Shanks. R. G.. a n d Pantridge. J. F. (1973). Mexileventricular muscle, but had a strikingly preferential tine (KO 1173) in the management of ventricular dyseffect where the APD was longest, so that APD rhythmias. Lancet, August 25. 404407. became more uniform. It has been suggested, (Wittig Campbell, R . W. F.. Talbot, R. C., Dolder, M. A,. Murray, et a / . , 1973) that this effect would induce a relatively A., Prescott, L. F., and Julian. D. G. (1975). Comparison o f procainamide and mexiletine in prevention o f ventrifaster conduction of premature impulses and the cular arrhythmias after acute myocardial infarction. greater uniformity of A P D (by reducing inhomoLancer, June 7. 1257- 1260. geneity) may contribute to an antiarrhythmic Hauswirth, 0.. a n d Singh. B. N. (1978). Voltage clamp in action. Whether such an effect of mexiletine demonheart muscle in relation to the genesis and the pharmacological control of cardiac arrhythmias. Pliurnracologic strated in isolated preparations at a low stimulation R V V ~ C 30, W . 5-63. frequency does indeed contribute to its overall Hoffbrand, B. I . . Shanks. R. G., a n d Bell, J. W. (1977). salutary effect in aborting ventricular arrhythmias Mexiletine in ventricular arrhythmias. PosrRrac/itarr remains unknown however, but merits further Mrrlical Joctrnnl (Suppl I ) 53, 1-169. 1977 Kupersmith. J., Antman, E. M.. and Hoffman, B. F. (1975). investigation. I n vivo electrophysiological effects of lidocaine in canine In summary, the present experiments have shown acute myocardial infarction. Circrtlurion Research. 36, that concentrations of mexiletine which are thera84-9 I . peutically relevant, have little effect on atrial intra- Lazzara, R . , Hope. R. R., El-Sherif, N., and Scherlag, B. J. (1978). Effects of lidocaine on hypoxic a n d ischaemic cellular potentials o r sinus node automaticity but cardiac cells. Anic,ricun Journal of Carcliolog~.41, 872-879. sino-atrial conduction is prolonged. In Purkinje McComish. M.. Robinson, C.. Kitson. D., and Jewitt. D. E. fibres the drug depresses conduction velocity, mem(1977). Clinical electrophysiological effects o f mexiletine. brane responsiveness and the v,, of phase 0 of the Postgra~luccteMcrlical Journal (Suppl I ), 53. 85-9 I. action potential associated with a shortening of the Paes d e Carvalho. A.. deMello, W. C., and Hoffman, B. F. (1959). Electrophysiological evidence for specialised fibre APD and the E R P but with an increase in the ratio types in rabbit atrium. Aniwicnn Journal of Phy.sici/ogy. APD/ERP. The drug decreased antegrade as well as 196,483488. retrograde conduction at the Purkinje fibre-ventricuRoos, J . C., Paalnian, A. C. A,. and Dunning. A. J. (1976). Electrophysiological effects of mexiletine in man. Bri/i.d/ tar muscle junction while producing uniform Hcarr Journal. 38, 1262- I27 I . shortening of the A P D in the distal conduction M. R., a n d Hoffman. B. F. (1973). Mechanisms o f system without a preferential effect at the ‘gate’. The Rosen. action of antiarrhythmic drugs. Circulation R i w a r r h , 32, mechanisms of action of mexiletine are thus comI. plex but resemble those of lignocaine, reflecting a Singh. B. N.. a n d Hauswirth, 0. (1974). Comparative mechanism of action o f anti-arrhythmic drugs. American simultaneous alterations in the depolarisation and H m r t Jorirnal, 87. 367-382. repolarisation processes in myocardial fibres. Singh, B. N.. and Vaughan Williams, E. M. (1971). Effect of Supported in part by SCOR Grant No. 17651, National Institutes of Health, Bethesda, Maryland. We are grateful to Ms Darlis Martin and Mrs Betty Garrigues for secretarial help, to Lance La-
altering potassium concentration o n the action of lidocaine and diphenylhydantoin on rabbit atrial and ventricular muscle. Circrrlarion Rrseurch. 29, 286-295. Singh. B. N.. a n d Vaughan Williams. E. M. (1972). Investigations of the mode o f action o f a new antidysrhythmic l ( i ~ 1-9. .,, drug, KO 1 173. BrirDh Jortrnul ( ~ ~ P l r c r r ~ / u c ~ o44,
Strauss, H. C.. Saroff, A. L.. Bigger, J. T., and Giardina, E. G. V. (1973). Premature atrial stimulation as a key t o the understanding of sinoatrial conduction in man. Circularion. 41,86-92. Talbot. R. G. (1978). Mexiletine. Anwricnn Hear1 Jovrnul, 89,537-538. TdlbOt, R. G.. Clark, R. A.. Nimnio. J., Neilson, J. M. M.. Julian, D. G., and Prescott. L. F. (1973). Treatment of ventricular arrhythmias with mexiletine (KO 1173). Luncc./. 2. 399-400.
