Electromechanical trachealis muscle: R. F. COBURN Department of Physiology, Philadelphia, Pennsylvania

coupling in canine acetylcholine contractions

University 19104

of Pennsvlvania J

COBURN, R. F. Electromechanical coupling in canine trachealis muscle: acetylcholine contractions. Am. J. Physiol. 236(3): C177-C184, 1979 or Am. J. Physiol.: Cell Physiol. 5(2): C177-Cl84, 1979.--0rgan bath and double sucrose gap experiments were performed on strips of canine trachealis muscle. With a slow increase in acetylcholine (ACh) concentration to 2 X lo-” M, onset of membrane depolarization coincided with onset of development of tension, and tension changes roughly paralleled membrane potential changes. Quasi-steady-state membrane depolarization-tension plots showed shifts in ACh data compared to elevated K+ data, so that ACh membrane depolarization for a given tension was smaller than seen during elevated K’ contractions. Verapamil, D-600, and La’]’ completely inhibited K+ contractions, but ACh contractions were resistant to these agents. During ACh contractions, plasma membrane hyperpolarization evoked with anodal current pulses resulted in contractions, and evoked depolarization usually resulted in relaxation. These anomalous tension responses to current injection were unchanged by verapamil, D-600, or La”’ (at concentrations that completely inhibited K+ contractions), or by tetrodotoxin or propranolol. During K* or Shydroxytryptamine (5HT) contractions, current injection gave tension responses that suggested electromechanical coupling mechanisms were operative. These data are interpreted as follows: membrane potential-dependent increases in plasma membrane Ca”+ transport that occur during EC+ contractions are inactivated or not activated during ACh contractions. ACh contractions appear to be largely independent of ACh membrane depolarization in contrast to K’ and 5HT contractions in which electromechanical coupling is evident and membrane potential-dependent Ca”+ transport operative.

airway

smooth muscle; verapamil;

D-600

A PRE VIOUS STUDY (7) the role of electromechanical upling (E-M couplin .g) in canine trachealis muscle was investigated during 5hydroxytryptamine (5HT) contractions. Measurements of membrane potential (Em)tension relationships during development of 5-HT contractions were compared with similar data during contractions caused by elevated K’ (K’ contractions). E-M coupling was determined directly by passing cathodal or anodal current across plasma membranes and measuring tension changes. Evidence for &,-independent tension (pharmacomechanical coupling) (26, 27) included different membrane depolarization-tension relationships with 5-HT and with K+ contractions, and the inability to completely reverse 5-HT tension by electrically reversing 5-HT depolarization, but E-M coupling mechanisms are IN

co

0363-6143/79/0000-OOOO$OI

.25 Copyright

@ 1979 the American

Physiological

School of Medicine I

also operative during 5-HT contractions (7). In the present study, I have extended the above data with similar experiments during acetylcholine (ACh) contractions. Because it is known that Ca’+ is a link in E-M coupling, I have also studied effects of Ca’+-antagonist agents in these experiments. Goals were to identify the relative importance of &-dependent and E,-independent tension. “E,-dependent tension” infers that at least one of the events linking contact of an agonist with the smooth muscle cell to activation of contractile proteins is controlled by changes in E,. It is generally accepted that membrane depolarization is a trigger of Ca2’ influx into the cell or release from sequestration sites, which then activates myofilaments. “Z&independent tension” infers that tension resulting from contact of the smooth muscle cell with the agonist is independent of E, changes. In all experiments, I compared ACh data with data obtained during K’ contractions in which tension development may all be E,, dependent. Data obtained in this study suggest that &-,-dependent changes in plasma membrane Ca” transport are not operative during ACh contractions and that E,-independent mechanisms are involved in ACh contractions. METHODS

Mongrel dogs, 15-20 kg, were anesthetized with pentobarbital and killed with intracardiac KCl. The trachealis muscle was prepared as described previously (7) for study in organ bath or double sucrose gap experiments. The sucrose gap apparatus was identical to that used by Btilbring and Tomita (3). Sucrose drip rates were adjusted to give a node width of 1 mm. Resistance of the sucrose channels containing muscle was usually -150 kG, and without muscle, -6 IMa. Previous experiments (7) have suggested that this apparatus measures -80% of E, changes, a value consistent with sucrose gaps used in other laboratories (6). The canine trachealis muscle (CTM) was reasonably stable in the double sucrose gap apparatus for 6-8 h (i.e., membrane resistance fell only -10%/h). Experiments were performed l-4 h after mounting the preparation in the apparatus. I performed two types of double sucrose gap experiments: a) E-M coupling was assesseddirectly by injecting anodal or cathodal current and determining effect on tension; or b) quasi-steady-state membrane depolarization-tension relationships were obtained by allowing ACh Society

