Guinea Pig Ventricular Muscle Research Articles

Influence of halothane on contraction at positive membrane potentials in single cells isolated from guinea-pig ventricular muscle.

Actions of halothane were investigated under voltage-clamp conditions in single cells from guinea-pig ventricular muscle. Contraction (measured by an optical method) evoked by step depolarization to 0 mV was consistently reduced by halothane. At positive membrane potentials (+60 mV) 2% halothane did not cause a consistent depression of peak contraction, and in the majority of cells contraction was enhanced. Two per cent halothane increased the time-to-peak contraction at +60 mV. However, when a pre-pulse to 0 mV was applied to inactive calcium current through L-channels, any effect of 2% halothane on the time-to-peak of contraction was reduced or abolished. A halothane-induced increase in time-to-peak contraction was also observed at membrane potentials in the range of the action potential plateau (+20 and +40 mV). In double-pulse experiments contraction during a 'test' depolarization to +60 was measured following a 'conditioning' depolarization to 0 mV. Contraction at +60 mV was slightly reduced at brief interpulse intervals (less than 400 ms) following the 'conditioning' depolarization to 0 mV, and recovered as the interval was prolonged; in cells exposed to halothane contraction at +60 mV was no longer influenced by the interval between the pulses. Isoflurane (3.2%) had qualitatively similar but less potent effects than halothane on contraction at +60 mV. These observations are consistent with the suggestion that mechanisms for calcium entry may vary with the membrane potential: at 0 mV, the major pathway for calcium entry may be through halothane-sensitive L-type calcium channels, while at +60 mV entry may be through additional pathways which are relatively resistant to halothane. Actions of halothane on the time-to-peak of contraction may be accounted for by its influence on the sarcoplasmic reticulum to decrease net uptake and release of calcium. These actions of halothane might be of importance during the action potential plateau.

MECHANISMS AND SIGNIFICANCE OF CALCIUM ENTRY AT POSITIVE MEMBRANE POTENTIALS IN GUINEA‐PIG VENTRICULAR MUSCLE CELLS

Possible mechanisms for calcium entry at positive membrane potentials were investigated in single cells isolated from guinea-pig ventricular muscle. The cells were voltage clamped and contraction was measured by an optical technique. When prolonged (200 ms to 2 s) depolarizations at +60 mV were applied, contraction amplitude increased with pulse duration, in contrast to the contraction at 0 mV. When a 'pre-pulse' to 0 mV was applied for 200 ms to inactivate current through 'L-type' calcium channels, contraction nevertheless increased with membrane potential during a subsequent test pulse applied over the range -40 to +60 mV. Contraction during the test pulse at +60 mV was abolished when extracellular calcium was reduced to zero. This effect developed more rapidly than abolition of the contraction in response to the pre-pulse to 0 mV. Reduction of extracellular calcium from 2.5 to 1 mM reduced the contraction at +60 mV to a greater extent than that at 0 mV and caused an inward shift in the current at +60 mV. Nifedipine (5 microM) substantially reduced the contraction during the test pulse to 0 mV but had little effect on the contraction at +60 mV. Conversely, dodecylamine (20 microM) caused little or no decrease in the contraction at 0 mV but substantially reduced the contraction at +60 mV. Following a conditioning pre-pulse to 0 mV the contraction at +60 mV was not consistently reduced by exposure to 3 microM-ryanodine. The interpolation of a single 200 ms pulse to +60 mV in a train of pulses to 0 mV potentiated the following contraction to 0 mV. This potentiation decayed over the first four steps to 0 mV following an interpolated pulse and increased with the voltage of the interpolated pulse over the range -20 to +60 mV. Potentiation was abolished on exposure to 3 microM-ryanodine. These observations are consistent with entry of calcium at positive membrane potentials through voltage-dependent, non-inactivating pathways which are insensitive to nifedipine but inhibited by dodecylamine. The observations support the hypothesis that calcium entry via this mechanism may contribute, at least under some conditions, to the loading of intracellular stores of calcium during the late plateau of the action potential, and thus influence subsequent contraction. Calcium entry through Na+-Ca2+ exchange is a possibility which would allow calcium entry to increase over the range of membrane potentials at which contraction was increased. However, additional calcium entry through other nifedipine-insensitive pathways, such as calcium-activated non-selective channels, cannot be excluded.