Vaughan Williams. E. M. (1977). Mexiletine in isolated tissue models. Pos/grarlnu/e Medicul Journal, (Suppl I ), 53. 30-34. Willox, S.. and Singh, B. N. (1976). A sensitive gas chromatographic method for the estimation of a new antiarrhythmic compound mexiletine (KO 1173) in hiological fluids. Jaiirnui of Chronruro/i~g.I~, 128, 196-197. Wittig, J. H.. Harrison, L. A.. and Wallace, A . G . (1973). Electrophysiological effects of lidocaine on distal Purkinje fihres o f canine heart. Anlerican Hcurr Jotrrnul. 86. 69-75.
Medical Ciwter,
inid
the Depurtmiwt o/’ MediciiriJ, UCLA
School of Medicine, Los Aiigeliv, Califbrtiia S U M M A R Y The electrophysiological effects of varying concentrations of mexiletine on isolated rabbit
sinus node and atrial preparations. Purkinje fibres, Purkinje-fibre-ventricular muscle junction and distal conduction system (‘gate’) were studied in oxygenated Tyrode solution at 35°C using standard microelectrode techniques. ‘Therapeutic’ concentrations of mexiletine (0.5 t o 2.0 pg. cm- 9 had little effect on the atrial action potentials or on sinus node automaticity but sino-atrial conduction was delayed. In Purkinje fibres, these concentrations of the drug depressed the dV/dt,,,, of phase 0, conduction velocity and membrane responsiveness accompanied by significant shortening of the action potential duration and the effective refractory period, the change in the former always being greater than that in the latter. Mexiletine decreased antegrade as well as retrograde conduction at the Purkinje-fibre-ventricular muscle junction, while producing a uniform shortening of the action potential duration in the distal conduction system without a preferential effect at the ‘gate’. The mechanisms of antiarrhythmic actions of mexiletine are thus complex but resemble those of lignocaine in superfused isolated cardiac muscle.
Mexiletine [KO I 1 73, 1-(2’, 6’-dimethyl-phenoxy) (1977) showed that concentrations of mexiletine 2-amio-propane] is a potent antiarrhythmic agent which were clinically meaningful reduced the which has recently undergone extensive experimental maximal rate of depolarisation of the transmem(Allen et al., 1972; Singh and Vaughan Williams, brane action potential in isolated rabbit atria and 1972; Hoffbrand et a/., 1977) and clinical (Campbell ventricular muscle strips without a change in the et al., 1973; Talbot et a/., 1973; Talbot, 1975; resting membrane potential or the action potential Campbell et a/., 1975; Hoffbrand et al., 1977) duration. However, there is little information on the evaluation. In contrast to lignocaine with which it effects of the drug in myocardial fibres from other shares close structural similarities as well as that in parts of the heart. The present study was therefore its antiarrhythmic spectrum of actions, mexiletine undertaken to characterise the antiarrhythmic has the advantage of being active following oral ad- actions of mexiletine in greater detail by determining ministration with a plasma half-life in excess of 10 to the electrophysiological effects of differing concen12 h (Campbell et a/., 1973; Talbot et a/., 1973; trations of the drug in isolated preparations of the Willox and Singh, 1976). The compound, therefore, sino-atrial node, atria, Purkinje fibres and Purkinjehas the pharmacokinetic characteristics of an agent fibre-ventricular muscle junctions. suitable for long-term prophylactic suppression of ventricular arrhythmias (Campbell et a/., 1975). Materials and methods Only limited data are however available with regard to the fundamental mechanism of action of the drug S I N 0 - A T R I A L N 0 D E P K E P A H A T I O N in cardiac muscle. Singh and Vaughan Williams This was the rabbit right atrial preparation dissected and isolated in the manner described by Paes de Address all correspondence end reprint requests l o : I’uhliCarvalho and co-workers (1959). Studies were percation Office, Department or Cardiology, Cedars-Sinai formed to define the effects of the increasing molar Medical Center, 8700 Beverly Boulevard, Los Angeles, concentrations of mexilctine on sinus node function. Calif. 90048. 288
28')
Mi~sileiitri~ oil isoluietl curtliuc t ~ r r t s c l ~ ~
Rabbits weighing between 1.5 and 3.0 kg were stunned with a blow on the head. The hearts were quickly excised and dissected in cold, modified Tyrode solution. The preparation was pinned in a wax-lined lucite bath gassed with 95 %8 0, and 5 % CO,. The composition of the Tyrode's solution was as follows: NaCI, 137; KCI, 4.5; NaH,PO,, 1.8; CaCI,, 2.7; dextrose, 5.5; NaHCO,, 12; in tripledistilled, deionised water. The tissue was superfused at a constant rate of 8 cm3.rnin-' with a temperature of 35.0 1 0.2 C. The pH of the bath was 7.40 i 0.2. Microelectrodes were prepared using standard techniques and filled with 3 mol.litre KCI. The electrodes had tip resistances between I5 to 30 MR. Microelectrodes were impaled in the sinus node and the adjacent crista terminalis. The cell considered to be representative of sinus node was chosen if it demonstrated earliest depolarisation, and slowest rate of phase 4 depolarisation with the