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R. F. COBURN

or K’ concentrations to increase slowly over a lO- to 1% min time period. Because steady-state E, and tension is approximated in 20-40 s, these quasi-steady-state data can be considered to be nearly steady state (7). Data were used in comparisons of K’ and ACh contractions only if repeated runs gave similar data, Changes in measured E, in elevated K+ experiments are complicated by changes in the liquid junction potential between sucrose and test solution. It was shown theoretically and experimentally (7) that changes in measured E, on adding K’ are only minimally exaggerated by junction potential changes. Advantages of the quasi-steady-state method are a) that depolarization-tension data over a wide range of drug and K+ concentrations can be obtained in a single preparation, and that b) it is particularly easy to see the depolarization-tension relationship at threshold drug concentrations. A disadvantage is that drug and K’ concentrations are not known except at the final steadystate concentration. Organ bath experiments were performed in a 5-ml chamber, as described previously (7). Drug concentration-tension relationships were determined using a cumulative technique. In K+ experiments a solution containing 59426 mM EC+was infused into the organ bath at a rate of 1-2 ml/min. Drugs were injected directly into the organ bath, Modified Krebs solution was utilized in most experiments, except for La3+ experiments in which the solution was buffered with Tris. Modified Krebs solution had the following composition in mM: Na’, 137.0; K’, 5.9; Mg2+, 1.2; Ca2’, 2.5; Cl-, 134; HzPO4, 1.2; HC03 , 15.4; glucose, 11.5. This solution was equilibrated with 97% 02 and 3% CO:! and had a pH of 7,33-7.38. The composition (mM) of “Tris solution” was Na’, 132.0; K’, 5.9; Mg2+, 1.2; Ca2+,2.5; Cl-, 146; glucose, 11.5; Tris, 5. The solution was equilibrated with 100% 02 and was adjusted with HCl to give a pH of 7.33-7.38. In elevated K’ experiments, K+ was substituted for Na’ in equimolar amounts. Drugs used in this study included acetylcholine (Sigma); 5-hydroxytryptamine creatinine sulfate monohydrate (Aldrich); atropine sulfate (Eli Lilly); verapamil HCl (Knoll Pharmaceutical); D-600 (Knoll AG.); propranolol hydrochloride (Ayerst); tetrodotoxin (Sigma).

FIG. 1. Dose-response relationship with acetylcholine (ACh) and 5hydroxytryptamine (5-HT). Mean data from four experiments. Small maximal 5-HT tension has been previously reported in airway smooth muscle from other species ( 13).

TABLE

1. Electrical

and mechanical data ACh

Maximal tension, g/g tissue Depolarization, mV Depolarization at 50% max. tension RM (% control)

897 k n= 10.3 -+ 12=6 3.0 zk n= 24 -t

RM, membrane

determined

resistance

59 mM K’

75 (5 x lo-” M) 4 2.4 (lo-” M) 2-l 6 7 with

anodal

790 t ?2=4 24.2 k n=6 14.5 k n=6 28 k current

82 4.2 2.1 9

pulses.

