Slow Action Potentials Research Articles

The Positive Inotropic Effect of Pimobendan Involves Stereospecific Increases in the Calcium Sensitivity of Cardiac Myofilaments

We have studied the effect of pimobendan (UD-CG 1 15 BS) on the electrical, mechanical, and biochemical activity of intact and detergent-skinned preparations of cardiac muscle. Racemic pimobendan increased the contractile force of guinea pig papillary muscle preparations and this positive inotropic action was associated with potentiation of the Ca:2+-dependent slow action potentials (APs). However, in the presence of 25 mM [K]o and maximally activating concentrations of isoproterenol, isometric force was increased further by addition of 50 μM pimobendan with no effect on the slow action potential. Experiments with chemically skinned heart μscle fibers showed that pimobendan. in a dose-dependent manner, increased active tension developed at submaximally activating concentrations of Ca The tension-cost (unit increase in ATPase rate/unit increase in force) was unchanged in the presence of pimobendan. Force-pCa and ATPase-pCa relations of skinned fiber preparations contracting isometrically were shifted to the left by 0.15–0.20 pCa units in the presence of 50 μ.M pimobendan. The mechanism for this effect was shown to be an increase in the Ca affinity of the regulatory binding sites of troponin C (TNC). These effects are due mainly to the /optical isomer of pimobendan. Addition of either the d or / isomer of pimobendan to preparations. maximally stiμlated by 1 μM isoproterenol, did not affect the slow AP parameters, but did increase contractile force to 124; of control by the d isomer and to 184“; of control by the / isomer. The Ca:2+-sensitizing effect of/-pimobendan on skinned liber preparations was substantially greater than that of the d isomer. Our results indicate that the Ca2+-sensitizing action of pimobendan (a) is due mainly to the / isomer, (b) involves direct effects on the Ca:2+-binding to TNC. and (c) involves specific interactions with domains of the regulatory protein complex

Antiarrhythmic effects of alpha-adrenoceptor antagonists in guinea pig ventricular myocardium

Antiarrhythmic effects of alpha-adrenoceptor antagonists were assessed in the reserpinized guinea pig ventricular myocardium. Both bunazosin (1 to 3 x 10(-7) M), a new alpha 1-adrenoceptor antagonist, and yohimbine (1 to 3 x 10(-7) M), another adrenoceptor antagonist, suppressed the transient depolarization and triggered activity induced by a train of rapid stimuli in the solution containing low potassium ion (K+), high calcium ion (Ca2+) and strophanthidin (1 to 5 x 10(-7) M). Bunazosin (3 x 10(-6) M) abolished the facilitatory effect of hypoxia on beta-adrenoceptor mediated abnormal automaticity. To clarify the mechanisms underlying the antiarrhythmic properties of alpha-adrenoceptor antagonists, their electrophysiologic effects on the fast and slow action potentials were investigated. Alpha-adrenoceptor antagonists (bunazosin, yohimbine and phentolamine) suppressed the slow response in a dose-related manner. The voltage-dependent block and use-dependent block of the maximal rate of rise (Vmax) of action potentials by bunazosin (10(-5) to 10(-4) M) and yohimbine (10(-6) to 10(-5) M) were studied. The analysis of the onset and recovery kinetics from the use-dependent block of drugs showed that both bunazosin and yohimbine act as slow kinetic drugs. It is concluded that alpha-adrenoceptor antagonists seem to have an antiarrhythmic effect through the inhibition of fast sodium ion (Na+) and slow Ca2+ currents of the cell membrane independently of blockade of myocardial alpha-adrenoceptors.

Effects of vasopressin on the somatic membrane of spinal ganglia neurons

The application of vasopressin (VP) in the isolated perfused dorsal root ganglia of 22-36 days old rats was studied by means of intracellular technique. 86.76% of cells have responded to the VP application. Depolarization was observed in 67.8% responded cells, the mixed response--in 16.95% cells, hyperpolarization--in 15.25% cells. All responses were dose-dependent and reversible. Input resistance (Rm) of the cell membrane decreased during depolarization and increased during hyperpolarization. The VP-evoked depolarization was accompanied by an increase in the action potential (AP) duration and decrease in the AP amplitude and after-hyperpolarization. Neurons with slow conduction velocity, high Rm and prolonged AP (small cells) had the lowest threshold of the sensitivity to VP (1.10(-11) M) and prolonged high-amplitude responses. Cells with the rapid conduction velocity, low Rm and rapid AP (large cells) responded to 1.10(-8) M, but sometimes even 1.10(-6) M had no effect. Depolarization in these neurons had smaller duration and low amplitude: sometimes hyperpolarization was observed. These results confirm the possibility that VP has effect on small neurons predominantly.