longest conduction time to the crista terminalis. The two microelectrodes were coupled to a high input impedance neutralising amplifier ( N F I ; Bioelectric) and displayed on a dual beam oscilloscope (RM 565, Tektronics). All events were photographed (Grass C4 camera) so that permanent records could be analysed. Recordings were obtained during spontaneous sinus rhythm and following atrial pacing. The atrial pacing was accomplished by the use of closelycoupled bipolar electrodes positioned immediately adjacent to the microelectrode at the crista terminalis. In addition, directly measured sino-atrial conduction time (antegrade and retrograde) as well as estimated conduction times were obtained by the extra-stimulus method of Strauss and co-workers (1973). Mexiletine, as a dilution of the crystalline drug, was then superfused by the method of cumulative additions to achieve the final bath concentrations 1 x I W ; and 1 x 10 of: 1 x 10 ';I x 10 s; 5 x mol.litre I . The characteristics of the sinus node action potential were analysed with respect to changes in spontaneous cycle length; the slope of phase 4, the maximum resting potential, action potential duration and estimated threshold potential.
Mongrel dogs of either sex weighing 10 to 20 kg were anaesthetised with intravenous pentobarbital sodium 30 mg.kg'. The heart was rapidly excised and dissected in oxygenated, cooled Tyrode solution. Preparations containing Purkinje fibres and ventricular muscle were dissected from either ventricle and placed in a wax-lined tissue bath which was perfused at a constant rate of 8 cm3.rnin-' with Tyrode solution, gassed with 95 "/, 0, and 5 % CO,. The temperature of the perfusate was maintained at 37 ! 0.5 C (mean 1 SEM). The techniques for the stimulation of the preparation and for the recording of the transmembrane potentials were as those described previously (Bigger and Mandel, 1970). The 'mapping' of the entire right bundle branch was accomplished by utilizing previously described techniques (Wittig ef u/., 1973). Microelectrodes were impaled in both ventricular muscle and Purkinje fibres with electrodes having the characteristics described above. The following action potential characteristics were evaluated: action potential durations at 50, 75, and 95 "/, of full repolarisation, action potential amplitude and its rate of rise, resting membrane voltage, conduction velocity, effective refractory period and membrane responsiveness. I n addition, antegrade and retrograde conduction characteristics across the Purkinje fibre-ventricular muscle junction were measured. All measurements pertaining to the transmembrane potential were obtained at a cycle length of 800 ms (75 per min) unless otherwise specified. The techniques used and definitions of effective refractory period and membrane responsiveness were as previously reported (Bigger et a/., 1968; Bigger and Mandel, 1970). All studies were performed under control conditions and following superfusion with solutions containing increasing concentrations of mexiletine as outlined above. All data were analysed for significance using Student's r test for unpaired values, using P=0.05 as the limit of statistical significance.
ATRIAL TISSUE
STUDIES WITH SINUS NODE A N D ATRIUM
Rabbit right or left atrial tissue was studied utilising methods similar to those described above. These preparations excluded pacemaker tissue and were studied at a basic driving rate of 800 ms (75 per min). The following action potential characteristics were measured : dV /dt,,,,, resting membrane potential, overshoot, amplitude, action potential duration at 50,75, and 90% of full recovery, and the effective refractory period.
The mean data from five experiments in which the effects of varying molar concentrations of mexiletine on various electrophysiological parameters of the sino-atrial node function were determined, are presented in Table I . Records from a typical experiment are reproduced in Fig. 1 . A concentrationdependent prolongation of sino-atrial conduction time and a lengthening of the APD,, were the most significant effects; there were trivial effects on the
PURKINJE FIBRE A N D P U R K I N J E FIBREVENTRICULAR MUSCLE PREPARATIONS
Results
Table I
Effects of varying concentrations of niexiletine on the electroph?isioloRicaI properties of rabbit sino-urrial node
SN-AT CT (ms) CL (ms) Phase 4 M R P (mVJ APD 90 (nis) TP (mV)
36.7 f I .7 540.0 f25.7 5.0 f 0 . 8 71.6 f 3 . 3
38.3f9.3 540.0 f26.3 2.9f0.4 72.1 f2.7
37.0f5.0 546.0 432.1 3.4fl.2 76.6f6.0
41.0f4.9' 555.0 f27.3 3.4f0.9 78.4f5.5
48.3f3.3+ 560.0 f25.3 3.4f0.4 62.9f7.6
63.3f6.0: 589.0 f23.8'' 3.4f0.9 61.0f7.6
123.3f1.3 53.3 f 2 . 3
117.7f1.7' 45.0f4.4
131.7f1.7' 53.3f2.3
133.3f8.8' 54.6*2.9
133.3f13.0' 44.1 f8.3
145.0f10.4' 49.1 f 2 . 8
Abbreviations: C= Control; SN-AT CT= sino atrial conduction time; CL=cycle length; M R P = membrane resting potential; APD,,,= action potential duration (at 90% repolarisation time) and TP= threshold potential. The data represent mean values f S E M from five experiments. The significant of differences from control: *P< 0.05; tP< 0.01 ; l P < 0.0005.