Effects of amiodarone on barium-induced automatic activity in guinea pig ventricular muscle

1. 1. The electrophysiological effects of a potent antiarrhythmic agent, amiodarone, on BaCl 2-induced automaticity of guinea pig ventricular papillary muscle were studied by means of conventional microelectrode techniques. 2. 2. BaCl 2 (5 × 10 −4 M) depolarized the maximum diastolic potential of muscle preparation to about -62 mV and induced the spontaneous activity at rate of about 62 beats/min. 3. 3. The application of amiodarone (4.4 × 10 −5 M to 1.1 × 10 −3 M) to the BaCl 2-induced automatic muscle fibers decreased the spontaneous rate and slope of slow diastolic depolarization in a dose-dependent manner, associated with insignificant changes in action potential duration. 4. 4. Amiodarone may exert the inhibitory effects on BaCl 2-induced automaticity by decreasing steady-state conductance for the Na + and Ca 2+, probably thereby permitting the manifestation of antiarrhythmic properties in combination of its other actions.

Rest- and stimulation-dependent changes in exchangeable calcium content in rabbit ventricular myocardium.

Ca2+ shifts in the isolated, perfused ventricular muscle of rabbit hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with 45Ca containing solution. The "cellular" 45Ca content was calculated by subtraction of 45Ca2+ dissolved in the free water of extracellular space from the total tissue 45Ca2+ content. The "cellular" content of 45Ca in stimulated (60 per min) preparation was 0.887 +/- 0.067 mmol/kg wet weight (w.w.). Six minutes of rest resulted in the drop of this content to 0.503 +/- 0.054 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. Contractile force (CF) decreased at that time to 23% of control. The first post-rest contraction (RSC) resulted in a gain of 0.073 mmol45Ca/kg w.w. Both the content of 45Ca and CF returned to the pre-rest values when stimulation was resumed. The difference between the 45Ca content of post-rest stimulated and rested preparations (0.384 mmol/kg w.w.) is equivalent to Ca2 fraction (Ca2), previously described in guinea pig ventricular myocardium (17, 12). However, the volume of rabbit Ca2 is only about 42% of that in guinea pig. Caffeine in concentration 12.5 mM, which did not displace Ca2 from guinea pig ventricular muscle, decreased Ca2 in the rabbit ventricle by 44%. CCCP, a protonofore destroying the mitochondrial protone gradient essential for Ca2+ uptake and maintainability, displaced Ca2 completely from rabbit ventricles. These results, although far from conclusive, do suggest that both the mitochondria and sarcoplasmic reticulum might be the site of the rate-dependent Ca2 fraction. The physiological meaning of differences in Ca2 content between rabbit, guinea pig, and rat ventricular myocardium is discussed.

O-201 - Electrophysiologicai effects of class III antiarrhythmic agents, dl-sotalol and E-4031, on guinea pig ventricular muscle during simulated ischemia

O-201 - Electrophysiologicai effects of class III antiarrhythmic agents, dl-sotalol and E-4031, on guinea pig ventricular muscle during simulated ischemia

O-214 - Effects of tolbutamide, a putative ATP-regulated K+ channel blocker, on the hypoxia-induced shortening of action potential duration in guinea-pig ventricular muscles

O-214 - Effects of tolbutamide, a putative ATP-regulated K+ channel blocker, on the hypoxia-induced shortening of action potential duration in guinea-pig ventricular muscles

P-235 - Effects of tetrodotoxin and lidocaine on ATX II-modified Na channels in guinea-pig ventricular muscle

P-235 - Effects of tetrodotoxin and lidocaine on ATX II-modified Na channels in guinea-pig ventricular muscle

P-236 - Effects of TYB-3823, a new antiarrhythmic drug, on the membrane potential of guinea-pig ventricular muscle

P-236 - Effects of TYB-3823, a new antiarrhythmic drug, on the membrane potential of guinea-pig ventricular muscle

Effects of L-propionylcarnitine on electrical and mechanical alterations induced by amphiphilic lipids in isolated guinea pig ventricular muscle.