The effect of Lambert-Eaton myasthenic syndrome antibody on slow action potentials in mouse cardiac ventricle.

Immunoglobulin G (IgG) from Lambert-Eaton myasthenic syndrome (LEMS) patients acts at motor nerve terminal Ca2+ channels. It was injected into mice to investigate effects on cardiac Ca2+ channels. Intracellular recordings were made of slow action potentials in right ventricular muscle cells in the presence of high K+ concentrations and isoprenaline (1 microM). Reduction in Ca2+ concentration reduced the rate of rise and amplitude, but not the duration, of slow action potentials whereas verapamil (1 microM) blocked them. They were not blocked by tetrodotoxin (10 microM), and 4-aminopyridine (1 mM) prolonged the decay phase without affecting the rate of rise and amplitude. The rate of rise, amplitude and duration of slow action potentials were not affected by LEMS IgG. These results show that LEMS IgG does not act on Ca2+ channel currents that underlie slow action potentials in mouse ventricles, suggesting antigenic differences between Ca2+ channels at motor nerve terminals and heart.

Positive inotropic action of novel vasoconstrictor peptide endothelin on guinea pig atria.

Endothelin (ET), a novel 21-amino acid peptide isolated from the culture supernatant of porcine aortic endothelial cells, has been shown to be the most potent of all known vasoconstrictor substances. The purpose of the present study was to investigate the effects and the mode of actions of ET on the heart. ET exerted a positive inotropic effect in a dose-dependent manner on the electrically driven left atria of guinea pigs. The ET-induced response was of slow onset and characteristically long lasting. The half-maximal effective dose of the ET-induced response was about 1 nM, indicating that ET is one of the most potent cardiotonic substances. The response was presumably caused through direct actions of ET on the myocytes, since adrenergic, histaminergic, and serotonergic antagonists showed no effect on the response. The dose-response relationship of ET for the positive inotropic effect was displaced to the right in a parallel fashion by 0.3 microM nicardipine. In the left atria depolarized with 22 mM KCl, ET produced gradually increasing contractions in conjunction with the slow response action potentials in response to the electrical pacing. These results suggest that the ET-induced response on the left atria is intimately related to the influx of extracellular Ca2+.

Positive inotropic effect of calcitonin gene-related peptide mediated by cyclic AMP in guinea pig heart.

The mechanism of cardiac actions of rat calcitonin gene-related peptide (CGRP) was analyzed on isolated guinea pig hearts. CGRP exerted a positive inotropic effect in a dose-dependent manner on the electrically driven left atria but not on the ventricles. Immunohistochemical studies demonstrated that CGRP-like immunoreactive nerves were distributed densely in the myocardia of the atria but only sparsely in those of the ventricles. The CGRP-induced augmentation of the contraction was accompanied by the shortening of the time to peak force and the increase in the relaxation velocity. The positive inotropic response to CGRP was significantly enhanced by isobutylmethylxanthine and was attenuated by adenosine. CGRP increased the action potential amplitude and prolonged action potential duration at the level of 50% repolarization in the left atria. In the preparations, which were partially depolarized with an increase in extracellular potassium, CGRP induced slow response action potentials. These electrophysiological results indicate that CGRP causes an increase in the slow inward Ca2+ current. The cyclic AMP content in the left atria significantly increased following the addition of CGRP, the time course of which was nearly consistent with that of the augmentation of the contractile force. In the membrane preparation of the atria, the activity of adenylate cyclase was enhanced by CGRP in a dose-dependent manner. These effects of CGRP are qualitatively similar to those of beta-adrenoceptor stimulation. It is concluded that the CGRP-induced response in the guinea pig atria is attributed to the activation of adenylate cyclase via stimulation of its specific receptor and the subsequent increase in the intracellular cyclic AMP level.