spontaneous cycle length except at the highest concentration mol*litre-l). There were no significant effects either on the slope of phase 4 or the estimated threshold potential. I n Table 2, the data relative to the effects of mexiletine on the various parameters of atrial transmembrane potentials are presented. Concentrations
A . Control
?Oms
,
B. 1 x 1 0 - ~ ~ r' 1 .
Fig. I Efiects of varying concentrations of inexiletine on the transnienibrane potentials from a cell in the sinoatrial node and that bani crista terminalis. The horizontal trace in each panel indicates zero poteiitiuls. Calibration: horizontal for tinie and vertical for transnienibrane voltage. Note the minimal change in the spontaneous cycle length even ajier 10 -'to 10 mol. litrt-I. Mexiletine did, however, reduce the amplitude of the action potential fiotii the crista.
of mexiletine
-
10 mol-litre had no significant effects. Higher concentrations (10 to 10 mol. litre-') produced striking reductions in dV/dt,, overshoot potential, action potential amplitude with a significant lengthening of the APD,, and the ERP; the resting membrane potential was reduced but not significantly. A. Control 100 control
B. 1
m~.rl
H
Fig. 2 Effects of increasing concentrutions 0f inexiletine on the transnienibrane potential recorded /rorii a canine Purkinje fibre. The upper trace in each punel represents zero potential and the lower, the differential s.igiial of the upstroke velocity of phase 0 (dV/df,,,axor Vmox).A concentration-dependent depression of dV/d~,,,,~und the abbreviation of the action potetitiul duration are rlenionstrutecl.
Table 2
E8ecis of wrying conceniraiions of nwxileiine on the iransniemhrane uciioir poioiiiu1.s in rahhii uiria
dV idtinax ( V d ) RMP (mv)
Control
10 7
10
194.8 f3.9
194.4f7.2 NS 81.2f4.3 NS 20.2f1.9 NS 101.4f5.2 NS 36.6f4.7 NS 57.2f7.4
187.2f14.7 NS 84.2f2.9
162.0f13.8
*
64.0f22.3
NS 82.6k1.9
NS
NS
17.8f1.8 NS 102.0f4.6
78.2f2.6 NS 14.0f2.7
NS
*
72.4f7.8 NS 6.0f3.3
101.2f5.1
92.2f3.2
78.4f6.3
84.4 f3.7
OSP
20.4 f 2 . 4
(mV) AMP (mV) APD
104.8 f3.4 37.0 f 4 . 3
184.2fll.2
18.6f3.8
10
4
+
*
NS
t
35.2f7.5 57.8f8.5 NS 199.0 f 19. I
43.4f8.5 NS 69.2f10.4 NS 191.4 f 18.6
192.6f22.5
208.2f23.7
233.3f31.7
174.2 f 18.3
193.4 f23.6
191.4f18.6
197.8f18.6
NS
10 *
fl
40.8f11.7 NS 63.0fl3.5 NS I74 4 f 12.5
164.2f18.7
5R.6 f5.0
x I I)
NS 39.4f4.8 NS 59.4f6.9 NS 179.2 f I 1.5
50 (ms) APD 75 (ms) APD 90 (ms) ERP (ms)
.5
h
NS
t
8
z
Abbreviations: dV /dtmax= first derivative of phase 0 of the action potential; R M P - - resting membrane potential; OSP=overshoot potential; A P D = action potential duration at 50. 75. 907; repolarisation times and ERP- effective refractory period. 3 ) : * P < 0.05; t P < 0.01 The data represent mean values ( f S E M ) from five experiments (except where 3 is indicated; here n : $P< 0.005. P U R K 1N J E FI B R E P R E P A K A T I 0 NS
The effects of increasing concentrations of mexiletine on the various parameters of the Purkinje fibre action potential were determined in 15 preparations. The mean data are summarised in Table 3 and records from a representative study are reproduced in Fig. 2. The earliest change was found with dV /dt,,, which was depressed by 9 % ( P - 0.005) by 10 mol*litre Table 3
mexiletine. with a concentration-dependent reduction in dV /dt occurring thereafter. Action potential amplitude was also reduced but no fall in R M P occurred until concentrations of mexiletine reached 10 to 10 mol*litre I . A concentration-dependent decrease also occurred in A P D and in the E R P and these were particularly striking at higher concentrations of mexiletine used. Changes in conduction
Efecis of varying conceniruiions ofnrexileiinc on /hi, iran.snieiiihrane nclion poiiwials in canine Purkitije fihrcv
Concwirru/ion of inc~silc~rint~ (nrol.lirrc, Control
10 7
5 2 5 . I f 15.7
508.0 f 2 I . 2
10
5
'5
RMP (mV)
93.4 f I .4
94.5 f I .0
477.0 f26.0 P < 0.005 90.9 f I . I
OSP
30.0 f I .9
