European Journal of Pharmacology, 55 (1979) 171--179

171

© Elsevier/North-Holland Biomedical Press

ELECTROPHYSIOLOGICAL EFFECTS OF DESIPRAMINE ON GUINEA PIG PAPILLARY MUSCLES J. TAMARGO, S. RODRIGUEZ and P. GARCIA DE J A L 6 N

Department of Pharmacology, School of Medicine, Universidad Complutense, Madrid 3, Spain Received 4 July 1978, revised MS received 29 December 1978, accepted 11 January 1979

J. TAMARGO, S. RODRIGUEZ and P. GARCIA DE JAL6N, Electrophysiological effects o f desipramine on guinea pig papillary muscles, European J. Pharmacol. 55 (1979) 171--179. The effects of desipramine (DMI) in concentrations between 1 × 10 -7 M and 1 x 10 -4 M on various electrophysiological parameters were evaluated in ventricular papillary muscles of guinea pig. At concentrations ,~ 5 × 10 -s M, DMI produced a significant shortening in the action potential duration (APD) measured at both 50% and 100% of repolarization. At 1 x 10 -4 M, the terminal portion of repolarization was so prolonged that the total APD was not significantly different from control values. DMI (I> 1 x 10 -s M) did not change the resting potential but significantly, decreased the overshoot potential, the amplitude, and the maximum rate of rise of phase 0 depolarization (Vmax) and shifted the membrane responsiveness and membrane reactivation curves downward and to the right. The effective refractory period (ERP) was shortened or lengthened, the effect being dependent on the concentration, but always made the ERP long relative to APD. DMI, (1 × 10 -s M and 5 x 10 -s M), attenuated and abolished the spontaneous activity and the Ca-mediated action potentials induced in ventricular muscle fibers. The mechanisms responsible for DMI's in vivo arrhythmogenic or antiarrhythmic effects are discussed. In terms of changes in ion conductance most effects can be explained by a reduction in sodium and calcium conductance. Ca-mediated action potentials Membrane reactivation Spontaenous activity

Cardiac action potential Membrane responsiveness

1. Introduction Desipramine (DMI) has been widely used in psychiatric therapy for the last fifteen years. Changes in ECG pattern,including sinus tachycardia, prolongation of the PQ and QT intervals, supraventricular tachycardia, widening of the QRS complex, ST and T wave abnormalities, ventricular arrhythmias, bradycardia and asystole, have been described both for the therapeutic dosage and after overdosage (Jefferson, 1975; Vohra et al., 1975; Thorstrand, 1976). Studies in anesthetized animals indicate that DMI may produce (Elonen and Mattila, 1972; Barth and Muscholl, 1974) and/or prevent cardiac arrhythmias (Fekete and Borsy, 1964; Schmitt et al., 1970; Garcia de

Desipramine

Guinea pig

JalSn et al., 1975). All this evidence suggests that DMI must alter the electrophysiological properties of the cardiac fibers. The present work was undertaken to evaluate the effects of DMI on the transmembrane potentials of isolated perfused guinea pig papilary muscles.

2. Materials and methods

Guinea pigs (300--400 g) were killed by a blow on the head and the hearts were rapidly removed and placed in oxygenated Tyrode solution at room temperature. The right ventricular papillary muscles were excised from the heart and pinned to the bottom of a 10 ml Lucite tissue bath. The chamber was con-