Pharmacological Actions of DPN 205–734, a Novel Cardiotonic Agent

DPN 205-734 [5-(4-cyanophenyl)-1,2-dihydro-6-methyl-2-oxopyridin-3-carbo nitrile] was investigated in vitro in Langendorff rabbit hearts, guinea pig and rabbit papillary muscles, and rat myocardium and in vivo in anesthetized and unanesthetized dogs, pithed open-chest cats, anesthetized rats, and cardiomyopathic hamsters. In vitro, this substance caused a concentration-dependent positive inotropic effect. Left ventricular dP/dtmax was increased in anesthetized dogs after intravenous injection of 0.02 and 0.2 mg/kg (35 +/- 10 and 130 +/- 13%, respectively) and in unanesthetized dogs after oral doses of 0.05-0.5 mg/kg (15 +/- 2 to 71 +/- 14%). DPN 205-734 lowered blood pressure and total peripheral resistance in several experimental models, indicating an afterload-reducing effect. It induced moderate tachycardia. The positive inotropic effect is not explainable by beta-stimulation as shown in pithed open-chest cats pretreated with propranolol. A phosphodiesterase-inhibiting activity (IC50 = 35.5 microM), measured in rat myocardium, may be primarily responsible for the positive inotropic action. In guinea pig papillary muscles partially depolarized with 22 mM K+, DPN 205-734 in a concentration of 1 microM restored slow action potentials, which were then blocked by carbachol. These actions can be explained by the increase in cardiac cyclic AMP level due to a phosphodiesterase-inhibiting effect. In rabbit papillary muscles the positive inotropic effect of DPN 205-734 (100 microM) was only moderately inhibited by carbachol (3 microM), suggesting an additional mechanism.

Potassium and calcium currents and action potentials in mouse Balb/c 3T3 fibroblasts.

The electrical properties of Balb/c 3T3 mouse fibroblasts were studied with the whole-cell patch clamp technique. In current clamp mode a resting potential of -75.5 +/- 2.1 mV was recorded. In voltage clamp mode an inward current was also observed at potentials negative to Vm. This current crossed the 0-current axis at a voltage near Vm, and rectified at more positive potentials; the degree of rectification was dependent on [K+]o. At potentials positive to -30 mV a transient inward current was observed, showing a peak amplitude of -193 +/- 36 pA at +10 mV; the current amplitude was dependent on voltage and [Ca2+]o, it was strongly increased by 20 mM BaCl2 and abolished by 2 microM verapamil and 1 microM nifedipine. These cells, in response to depolarizing stimuli, develop slow action potentials, probably supported by the Ca2+ current.

Tiamdipines: A new class of Ca++-antagonists

BBR 2160 is a new dihydropyridine derivative belonging to the group of the so-called tiampidines. We used intracellular microelectrodes to characterize the electrophysiological properties of BBR 2160 on sheep Purkinje fibres and guineapig papillary muscle. BBR 2160 (10−7 and 10−6 m) dose dependently decreased the contractility of driven Purkinje fibre and papillary muscle. This effect was associated with a lowering of the plateau phase and a shortening of action potential duration in papillary muscle. The effect of the drug developed quite slowly over time. The amplitude and V̇max of normal action potential were not affected by BBR 2160. Instead BBR 2160 reduced the amplitude and V̇max of the slow action potentials (which are a relatively good index of the slow inward current) induced by histamine (10−5 m) in K+-depolarized (22 mm) papillary muscle. The results suggest that BBR 2160 has calcium-antagonistic properties in cardiac tissue.

Electrophysiological studies on the effects of mianserin, a tetracyclic antidepressant agent, on isolated guinea-pig papillary muscle

We studied the effects of mianserin on the action potentials of papillary muscle from the guinea-pig. Mianserin (above 10μM) reduced the maximum rate of the rise (⊢otV max) of the action potential, and shifted the ⊢otV max E m relationship to more negative potentials. The compound also depressed the slow action potentials of K +-depolarized papillary muscles. It is concluded that relatively high concentrations of mianserin had an inhibitory action on the electrophysiological properties of both the fast- and slow-response fibers of the heart.

Inhibition of cardiac slow action potentials by 8-bromo-cyclic GMP occurs independent of changes in cyclic AMP levels.