26.0 f 2 . 2
30.7 f 2 . 1
122.8 f2.7
120.5 f2.3
122.0 f 2 . 5
dV /dt
(mV) AMP (mV) A P D 50":, (ms) A P D 75y0 (ms) A P D 95",, fms) ERP (ms)
*cv
251.9 f 10.6
237.7 f 8 . 2
242.1 f9.0
x II)
380.6 f26.0 P< 0.0005 89.3 f I .8 26.8 412.3
288.5 f7.6
299.9 f9.4
351.1 flO.O
340.9 f8.0
347.5 f9.0
281.4 f 13.9
285.2 f 14.2
274.3 f 14.0
2.37 fO. 18
2.55 f0.23
2.49 f 0 . 2 5
~~~~~
Abbreviations: Meanings o f symbols as in Table 2. The data represent mean values f S E M from I5 experiments. 'n-
-
228.3 f 18.8
P< 0.0005
P < 0.0005
87.1 f3.9 P< 0.05 26.7 f 4 . 9
82.2 f4.3 P< 0.01 22.7 f 4 . 5
116.5 f 2 . 7
114.2 f3.4
104.9 f 3 . 7
P< 0.01
214.4 *9.1
203.3 f9.7 P< 0.005 265.9 f7.5 P < 0.005 309.4 f9.3
P< 0.0005 I I I .9 f9.2 P< 0.0005
272.5 f6.9 P< 0.005 3 17.9 f 7 . 3 P -0
300
*
l-
0
1
100 Fig. 3 Depression of conduction velocity by rirexiletitre in linear strands of Purkinje fibres. Ordinate: molar concentrations of niexiletine. Reduction in conduction velocity is concentration-depenciit with the threshold at I x I&' nrol~litrc',
. L
60
80
5
100
Fig. 5 Changes in itrenibratre responsiveness in a Purkinje fibre following increasing concentrations of nrexilerine. Ordinate: dVlrlt (Ves-1) of action potentials elicited bv stimulating the fibre at varying levels of inenrbrane potential during repolarisation before (control) and after mexiletine. Abscissa: Levels of membrane potential ( i n V ) during repolarisation of a driven action potential. Mexiletine shifred the membrane responsiveness curve ttownward~and to the left.
EFFECTS OF MEXILETINE ON PURKINJE FIBRE-VENTRICULAR MUSCLE JUNCTION
Fig. 4 The relationship between the changes in ERP on abscissa) and those in the APD ( A APD on ordinate) in a Purkinje fibre preparation following superfusion with varying concentrations of mexiletine. The shortening of the APD was greater at all concentrations of mexiletine.
(A ERP
These were studied in 12 preparations and results from a typical experiment with respect to the effects of different concentrations of mexiletine on antegrade and retrograde conduction at Purkinje fibreventricular muscle junction are shown in Fig. 5 . Significant reduction in both antegrade and retrograde refractoriness across the ventricular musclePurkinje fibre junction were apparent. However, substantial changes in the configuration of the refractory period curve were only apparent when the preparations were exposed to clearly toxic ( I x 10 mol*litre--')concentrations of mexiletine.
Mrxiletitie
011
A
isolateil carrliac tiiii,wli#
293 B
Ant e g rade
400 c
2 N -
s
Ret rogrcde
400 h
ffl
-E 300-
300
N
LL
a
I
I
€
li-
a
> 200
200 PF,-PF2
300 (ms)
20c -
300
200
400
VMI-VMz
400
(mS)
Fig. 6 A typical example of the effect of increasing concentration of mexiletine on the Purkinje fibre- papillary muscle junction. Panel A shows the effect of the drug on antegrade conclucrion. Note the prolongation of the effective refractory period at low doses with niarked alterations at higher dose level. Panel B shows inexiletine's effect on retrograde refractoriness. Note the prolongation of the refraciory period in a step-wise fashion related to the concentration of niexiletitie. The horizontal axis show the corcplin~interval at the stiniidation site and the vertical axis show the response at the sattipling sire
EFFECTS OF MEXILETINE O N THE ACTION POTENTIAL DURATION IN DIFFERENT PARTS O F T H E RIGHT BUNDLE BRANCH
These were studied utilising varying concentrations of mexiletine across the total length of the right bundle branch in five preparations. Action potential durations were measured at seven sites along the course of the right bundle branch stimulated at a cycle length of 800 ms. The mean data from these studies are summarised in Fig. 6. The shortening of the A P D that was observed was uniform along the preparation, there being no significant differential effect a t the 'gate' at both test concentrations (5x and I x mol*litre I ) used.