172 tinously superfused at a rate of 4 ml/min with Tyrode solution of the following composition (mM): NaCl 137;NaHCO3 12; KCL 2.7; CaC12 1.8; MgC12 1.0; NaH2PO4 0.2; dextrose 5.5. The pH of the solution was adjusted to 7.3 by gassing with 95% 02 and 5% CO2. The temperature of the perfusate was maintained at 34 ° C. Preparations were stimulated regularly at a basal rate of 1 Hz. Squarewave pulses (2--3 msec. duration and threshold strenght) were generated by a multipurpose programable stimulator and were delivered through a pair of Teflon-coated silver electrodes. Each preparation was allowed to equilibrate for at least one hour before control measurements were made. After equilibration the stimulation rates ranged from 0.5 to 3.3 Hz. The rates of stimulation increased stepwise and records were taken approximately 1 min after each change. Transmembrane potentials were recorded through glass microelectrodes filled with 3 M KC1 (15--40 M~2 resistance). The microelectrodes were coupled to a dual beam storage oscilloscope (Tektronix 5 1 0 3 N ) v i a Ag--AgC1 connections and high-impedance, capacity-neutralizing amplifiers (WPI). Transmembrane potentials were photographed on 35 mm film with a Grass K4 Kymograph camera. The m a x i m u m rate of rise of the action potential ('~max) was obtained by electronic differentiation. Other parameters measured were resting membrane potential, amplitude, overshoot and duration of the action potential at 50 (ADPs0) and 100% (ADPI00) of repolarization. The definitions of effective refractory period, membrane responsiveness and membrane reactivation are based on those described by Strauss et al. (1968) and Carmeliet et al. (1976). The effective refractory period and membrane reacttivation after the end of the preceding action potential were measured by delivering a test stimulus at twice threshold strength at the desired intervals after every eighth action potential. Spontaneous activity was induced by adding 0.2 mM BaCl2 to the Tyrode solution. Ca-mediated action potentials were elicited by adding 0.2 mg/1 isoproterenol to

J. TAMARGO ET AL. high K (27 mM) Tyrode solution (Carmeliet et al., 1976). Powdered DMI was dissolved in distilled and deionized water. Further dilutions were carried out in Tyrode solution to provide the desired final concentrations. Five different molar concentrations were used in these experiments: 1 X 10 -7 M, 1 × 10 -6 M, 1 X 10 -s M, 5 X 10 -s M and 1 X 1 0 -4 M (0.02--26 pg/ml). Statistical analysis was performed using Student's t-test for paired data.

3. Results

3.1. Action potential characteristics The effect of DMI in concentrations between 1 X 1 0 -TM and 1 X 1 0 -4M was studied in ventricular fibers. After control measurements each preparation was exposed to successive concentrations of the drug or the effects of a single concentration were determined and the preparatio.n discarded. The effects of DMI were apparent within 5 min after the beginning of the perfusion and stabilized in 30 min or less. The control values of the parameters and the results obtained after 30 min exposure to the drug are summarized in table 1 and records of action potentials obtained in one experiment are shown in fig. 1. Concentrations from 1 X 10 -7 M to 5 × 10 -6 M exerted no significant effect on resting membrane potential or overshoot and thus there were no changes in the total amplitude of the action potential. At higher concentrations of DMI (1 X 10 -s M to 1 X 10 .4 M), the m a x i m u m rate of rise of phase o (~'ma x ), amplitude and overshoot were markedly reduced but no change was observed in the resting potential. At a concentration of 1 X 10 -4 M all fibers became inexcitable after 40 min. The action potential duration was significantly shortened at concentrations ~< 5 X 10 -s M. This shortening was due predominantly to an increased slope of phase 2 and the plateau level was shifted to more negative

ELECTROPHYSIOLOGICAL EFFECTS OF DESIPRAMINE

173

TABLE 1 E l e c t r o p h y s i o l o g i c a l e f f e c t s o f d e s i p r a m i n e ( m e a n -+ S.E.M.). T h e n u m b e r s in p a r e n t h e s e s r e p r e s e n t the n u m b e r o f observations. Resting potential

Overshoot (mV)

Amplitude (mV)

APDs0 (msec)

APD 100 (msec)

Vmax (V/sec)