response to ACh is similar to that described by others (11, 30). Maximal depolarization is less than that found using microelectrode techniques (II, 3O), probably due, at least in part, to the inability of the sucrose gap to measure the entire E, change. Membrane potential oscillations were only rarely seen in our preparations. The onset of tension during slow increase in the K’ of the perfusing solution generally occurred when the membrane was depolarized 5-8 mV. The largest increases in K+ tension were seen when E, was reduced lo-20 mV and maximal tension occurred at 20-25 mV depolarization. Figure 3 shows a typical comparison of depolarization-tension data taken from “slow” ACh and K’ runs on the same preparation. In all six experiments, ACh RESULTS data were markedly different from K+ data in that memACh contractions, coupling mechanisms. ACh, 5 x brane depolarization for a given tension was smaller. lo-* to 5 x 1O-4 M, caused sustained contractions in Table 1 gives mean data for the six experiments. Similar canine trachealis muscle (CTM), as reported by others plots were obtained using steady-state data obtained at (11, 17, 29, 30). Steady-state dose-response relationsh!>ps different [ACh] and [K’]. The quasi-steady-state K’ determined in organ bath experiments are shown in Fig. depolarization-tension relationship might be influenced 1. In the sucrose gap, the onset of tension during slowly by release of neurotransmitters from intramural nerves. increasing ACh concentrations always occurred almost Propranolol (4 FM) had no measurable effect on these simultaneously with the onset of depolarization of the plots nor did 1.4 PM atropine studied in the last EC+run membrane, and tension developed roughly in parallel in several experiments. The latter finding was surprising with a fall in E,. No action potentials were seen during because it was expected (11) that atropine would decrease this graded E, response to ACh. After maximal tension K’ concentrations slightly at low [K+], so that the actual and depolarization was developed, E, increased several difference between K+ and ACh depolarization-tension millivolts, whereas tension remained constant. After plots would be greater than found in experiments in the wash, tension relaxed as Em returned to control. Mean absence of atropine. data for ACh are listed in Table 1. Figure 2 illustrates The importance of E-M coupling during ACh contracACh data obtained in the sucrose gap. The graded E, tions was evaluated by displacing E, with anodal or Downloaded from www.physiology.org/journal/ajpcell by ${individualUser.givenNames} ${individualUser.surname} (130.070.008.131) on January 14, 2019.

E-M

COUPLING

IN

CANINE

TRACHEALIS

Cl79

MUSCLE

1 IOmV

I 150 mg I 1 60 set

ACh 2 x10a7 g/ml

FIG. 2. Effect of ACh administration on membrane potential f&J and isometric tension (T). ACh was infused at a constant concentration. (Data obtained in sucrose gap apparatus.)

1 IOmV

I 150 mg Wash

h K+

01

0

I G

5

Membrane

M-$ IO

/O

, 15

Depolarization

I 20

25

(mV)

FIG. 3. Relationship of membrane depolarization and tension during “slow” increase of [ACh] and [K’], and after removal of agents from perfusing solution. During development of tension, E, and tension were nearly steady state due to slow increase in ACh and K* concentration, but during washout this was not the case. Washout data are shown to illustrate the finding that even under conditions in which steady state did not exist, depolarization-tension plots were markedly different with ACh and with K+. In K’ run shown here, maximal tension was not achieved as occurred in most other 59 mM K’ runs.

cathodal current and measuring effects on tension. This approach has demonstrated tightly coupled Em-tension relationships during K’ and 5-HT contractions (7). Figure 4 illustrates typical data. Instead of reversing ACh tension, anodal current always caused the opposite tension response, i.e., a phasic contraction. In a few strips the response was biphasic, a phasic contraction followed by a small relaxation during the IO- to 15-s current pulse. Cathodal current usually caused a relaxation, but occasionally a small contraction. Onset of these evoked tension changes had a latency period of about one second, and were not altered by 6.4 PM propranolol, 1 PM tetrodotoxin, La3+, 1-5 mM, 2.5 x 1W5 M verapamil, or 6 x 10B6 M D-600 (Fig. 5). Calcium Antagonist Agents. I hoped to separate plasma membrane Em-dependent Ca”-transport processes from other cellular J&-dependent processes that

may influence ACh contractions. As will be discussed in a later section, La”‘, verapamil, and D-600 are known to uncouple E-M coupling in K’ contractions in other graded smooth muscles, probably by blocking &dependent plasma membrane Ca2* transport. Eight-minute pretreatment with verapamil inhibited EC+ contractions in a dose-dependent manner, 50% inhibition occurring at 5 x lop6 M and maximal inhibition at 2 x lo-’ M. Verapamil injections (2 x 10B5 M) during fully developed K’ contractions caused a fall in tension to base line (Fig. 6). Verapamil(2 X low5 M) had either no effect on resting Em (n = 4) or caused a I- to 2-mV hyperpolarization of the membrane (n = 4), without a change in the size of electrotonic potentials produced with brief anodal current pulses. The membrane was depolarized by 59 mM K+ to a similar extent in the presence and absence of verapamil (2 x lo-" M). ACh contractions were resistant to verapamil (8-min exposure) at concentrations that completely inhibited K’ contractions, averaging 82 t 5% of control, and had a similar configuration. Injection of verapamil during ACh contractions caused a small relaxation (Fig. 7), in contrast to the complete relaxation seen when this agent was injected during EC+ tension. ACh given after pretreatment with verapamil resulted in membrane depolarization, similar to that observed in control runs; and verapamil given during ACh contractions did not have an effect on E, that I could detect. Effects of verapamil on ACh contractions were similar in Krebs and Tris solutions. For comparison, effects of verapamil on 5-HT contractions were also determined. Verapamil (2 x lo-’ M) completely or nearly completely inhibited development of 5-HT tension without inhibiting 5-HT depolarizations of the membrane. Verapamil injections during 5-HT contractions caused complete relaxations, with a time course similar to relaxations seen when verapamil was injected during EC+ contractions. D-600 (5 x 10S6 M) had identical effects on K+, ACh,