We examined the effects of L-propionylcarnitine (Prop. C), a short-chain acylcarnitine, on amphiphile (L-lysophosphatidylcholine or L-palmitoylcarnitine)-induced electrophysiological and ultrastructural changes in isolated guinea pig ventricular papillary muscles, under acidic conditions (pH 6.9). Conventional microelectrode, tension-recording, and electron microscope techniques were used. Both amphiphiles, at a concentration of 10(-4) M, significantly decreased the resting membrane potential, action potential amplitude, and action potential duration, but increased the developed and resting tension. Such amphiphile-induced electrical changes were not observed in muscles pretreated with the beta-blocker, atenolol, although the mechanical changes remained unaffected. The application of Prop. C (10(-2) M), in the continued presence of the amphiphiles caused a return of the action potential duration and the developed tension to the control level. However, the resting potential and action potential amplitude remained unaffected; in fact, the maximum upstroke velocity (Vmax) of the action potential tended to decrease further. Pretreatment with Prop. C prevented all the amphiphile-induced electrophysiological and mechanical changes, except for Vmax. Electron microscopic studies revealed that amphiphile-induced ultrastructural changes were prevented, at least in part, in the presence of Prop. C. Thus, Prop. C antagonizes some of deleterious effects of amphiphiles, such as lysophosphatidylcholine and palmitoylcarnitine, upon the electrical and mechanical activities of the ventricular muscle, under acidic conditions.

Evidence for voltage-dependent block of cardiac sodium channels by tetrodotoxin

The effects of tetrodotoxin (TTX) on cardiac sodium channels in guinea-pig ventricular muscle were investigated. Membrane potential was controlled using a single sucrose gap voltage clamp method, and the maximum upstroke velocity of the ventricular action potential ( V ̇ max ) was used as an indicator of drug-free sodium channels. Reduction of V ̇ max by TTX was found to be both voltage- and time-dependent, similar to the effects of many local anesthetic drugs, with the exception that TTX concentrations high enough to produce significant use-dependent block (e.g. 2 μ m), also produced significant tonic block, even at potentials negative to −85 mV. The mechanism underlying use-dependent block was determined by defining the time course of block development at potentials between −40 and +20 mV, and the time course of recovery at −85 mV. In 2 μ m TTX, the time course of block development at +20 mV contained two phases, a fast phase (τ < 3 ms) having a mean amplitude of 8.1 ± 3.2% of control V ̇ max , and a slow phase (τ = 429 ± 43 ms) having an amplitude of 35 ± 2% of control V ̇ max ( n = 5). Recovery from use-dependent block at −85 mV occurred with a time constant of 324 ± 58 ms ( n = 5). The effects of TTX could be well-described by a modulated receptor model with an estimated 12 mV drug-induced shift of inactivation, and state-dependent dissociation constants of 10, 4 and 0.3 μ m for rested, activated and inactivated channels. These same drug rate constants could also be used to adequately simulate the reported effects of TTX on plateau sodium currents in a variant model with slow inactivation kinetics.

Tonic and Phasic Vmax Block Induced by 5-Hydroxypropafenone in Guinea Pig Ventricular Muscles

The effects of 5-hydroxypropafenone (5-OH-P) on transmembrane action potentials were studied in isolated guinea pig papillary muscles. 5-Hydroxypropafenone, 10(-7) M to 5 X 10(-5) M, produced a dose-dependent decrease in the Vmax and amplitude of the action potential and a depolarization of the resting membrane potential, whereas the action potential duration was not affected by the drug. In the presence of 5-OH-P, trains of stimuli at rates between 0.2 and 2 Hz led to an exponential decline in Vmax (onset rate at 1 Hz 0.07 +/- 0.01 per action potential) to a new steady-state level. This phasic, use-dependent Vmax block was augmented at higher stimulation frequencies. The time constant for the recovery of Vmax from the phasic block (offset) was 200.0 +/- 9.3 s. In papillary muscles depolarized with 10 mM K the inhibitory effect of 5-OH-P on Vmax of the first action potential after a long quiescent period (tonic block) was markedly increased, but the rates of onset and offset of the phasic block were similar to those found in normally polarized fibers under 5.4 mM K. 5-OH-P also shifted the curves relating membrane potential and Vmax in the hyperpolarizing direction. These findings suggest that 5-OH-P exhibited similar phasic inhibitory action of the fast sodium channels than other class Ic antiarrhythmic drugs. This phasic Vmax block, and its greater inhibition of Vmax in depolarized muscles through the increase of tonic block, would play a major role for the antiarrhythmic effect of the drug.

Frequency and voltage dependent effects of AN-132, a new class I anti-arrhythmic agent, on Vmax of action potential upstroke in guinea pig ventricular muscles.