(mY) C o n t r o l (12) DMI, 1 × 10 -7 M

89.0 -+ 1.7 89.0 + 1.5

32.6 -+ 0.5 31.5 -+ 0.5

123.6 -+ 2.1 123.0 -+ 2.5

117.6 + 5.9 88.3 + 6.5 2

159.0 + 8.7 130.5 + 4.8 2

164.0 + 4.9 162.6 -+ 3.2

C o n t r o l (12) DMI, 1 × 10 -6 M

89.0 -+ 1.7 89.0 -+ 1.5

32.6 -+ 0.5 31.5 -+ 0.8

123.6 -+ 2.1 121.6 -+ 2.8

117.6 + 5.9 80.3 -+ 3.4 3

159.0 + 8.7 117.3 + 5.5 3

164.0 + 4.9 160.2 + 4.0

C o n t r o l (12) DMI, 1 × 10 -s M

90.7 -+ 1.9 90.5 -+ 1.7

33.7 -+ 2.3 24.8 -+ 2.6 2

124.4 + 3.0 116.1 -+ 3.0 1

115.1-+6.3 80.0 + 5.9 3

157.9+8.0 124.8 -+ 6.1 3

176.6-+3.2 160.6 + 4.2 2

C o n t r o l (14) DMI, 5 X 10 -s M

86.7 -+ 2.6 86.5 -+ 2.2

34.9 + 1.1 21.1 + 2.7 3

122.5 -+ 2.2 108.2 -+ 2.6 3

109.2 -+ 6.1 84.5 -+ 3.2 2

154.2 -+ 3.3 135.1 + 4.8 2

168.4 -+ 8.7 124.5 -+ 9.2 3

C o n t r o l (8) DMI, 1 × 10 -4 M

86.8 + 2.2 84.3 -+ 1.4

34.3 + 2.1 13.3 -+ 3.1 3

120.3 + 2.4 96.3 -+ 2.2 3

112.6 + 2.5 108.9 -+ 5.9

137.3 -+ 9.8 135.0 + 3.2

168.4 + 8.6 108.2 -+ 6.4 3

I P "~ 0 . 0 5 . 2 P (0.01. 3 p ~ 0.001.

values. As a consequence the onset of phase 3 occurred earlier and its slope increased leading to a shortening in the ADPs0 and APD100 values. At 1 × 10 -5 M the slope of

50 rnv[ IO0 V/sec 50 rnsec Fig. 1. Effects of DMT on action potential characteristics o f ventricular muscle fibers. In each panel, t h e u p p e r tracing is t h e a c t i o n p o t e n t i a l a n d t h e b o t t o m tracing is t h e d i f f e r e n t i a t e d signal o f t h e a c t i o n potential u p s t r o k e (Ymax). A: c o n t r o l a c t i o n p o t e n t i a l . B, C a n d D: a c t i o n p o t e n t i a l s o b t a i n e d following t h e e x p o s u r e t o 1 × 10 -s M, 5 × 10 -s M a n d 1 × 10 -4 M, respectively.

phase 2 increased still further, the inflection between phases 2 and 3 became less distinct and the slope of phase 3 was decreased so that the action potential presented a triangular configuration. At concentrations between 5 × 10 -s M and 1 × 10 -4 M the slope of the terminal phase o f repolarization decreased progressively and its duration was so prolonged that even though there was a proportionately faster repolarization during the early phases, the APD,00 did not differ significantly from control values. The effects of DMI on ~'rmax and APD,00 were dependent on the time and frequency of stimulation. Fig. 2 shows the changes in Vm~ in 6 preparations driven at cycle lengths of 3 0 0 - - 2 0 0 0 m s e c . Before treatment the "¢max was slightly decrease (less than 10%) at the t w o more rapid rates of stimulation. DMI, 1 × 1 0 - s M and 5 × 1 0 -sM, decreased the Vmax at all stimulation rates b u t the decrease was greater at rapid than at slow rates. Action potentials were shortened more at slow rates where the control APD~00 was longer, b u t the percent change in (APDh00 at each rate of stimulation was virtually identical at any given DMI concentration.