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R. F. COBURN

A

5 HT

B 10mV

60

C

ACH

I

SEC

ACH

I

10 mv

ISOmg

I

60

SEC

4. Effects of anodal and cathodal current on membrane potential and tension during 5-HT and ACh contractions. A, SHT, 5 X IO-‘j M; B, ACh, 2 x W” M; C, ACh, 2 x 10-” M. In C after initial anodal

and two cathodal current injections, anodal and cathodal pulses were alternated so that E,, oscillated from hyperpolarization to depolarization to hyperpolarization. I = 5-10 PA.

and 5-HT contractions to those seen with 2 x lo-” M verapamil. I did not study effects of this agent on E, or E, changes with the various agonists. Like verapamil, 1.5-3 mM La”’ had a negligible effect on resting E, (mean effect -1.2 t 0.6 mV, YL= 5) and did not inhibit E, changes resulting from injection of ACh. At this concentration (8-min treatment) La3+ completely inhibited development of K+ contractions and, when injected during fully developed K’ contractions, caused a fall in tension with a similar time course as with verapamil or D-600 injection. ACh contractions were resistant to pretreatment with La”+, as described previously (l7), but there were differences between effects of La3’ and the other Ca2+-antagonist agents. ACh contractions became more phasic after pretreatment with La”+ and tension slowly decreased so that the tonic or steadystate tensions were only l&30% of control. Administration of 3 mM La3+ during an ACh contraction resulted in a more prolonged relaxation than seen with verapamil or D-600. The addition of 3 mM La3+ after verapamil (2 x lo-” M) during an ACh contraction proved an effect of La”’ in addition to the verapamil effect. Depolarized Preparations. In canine trachealis muscle depolarized with 126 mM K’, administration of ACh caused additional tension development, as shown in Fig. 8. Similar findings were also demonstrated with 5-HT.

development of tension might be E, independent and/or Em dependent. ACh data were compared with data obtained during K+ contractions because it is likely that the mechanism or mechanisms involved in generating K+ tension are Em dependent. This concept is supported by the finding (7) that electrical reversal of 59 mM K+ depolarization almost completely, or completely, reversed 59 mM K’ contractions in CTM. EC+ contractions in other graded smooth muscles are dependent on extracellular Ca2+ and the Ca2+ permeability of the plasma membrane during K+ contractions is E, dependent (12, 15, 23, 31, 32). The mechanism of this J&-dependent Ca2+-transport mechanism is unknown but probably involves Ca2+ channel-gating in which the gate is charged; but it could also involve an &-dependent carrier or another mechanism. The Ca2+-antagonist agents, La3+, verapamil, and D-600, which appear to block &-dependent Ca2’ transport so that the plasma membrane Ca2’ permeability does not increase with depolarization (1, 2, 12, 14-16, 18, 19; 21, 23, 25), completely inhibit K+ contractions in smooth muscle (2, 14, 16, 19, 23, 31-33). There is some evidence that verapamil and La3+ do not inhibit contractions resulting from Ca2+ release from intracellular stores (I, 21), These findings imply that with K+ contractions there is no J&dependent release of activating Ca2+ or Z&-dependent Ca2’ uptake by the cell that is not inhibited by these drugs. The K’ contractions in CTM appear to have the same characteristics as found in other graded smooth muscles, i.e., dependency on extracellular Ca” (17) and complete

FIG.