activity or by a combination of these mechanisms (Hauswirth and Singh, 1978). Our observations with mexiletine in isolated atria
"Oar 300 .
h
ffl
E
v
Discussion The fundamental mode of action of the available antiarrhythmic drugs have been interpreted for many years from an analysis of their effects on the various parameters of the action potential from tissues from different parts of the heart (Bigger and Mandel, 1970; Singh and Vaughan Williams, 1971; Rosen and Hoffman, 1973; Singh and Hauswirth, 1974). From such studies, it has been demonstrated that different antiarrhythmic agents exert their salutary effects essentially by depressing phases 0 and 4 of the cardiac action potential, by inhibiting sympathetic excitation, by affecting the repolarisation phase and by blocking the slow-channel
200
'
4' 'O0
P 1
2
k- RBB -14
3
4 5 6 7 False tendon-1Muscle
Fig. 7 Eflects of niexiletine on APD in differentparts of the ventricular conducting system. Ordinate: duration of APD to 90% of repolarisation in nisec. Abscissa: location of the recording sites between right bundle branch ( R B B ) and ventricular muscle. Nore lack of preferential shortening of the APD at the point where the APD is longest.
294
and canine Purkinje fibres extend previous observations (Singh and Vaughan Williams, 1972) which indicated that the drug acted predominantly by depressing the upstroke velocity of phase 0 of the action potential in rabbit atria and ventricular muscle. The present studies, utilising a wide range of concentrations, have demonstrated that concentrations of the drug corresponding to its therapeutic plasma levels to I x in man (0.5-2.0 pgacm-' or 2.21 x mol-litre-') exerted relatively trivial depressant effects on atrial intracellular potentials and on sinus node automaticity. This is consistent with the observation that the drug has variable effects on the spontaneous sinus rate at therapeutic plasma levels in man (Roos e/ a/., 1976). It is of interest, however, that sino-atrial conduction was depressed by high concentrations of mexiletine; it is thus conceivable that the depressant effect of the drug on sinus node automaticity and intra-atrial conduction may be more pronounced in diseased tissues. Indeed, sinus arrest and failure of sino-atrial conduction have been encountered in patients with the sick sinus syndrome even in the context of therapeutic serum levels of mexiletine (Roos et al.. 1976). The observed lack of significant effects of therapeutic concentrations of mexiletine in atrial tissues, however, suggests that the drug is unlikely to be of value in correcting supraventricular tachyarrhythmias but no systematic clinical studies have been reported. The lack of effect of mexiletine on normal sinoatrial automaticity emphasise the possibility that the fast sodium channel does not play a significant role in the genesis of the normal pacemaker potential. However, the fact that mexiletine does abort ectopic ventricular arrhythmias in varying clinical and experimental conditions (Hoffbrand e/ a/., 1977) not only suggests that abnormally generated phase 4 depolarisation is suppressed by the drug, but that the ionic mechanisms underlying such ectopic activity may differ from those in the normal pacemaker tissues. Clearly, the effects of mexiletine on automaticity produced by ischaemia, digitalis, o r other arrhythmogenic interventions will be of interest. Perhaps of the greatest clinical relevance are our studies which have examined the effects of mexiletine on Purkinje fibres, Purkinje fibre-ventricular muscle junction and on the so-called gating mechanism in the distal conduction tissues particularly in light of the observation that the drug is specially potent in controlling ventricular arrhythmias (Hoffbrand e/ a/., 1977). The effects here were striking with respect to the depression of V,, membrane responsiveness, and to a lesser extent, of directly measured conduction velocity. All the observed effects are accountable in terms of the drug's de-
Iwuo YuiiiuKuchi, Bruniuli N . Singh, untl Williuiii J. Muiiclc.l