174

J. T A M A R G O E T AL.

100-

200

:o

! _: ' 300

' 500

' 800

/1 1000

,oo • APD

t 2000

O ERP

C y c l e l e n g t h (msec)

Fig. 2. E f f e c t o f DMI o n rate d e p e n d e n t c h a n g e s in m a x i m u m rate o f rise. T h e m a x i m u m rate of rise was p l o t t e d o n t h e o r d i n a t e a n d t h e driven cycle l e n g t h o n t h e abscissa. O b s e r v a t i o n s are s h o w n for c o n t r o l c o n d i t i o n s (solid circles) a n d d u r i n g e x p o s u r e t o DMI at c o n c e n t r a t i o n s o f 1 × 10 -s M ( o p e n circles) a n d 5 x 10 -s M (solid triangles). Each p o i n t r e p r e s e n t s t h e m e a n o f 6 e x p e r i m e n t s ; vertical bars r e p r e s e n t t h e s t a n d a r d e r r o r of t h e m e a n .

This rate dependence of the Vrnax was observed not only at constant rates of stimulation, but also in premature action potentials elicited at different intervals after the end of the preceding action .potential. At t h a t m o m e n t the decrease in V m a x w a s comparable to that obtained at similar constant rates when the cycle length was the same. All changes induced by DMI concentrations up to 1 X 10 -s M were reversed within 40--60 min of perfusion with drug-free Tyrode solution. When concentrations higher than 1 × 10 -s M were used the changes were only partially reversible or irreversible.

3.2. Effective refractory period The effective refractory period (ERP) of ventricular muscle, defined as the shortest interval between two propagated responses, was determined in 7 preparations. The results were plotted as a function of DMI concentrations (fig. 3). At concentrations between I ×

C

|

I

I



7

6

5

4

Fig. 3. Effective r e f r a c t o r y p e r i o d ( E R P ) a n d a c t i o n p o t e n t i a l d u r a t i o n ( A P D ) o f v e n t r i c u t a r m u s c l e fibers as a f u n c t i o n o f DMI c o n c e n t r a t i o n in t h e p e r f u s a t e . E R P (solid circles) a n d APD ( o p e n circles) were p l o t t e d in t h e o r d i n a t e as p e r c e n t of c h a n g e a n d t h e negative c o m m o n l o g a r i t h m of DMI c o n c e n t r a t i o n was p l o t t e d o n t h e abscissa. Each p o i n t r e p r e s e n t s t h e m e a n of 7 e x p e r i m e n t s ; vertical bars r e p r e s e n t t h e S.E.M. Ordinate: % of c h a n g e ; abscissa: --tog [desip r a m i n e ] (M).

10 -7 M and 1 × 10 -6 M a significant shortening in ventricular ERP was observed (P < 0.01 in each case). At 1 × 10 -5 M, ERP was n o t signficantly different from control; at 1 × 10 -4 M, the mean ventricular ERP was significantly longer than control values (P < 0.001). The change in action potential duration relative to change in ERP is also shown in fig. 3. It is clear that at all concentrations the ERP was prolonged out of proportion to any change in total action potential duration.

3.3. Membrane responsiveness and membrane reactivation To further examine the effects of DMI on the rate of rise, the relationship between membrane potential at the time of activation and Vmax (i.e. membrane responsiveness) was studied in 4 experiments by recording the

ELECTROPHYSIOLOGICAL EFFECTS OF DESIPRAMINE

electrical activity of different external concentrations of potassium (2.7--13.5.M). A typical S-shaped curve relating the Vmax to membrane potential is illustrated in fig. 4. DMI in concentrations ~> 1 × 10 -s M consistently decreased the ~'max at any given level of membrane potential, as demonstrated by the shift of the curve downward and to the right. This indicates that in the presence of DMI the membrane had to be repolarized to a greater extent in order to obtain the same rate of rise as in control conditions. Membrane reactivation, defined as the relationship between Ym ax of premature responses and the coupling interval was determined in 6 experiments. DMI, 1 × 10 -s M and 5 × 10 -s M, caused a depression of membrane reactivation shifting the curve downward and to the right (fig. 5).

3.4. Ca action potentials

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Electrophysiological effects of desipramine on guinea pig papillary muscles.

European Journal of Pharmacology, 55 (1979) 171--179 171 © Elsevier/North-Holland Biomedical Press ELECTROPHYSIOLOGICAL EFFECTS OF DESIPRAMINE ON G...
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