The in vitro electrophysiological properties of a newly synthesised antiarrhythmic agent, AN-132, were evaluated by recording transmembrane action potentials from guinea pig papillary muscles. AN-132 (10-100 mumol.litre-1) caused a dose dependent decrease in the maximum upstroke velocity (Vmax) of the action potential without affecting the resting potential. In the presence of AN-132, trains of stimuli at rates greater than or equal to 0.1 Hz led to an exponential decline in Vmax. This use dependent block was enhanced at higher stimulation frequency. The time constant for the recovery of Vmax from the use dependent block was 39.5-41.2 s. The curves relating membrane potential and Vmax were shifted by AN-132 (100 mumol.litre-1) in the direction of more negative potentials (6.1 mV). In preparations treated with AN-132 (30 and 100 mumol.litre-1), the Vmax of test action potentials preceded by conditioning clamp pulses to 0 mV was progressively decreased with an increasing number of pulses. A single prolonged clamp pulse to 0 mV reduced Vmax much less than multiple brief clamp pulses. These findings suggest than AN-132 has use dependent inhibitory action on the fast sodium channel by binding to the channel mainly during its activated state and that the unbinding rate of the drug during diastole is very slow. This use dependency and its greater inhibition of Vmax in depolarised muscles through the increase in tonic block may play a major role in preventing ventricular arrhythmias.

Comparative study of the block of V max by aprindine and quinidine in the guinea-pig heart muscle

The depression of V max of the action potential in guinea-pig ventricular muscle by aprindine and quanidine was compared. Aprindine caused a more pronouced rate-dependent block (K d = 10 −6 M at 3.3 Hz) than did quinidine (K d = 1.6 × 10 −5 M at 3.3 Hz). Aprindine shifted the relationship between V max and resting potential to a more negative potential (mean 9.2 mV: 10 μM) than did quinidine (mean 5.7 mV: 10 μM). In addition, aprindice caused a more pronounced resting block (K d = 1.3 × 10 −5 M) than quinidine (K d = 8.6 × 10 −5 M). It is concluded that aprindine has a higher affinity for activated and/or inactivated and resting state channels than quinidine, making channels unavailable for conduction upon activation.

Influence of halothane on electrical coupling in cell pairs isolated from guinea-pig ventricle.

1. The actions of halothane on electrical coupling between cells were investigated in cell pairs isolated from guinea-pig ventricular muscle. 2. Under voltage-clamp conditions a step depolarization applied to one cell caused a similar change in potential in the second. Application of halothane led to the appearance of double peaks in inward current evoked by step depolarizations. These observations were interpreted in terms of uncoupling of the cells leading to escape of the second cell from the influence of the voltage-clamp in the first cell. 3. This suggestion that uncoupling in the presence of halothane led to differences in electrical activity in the two cells was confirmed in experiments in which independent electrodes were used to measure membrane potential in the two cells. 4. The voltage responses of both cells of the pair were recorded in response to constant current pulses. Administration of halothane led to abolition of the response recorded from the second cell while that of the first was enhanced. The actions are consistent with an action of halothane on gap junctions to block electrical coupling. 5. Qualitatively similar observations, consistent with electrical uncoupling, were observed with isoflurane. 6. These findings may be significant in relation to the arrhythmogenic actions of halothane.

Electrophysiological Effects of KT-362, a New Antiarrhythmic Agent with Vasodilating Action, on Isolated Guinea Pig Ventricular Muscle

The effects of KT-362, a newly synthesized vasodilating and antiarrhythmic agent, on transmembrane action potentials were examined in isolated papillary muscles of guinea pig. KT-362 (3 x 10(-6) to 3 x 10(-5) M) caused a dose-dependent decrease in the maximum upstroke velocity (Vmax) of the action potential without affecting the resting potential. In the presence of KT-362, trains of stimuli at rates greater than or equal to 0.2 Hz led to an exponential decline in Vmax. This use-dependent block was enhanced at higher stimulation frequency. The time constant for the recovery of Vmax from the use-dependent block was 2.8-2.9 s. The curves relating membrane potential and Vmax were shifted by KT-362 (10(-5) M) in the direction of more negative potentials (8.6 mV). In papillary muscles treated with KT-362 (10(-5) M), the Vmax of test action potential preceded by conditioning clamp pulses to 0 mV was progressively decreased with increasing number of pulses, while it was little affected by the duration of each clamp pulse within the range from 10 to 700 ms. These findings suggest that KT-362 has quinidine-like use-dependent inhibitory action on the fast sodium channel of cardiac muscles by the binding to the channel mainly during its activated state. At high concentrations (greater than or equal to 3 x 10(-5) M), KT-362 also suppressed the slow action potential in K+ depolarized papillary muscles.

Substrate-induced alterations of heat production in isolated guinea-pig ventricular muscle

Substrate-induced alterations of heat production in isolated guinea-pig ventricular muscle

Heat production of quiescent ventricular trabeculae isolated from guinea-pig heart.