pressant effect on the fast sodium channel consistent with its known local anaesthetic properties on nerve (Singh and Vaughan Williams, 1972), the reduction in the resting membrane potential occurring only in potentially toxic concentrations of the drug. O n the basis of these effects, one might have expected a lengthining of the E R P with a delayed return of excitability in the myocardium. I t was, therefore, somewhat surprising that the drug produced a concentration-dependent abbreviation of the ERP, the nature of which is not understood. However, the shortening of the ERP, was accompanied by a greater abbreviation of the APD. The precise mechanism of accelerated repolarisation induced by mexiletine is uncertain and will require voltage clamp studies (Hauswirth and Singh, 1978) for the elucidation of the nature of the changes in the underlying ionic conductances. Nevertheless, it is noteworthy that the ratio of E R P / A P D was always prolonged by mexiletine which, in association with depressed conduction velocity, may be significant in terminating re-entrant ventricular arrhythmias. The possibility must also be considered that mexiletine might exert a differential effect on the E R P in the normal and ischaemic tissues, shortening it in the former and lengthening it in the latter, a possibility supported by His bundle studies in man. For example, in patients with relatively normal conduction, McComish and co-workers (1977) found that the E R P was shortened by mexiletine, whereas in those with pre-existing but clinically occult conduction system disease, the drug produced a significant lengthening of the E R P in His-Purkinje system (Roos e / a/., 1976). The electrophysiological effects of mexiletine in ischaemic tissues will clearly be of much interest especially in view of the fact that many of the electrophysiological properties of mexiletine described here closely resemble those of lignocaine reported previously. Particularly significant in this context are the recent observations (Kuppersmith et a/., 1975; Lazzara et al., 1978) which have indicated that very much lower concentrations of lignocaine reduce the upstroke velocity of phase 0 in hypoxic and ischaemic cardiac cells than those which produce comparable effects in normal fibres. Thus, quantitative differences in the effects of mexiletine in normal and ischaemic tissues are also likely, a feature that is of potential significance in the overall action of the drug in the control of arrhythmias complicating myocardial ischaemia. In our studies, it is also noteworthy that the drug produced moderate decreases in retrograde as well as antegrade conduction at Purkinje fibre-ventricular muscle junction. Nevertheless, the basis for reentrant ventricular arrhythmias have been repeatedly demonstrated to be due to the presence of slowed
Mc.xilctini~on isolutcrl cirrcliuc muscli~
295
conduction, unidirectional block, reflection o r forteza for illustrations and to Mrs Bernadette summation. These basic reentrant mechanisms Fluharty for photography. Mexiletine was supplied would be expected to be more readily operative in as a gift for experimental studies by Dr W. Benson of areas where inherent electrophysiological differences Boehinger-lngleheim, Ltd., New York. exist in Purkinje-papillary muscle junction. Mexiletine’s effects at this junction may, in the setting of abnormalities such as those associated with ischaemia, be antiarrhythmic. However, our data have not proved conclusive in this regard. Further- References more, in our studies, the effects of the drug on the Allen, J. D., Kofi Ekue, J . M., Shanks, R. G., and Zaidi, APD were distributed uniformly in the major areas S. A. (1970). T h e effect on experimental cardiac arrhythof the right bundle branch when the preparation was mias of a new anticonvulsant agent. KO 1173. and its comparison with phenytoin and procainaniide. British stimulated at a frequency of 75 per min: no preJournal of Pharntacologv. 39, 183- 184. ferential change with respect to the shortening of the Bigger, J. T., Jr., Bassett, A. L., a n d Hoffman, B. (1968). APD at the level where the duration of the action Electrophysiological effects of diphenylhydantoin o n potential was maximal (‘gate’) was demonstrated. In canine Purkinje fibres. Circulation Research. 22, 2 13-224. contrast, Vaughan Williams (1977) who used a Bigger. J. T.. Jr., and Mandel, W. J. (1970). The electrophysiologic effects of lidocaine on canine Purkinje fibre lower stimulation frequency, found that mexiletine and ventricular muscle. Jocirnal of Clinicul Invvsfi#arion, like lidocaine (Wittig rt u/., 1973) shortened the 49,63-77. APD throughout the conduction pathway and in the Campbell, N. P. S., Chaturvedi. N. C.. Kelly, J. G., Strong. J. E.. Shanks. R. G.. a n d Pantridge. J. F. (1973). Mexileventricular muscle, but had a strikingly preferential tine (KO 1173) in the management of ventricular dyseffect where the APD was longest, so that APD rhythmias. Lancet, August 25. 