DISCUSSION

I investigated the possibility that the sequence of events leading from contact of the CTM cell with ACh to

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E-M

COUPLING

IN

CANINE

TRACHEALIS

inhibition by La3’, verapamil, or D-600. These agents may have effects other than inhibition of Em-dependent Ca2’ channels (9, 20, ZZ), but the strength of this phar-

ACH

CONTRACTIONS VERAPA

CONTROL

I I

MIL

IO MV 0-t G D 600

CONTROL

Cl81

MUSCLE

IO MV 0.1 G

b - 4 IO SEC FIG. 5. Effect of verapamil (2 X IO-” M) and D-600 (7 x lo-” M) on tension responses to electrically evoked hyperpolarization. After control data were obtained, the calcium-antagonist agent was added to the perfusing solution, and after 8 min, effects of anodal current were again determined. This figure shows lack of an effect of Ca”+-antagonist agents on anomalous tension response to evoked changes in membrane potential.

macological approach is the similar effect of the three agents. One of the agents, La3’, probably does not enter the cell, which suggests that the Ca2+-antagonist effect is at the level of the plasma membrane. The La”’ and other agents did not uncouple cell-to-cell electrical connections because electrotonic potentials were not decreased by these agents. Each of the Ca2’ antagonists caused relaxation of K+ contractions, despite persistance of membrane depolarization. This suggests that inhibition of the Em-dependent calcium transport mechanism utilized during K’ contractions is a necessary step for relaxation and that mechanisms of intracellular or plasma membrane Ca2’ scavenging are operative even though the membrane remained depolarized. ACh contractions had markedly different characteristics from K’ contractions, E,-tension relationships for ACh contractions were significantly different. More tension was always observed for a given depolarization, as has been previously reported by Farley and Miles (11) and by Suzuki et al. (30), a finding similar to that seen during 5-HT contractions in CTM (7). These data do not prove there are mechanisms for development of tension that are completely independent of E, changes, because it is possible that ACh or other agents can increase the number of Ca2’ sequestration or Ca2+-transport sites influenced by a given Em change or have other E,amplifying effects. This possibility has been tested for 5HT contractions in CTM in which, as with ACh contractions, Em-tension relationships are displaced from those obtained during K’ contractions (7). If &-,-tension displacement is due to changes in E, amplification, electrical reversal of depolarization should reverse tension. During 5-HT contractions, electrical reversal of 5-HT depolarization did not completely reverse tension (7), although reversal of depolarization in the same preparation did completely reverse tension during K+ contractions. Thus it is possible that the displaced I&tension

FIG. 6. Effect M) during a K’ caused complete slon.

of verapamil contraction. relaxation

(2 x lo-” Verapamil of K’ ten-

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Cl82

R. F. COBURN

plot may reflect &-,-independent mechanisms of generating tension. The approach of assessing E-M coupling by electrically displacing E, and measuring effects on tension gave unexpected results during ACh contractions, in that electrical reversal of ACh depolarization resulted in a phasic contraction, rather than a relaxation. Contraction seen during evoked hyperpolarization and, also, relaxation seen during further evoked depolarization of the mem-= brane, might be related to release of a neurotransmitter from nerve cells in the preparation, but neither propranaltered these responses. These re0101 or tetrodotoxin sponses might be caused by current flux across intracellular membranes (28), but evoked E, displacements during Kt and during 5-HT contractions gave appropriate responses consistent with E-M coupling. It can be argued that because current injections were similar in K+, 5-HT, and ACh contractions, effects on internal membranes or neurotransmitter release should be similar and that anomalous tension responses are not due to these phenomena. It is possible that anomalous responses were due to changes in intracellular-extracellular ratios of the positively charged ACh molecule, resulting from changes

Control

After Verapamil

1 #Ver&milT

ACh

5 min

ACh

FIG. 7. Effect of verapamil (2 x 10-:’ M) on ACh contractions. Figure shows that verapamil injected during an ACh contraction causes a relatively small relaxation and, if given prior to ACh injection, results in a smaller ACh contraction.

FIG. 8. 5HT and ACh in canine trachealis muscle 126 mM K. Tension scale decrease of sensitivity.