1. A new calorimetric technique has been developed which allows continuous measurement of the rate of energy expenditure in superfused preparations of cardiac muscle. Thin trabeculae of guinea-pig ventricular muscle were mounted in a Perspex tube of 0.8 mm inner diameter and the temperature difference of the perfusate upstream and downstream of the preparation was measured. 2. The resting heat rate of trabeculae of 240-575 microns diameter from guinea-pig heart was determined repeatedly for up to 6 h after cardiectomy. It did not vary with time during the course of the experiment. 3. The average resting heat rate measured in HEPES-buffered Tyrode solution containing 20 mM-glucose and 2 mM-pyruvate as substrates was 130 +/- 29 mW/g dry weight or 36 +/- 8 mW/cm3 of tissue (n = 15). This is an order of magnitude larger than the resting heat rate reported in the literature for isolated cardiac preparations. 4. After omitting the pyruvate from the superfusate the resting heat rate decreased to 60-70% of its steady value within 4 min. After readmission of pyruvate this effect was reversed. The average resting heat rate with glucose as sole substrate was 23 +/- 4 mW/cm3. 5. Uncoupling of the mitochondria by 50 microM-2,4-dinitrophenol (DNP) increased the heat rate up to 170 mW/cm3. This effect could be maintained for several minutes and was fully reversible. Raising the external K+ concentration to 150 mM (NaCl replaced by KCl) induced a transient rise in the rate of heat production up to 115 mW/cm3. 6. The heat production during uncoupling of the mitochondria and during potassium contractures was inversely related to the diameter of the preparation. Calculation based on Hill's equation (Hill, 1928) indicated that this was caused by the development of anoxia at the core of the preparation. 7. In contrast, the rate of heat production of quiescent preparations was not correlated with diameter and calculation indicated that at rest there was no anoxic core. The high value of resting heat rate found in the present study is discussed within the context of the large variation of 1.7-25 mW/g reported in the literature for resting metabolic rate of cardiac muscle.

Frequency- and Voltage-Dependent Depression of Maximum Upstroke Velocity of Action Potentials by Pirmenol in Guinea Pig Ventricular Muscles

The frequency-dependency and voltage-dependency of the suppressing effect of pirmenol, a novel antiarrhythmic agent, on the maximum upstroke velocity (Vmax) of action potentials were examined and compared with those of disopyramide in guinea pig papillary muscles. Pirmenol in concentrations higher than 3 μM decreased Vmax with a slight increase in action potential duration. The reduction of Vmax by pirmenol was enhanced in a frequency-dependent manner over the range of 0.1-2.0 Hz. Pirmenol (30 μM) produced a small resting block (5.5%), whereas disopyramide (100 μM) produced a greater one (25.8%). The onset of frequency-dependent Vmax reduction at 2.0 Hz followed a monoexponential function with a slow rate constant (0.308±0.055 AP-1). The time constant for the recovery from the frequency-dependent block by pirmenol was also slow (33.5±5.4 sec), but faster than that of disopyramide (82.5±12.3 sec). At 1.0 Hz, pirmenol caused a shift (9.5 mV) of the curve relating the resting membrane potential and Vmax along the voltage axis in the hyperpolarizing direction. Thus, pirmenol is a Class la drug that has frequency- and voltage-dependent inhibitory actions on Vmax, and its onset and offset kinetics are relatively slow.

O-5 - Electrophysiological effects of OPC-88117, a new antiarrhythmic agent, on isolated rabbit hearts and guinea pig ventricular muscles

O-5 - Electrophysiological effects of OPC-88117, a new antiarrhythmic agent, on isolated rabbit hearts and guinea pig ventricular muscles

Inhibition by amiodarone of slow response action potentials and contraction in guinea pig ventricular muscle

1. 1. Effects of an antiarrhythmic agent, amiodarone (AM), on slow response action potentials and contraction of guinea pig ventricular muscle were examined. 2. 2. AM (4.4 × 10 −5 M, 1.1 × 10 −4 M) significantly inhibited the maximum upstroke velocity of slow response action potentials and contraction. 3. 3. As for action potential duration (APD), higher concentration (1.1 × 10 −4 M) of AM significantly shortened the APD, while low concentration (4.4 × 10 −5 M) had little effect on it. 4. 4. These findings strongly suggest that AM blocks slow inward current inflow via Ca 2+ channels in the ventricular muscle.