404407. became more uniform. It has been suggested, (Wittig Campbell, R . W. F.. Talbot, R. C., Dolder, M. A,. Murray, et a / . , 1973) that this effect would induce a relatively A., Prescott, L. F., and Julian. D. G. (1975). Comparison o f procainamide and mexiletine in prevention o f ventrifaster conduction of premature impulses and the cular arrhythmias after acute myocardial infarction. greater uniformity of A P D (by reducing inhomoLancer, June 7. 1257- 1260. geneity) may contribute to an antiarrhythmic Hauswirth, 0.. a n d Singh. B. N. (1978). Voltage clamp in action. Whether such an effect of mexiletine demonheart muscle in relation to the genesis and the pharmacological control of cardiac arrhythmias. Pliurnracologic strated in isolated preparations at a low stimulation R V V ~ C 30, W . 5-63. frequency does indeed contribute to its overall Hoffbrand, B. I . . Shanks. R. G., a n d Bell, J. W. (1977). salutary effect in aborting ventricular arrhythmias Mexiletine in ventricular arrhythmias. PosrRrac/itarr remains unknown however, but merits further Mrrlical Joctrnnl (Suppl I ) 53, 1-169. 1977 Kupersmith. J., Antman, E. M.. and Hoffman, B. F. (1975). investigation. I n vivo electrophysiological effects of lidocaine in canine In summary, the present experiments have shown acute myocardial infarction. Circrtlurion Research. 36, that concentrations of mexiletine which are thera84-9 I . peutically relevant, have little effect on atrial intra- Lazzara, R . , Hope. R. R., El-Sherif, N., and Scherlag, B. J. (1978). Effects of lidocaine on hypoxic a n d ischaemic cellular potentials o r sinus node automaticity but cardiac cells. Anic,ricun Journal of Carcliolog~.41, 872-879. sino-atrial conduction is prolonged. In Purkinje McComish. M.. Robinson, C.. Kitson. D., and Jewitt. D. E. fibres the drug depresses conduction velocity, mem(1977). Clinical electrophysiological effects o f mexiletine. brane responsiveness and the v,, of phase 0 of the Postgra~luccteMcrlical Journal (Suppl I ), 53. 85-9 I. action potential associated with a shortening of the Paes d e Carvalho. A.. deMello, W. C., and Hoffman, B. F. (1959). Electrophysiological evidence for specialised fibre APD and the E R P but with an increase in the ratio types in rabbit atrium. Aniwicnn Journal of Phy.sici/ogy. APD/ERP. The drug decreased antegrade as well as 196,483488. retrograde conduction at the Purkinje fibre-ventricuRoos, J . C., Paalnian, A. C. A,. and Dunning. A. J. (1976). Electrophysiological effects of mexiletine in man. Bri/i.d/ tar muscle junction while producing uniform Hcarr Journal. 38, 1262- I27 I . shortening of the A P D in the distal conduction M. R., a n d Hoffman. B. F. (1973). Mechanisms o f system without a preferential effect at the ‘gate’. The Rosen. action of antiarrhythmic drugs. Circulation R i w a r r h , 32, mechanisms of action of mexiletine are thus comI. plex but resemble those of lignocaine, reflecting a Singh. B. N.. a n d Hauswirth, 0. (1974). Comparative mechanism of action o f anti-arrhythmic drugs. American simultaneous alterations in the depolarisation and H m r t Jorirnal, 87. 367-382. repolarisation processes in myocardial fibres. Singh, B. N.. and Vaughan Williams, E. M. (1971). Effect of Supported in part by SCOR Grant No. 17651, National Institutes of Health, Bethesda, Maryland. We are grateful to Ms Darlis Martin and Mrs Betty Garrigues for secretarial help, to Lance La-
altering potassium concentration o n the action of lidocaine and diphenylhydantoin on rabbit atrial and ventricular muscle. Circrrlarion Rrseurch. 29, 286-295. Singh. B. N.. a n d Vaughan Williams. E. M. (1972). Investigations of the mode o f action o f a new antidysrhythmic l ( i ~ 1-9. .,, drug, KO 1 173. BrirDh Jortrnul ( ~ ~ P l r c r r ~ / u c ~ o44,
Strauss, H. C.. Saroff, A. L.. Bigger, J. T., and Giardina, E. G. V. (1973). Premature atrial stimulation as a key t o the understanding of sinoatrial conduction in man. Circularion. 41,86-92. Talbot. R. G. (1978). Mexiletine. Anwricnn Hear1 Jovrnul, 89,537-538. TdlbOt, R. G.. Clark, R. A.. Nimnio. J., Neilson, J. M. M.. Julian, D. G., and Prescott. L. F. (1973). Treatment of ventricular arrhythmias with mexiletine (KO 1173). Luncc./. 2. 399-400.
Vaughan Williams. E. M. (1977). Mexiletine in isolated tissue models. Pos/grarlnu/e Medicul Journal, (Suppl I ), 53. 30-34. Willox, S.. and Singh, B. N. (1976). A sensitive gas chromatographic method for the estimation of a new antiarrhythmic compound mexiletine (KO 1173) in hiological fluids. Jaiirnui of Chronruro/i~g.I~, 128, 196-197. Wittig, J. H.. Harrison, L. A.. and Wallace, A . G . (1973). Electrophysiological effects of lidocaine on distal Purkinje fihres o f canine heart. Anlerican Hcurr Jotrrnul. 86. 69-75.