t 5-HT

t26 mM

K+

1

I 5 min

in E,. During current application in the double sucrose gap, there is expected to be heterogeneity of E, and E, displacement in different cells. This is due to decremental polarization across the node and to geometric complexity of the muscle preparation that causes uneven current densities (largely due to nonuniform extracellular resistance in series with membrane resistance) (24). However, I cannot relate heterogeneity of E, displacement to the anomalous tension change during ACh contractions. Furthermore, heterogeneity of E, displacement may be similar with ACh and with K+ and 5-HT contractions. The finding of anomalous tension responses to evoked E, displacements that were not altered by the three Ca”-antagonist agents suggests that the plasma membrane Em-dependent Ca2’ -transport mechanism is not operative during ACh contractions. Consider the possibility that E-M coupling was masked during the anomalous tension response. Addition of Ca”+-antagonist agents is then expected to increase the size of the anomalous response, but this did not occur. Data obtained in this study all seem consistent with the conclusion that the plasma membrane J&dependent Ca2+-transport mechanism is not operative during ACh contractions. These data include the lack of an effect of Ca2+-antagonist agents on anomalous tension responses to evoked E, displacements, the resistance of ACh contractions to Ca2+-antagonist agents, the displaced depolarization-tension plot that compared ACh to EC+contractions, and the finding that ACh contracted CTM that was depolarized by high extracellular K’. These data are supported by the previous finding that ACh contractions are resistant to a decrease in perfusing solution Ca2+ (no added Ca2’, 0.1 mM ethyleneglycol-bis(P-aminoethyl ether)N,N’-tetraacetic acid (EGTA), 30 min at 37°C) (17) It’is still possible that Em-dependent processes play a minor role during ACh contractions and that there is Emdependent Ca2’ release from cell membranes that is not inhibited by Ca2’ -antagonist agents and not utilized during K’ contractions, Current injection runs were performed during complete development of ACh contractions, and the possibility that E-M coupling is of greater importance early in the developing contraction needs to be further evaluated. Ca”‘-induced Ca2+ release from

contractions treated with is that after

W&l

126mM

K+

1

I 5 min

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E-M

COUPLING

IN

CANINE

TRACHEALIS

Cl83

MUSCLE

intracellular stores could still be operative in CTM, although there is evidence that this is not important in another graded smooth muscle (31) My data gave no informa tion about E,-independ .ent events involved in development of ACh contractions or even whether E,independent events occur in the plasma me mbrane or in intracellular structures. Farley and Miles have previously reported (11) ACh contractions in CTM (at ACh < lop7 M) that occurred without membrane depolarization and that indicate a membrane potential-independen t mech.anism. In my studies I was unable to find this type of evidence. ACh contraction did not precede membrane depolarization as ACh concentration slowly increased, and in most experiments depolarization roughly paralleled tension development. The biophysics of ACh contractions in CTM may be similar to norepinephrine contractions in graded vascular smooth muscle where the plasma membrane Z&,-dependent Ca”+-transport mechanism is not a primary mechanism in generating contractions, as judged by Ca’+-antagonist and 45Ca-uptake experiments (2, 12, 14, 15, 23, 31-33) and studies of E-M coupling (5, 8, 26). In contrast to ACh contractions, 5-HT contractions apparently utilize the plasma membrane Em-dependent Ca’+-transport mechanism, as indicated by close E-M coupling during current application (7), and by complete inhibition of 5-HT contractions by all three Ca2+-antag-

l

onist agents. I could not study effects of Ca2+-antagonist agents on electrically evoked tension responses during exposure to 5-HT because the muscle completely relaxed; electrically evoked contractions cannot be detected in relaxed strips, presumably due to series elastic component, There is, however, evidence of Em-independent tension during 5-HT contractions; i.e., depolarizationtension plots that are similar to those reported for ACh (7), the inability to completely reverse 5-HT tension by reversing 5-HT depolarization or hyperpolarizing the membrane (7), and the presence of 5-HT contractions in depolarized preparations. My results suggest that there may be interaction of different drug effects at the level of the plasma membrane E,-dependent Ca2’ -transport mechanism, An agent such as ACh may turn off plasma membrane J&dependent mechanisms that are utilized by SHT, or, alternatively, it may be possible for an agent such as 5-HT to turn on this mechanism and potentiate actions of agents, such as ACh, that normally may not use this mechanism but that have effects on E,. Further work is needed to evaluate this interesting possibility, This investigation was supported by National Heart and Lung Institute Grant I PO1 HL 19737-01. Some of the data reported here have been published in a preliminary publication (Federation Proc. 36: 2692, 1977.) Received

12 June

1978; accepted

in final

form

23 October

1978.

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Electromechanical coupling in canine trachealis muscle: acetylcholine contractions.

Electromechanical trachealis muscle: R. F. COBURN Department of Physiology, Philadelphia, Pennsylvania coupling in canine acetylcholine contractions...
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