Cyclic GMP inhibits the slow inward Ca current of cardiac cells. This effect could be due to a cyclic GMP-mediated phosphorylation of the Ca channel (or some protein modifying Ca channel activity), or alternatively, to enhanced degradation of cyclic AMP owing to stimulation of a phosphodiesterase by cyclic GMP. To test the latter possibility, we examined the effect of extracellular 8-bromo-cyclic GMP on cyclic AMP levels in guinea pig papillary muscles, in parallel with electrophysiological experiments. Isoproterenol (10(-6) M) significantly increased the cyclic AMP levels and induced Ca-dependent slow action potentials. Superfusion with 8-bromo-cyclic GMP (10(-3) M) inhibited the slow action potentials induced by isoproterenol. However, muscles superfused with 8-bromo-cyclic GMP had cyclic AMP levels identical to those of muscles superfused with isoproterenol alone. Similarly, 8-bromo-cyclic GMP had no effect on the increase in cyclic AMP levels of muscles treated with forskolin (10(-6) M) or histamine (10(-6) M). We conclude that the inhibitory effect of cyclic GMP on slow Ca channels in guinea pig ventricular cells is not due to a decrease in the cyclic AMP levels. We hypothesize that a cyclic GMP-mediated phosphorylation is the most likely explanation for the Ca channel inhibition observed in this preparation.

Inhibitory actions of amoxapine, a tricyclic antidepressant agent, on electrophysiological properties of mammalian isolated cardiac preparations.

1. The electrophysiological effects of amoxapine were examined in guinea-pig isolated papillary muscles and rabbit sinoatrial nodes using a conventional microelectrode technique. 2. In papillary muscles, amoxapine above 10 microM caused a dose-dependent decrease in the maximum upstroke velocity (Vmax) of the action potential and in the action potential amplitude (APA), whereas the action potential duration at 90% repolarization (APD90) was significantly prolonged. For a decrease in Vmax, amoxapine produced a negative shift of the curve relating Vmax to the resting potential (Em) along the voltage axis to more negative membrane potentials. 3. Amoxapine also decreased Vmax and the overshoot potential of K+-depolarized slow action potentials of papillary muscle preparations. 4. In spontaneously beating sinoatrial node preparations, amoxapine above 3 microM reduced the heart rate, Vmax, APA and the slope of phase 4 depolarization in a dose-dependent manner. 5. It was concluded that amoxapine exerts inhibitory actions on fast- and slow-response fibres of the heart and these actions can be mainly explained by inhibition of both fast Na+ and slow Ca2+ channels.

Action potentials of cardiac muscle in healing infarcts: Response to norepinephrine and caffeine

Previous studies have shown that the muscle fibers that survive on the epicardial surface of a 5- or 14-day-old infarct in canine hearts have reduced or absent plateau phase during repolarization. Since the absence of a plateau phase could be related to a decrease in the slow inward current, we determined whether norepinephrine (NE) (3.13 x 10(-5) M) or caffeine (2.5 to 5.0 mM) restored the reduced plateau phase of action potentials of fibers from the 5- to 14-day infarct (4 mM K+). In addition, we compared the slow inward current-mediated slow response action potentials induced by NE or caffeine in K+ depolarized fibers from the 5- and 14-day infarcts to those induced in control fibers. Our results show that NE and caffeine had little or no significant effects on repolarization or the plateau area of action potentials of fibers from infarcted preparations. In addition, NE and caffeine both induced slow response action potentials in normal fibers, but in some fibers from infarcts, these agents failed to elicit slow responses. In other fibers from infarcts, NE and caffeine induced slow response action potentials but they had reduced mean areas when compared to the control potentials. In conclusion, the alteration and absence of slow responses in fibers from infarcted preparations suggest that there may be a chronic abnormality in the slow Ca current channel in fibers from the 5- and 14-day infarct.

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.

Electromechanical Effects of bPTH-(1-34) on Rabbit Sinus Node Cells and Guinea Pig Papillary Muscles

The effects of bPTH-(1-34), a synthetic preparation of bovine parathyroid hormone containing the first 34 amino acids, on electromechanical activity of isolated rabbit sinus node cells and guinea pig papillary muscles were examined by microelectrode techniques. In sinus node cells, bPTH-(1-34) (10(-7) M) decreased the cycle length of spontaneous firing (SPCL) accompanied by an increase in the maximum upstroke velocity at phase 0 (dV/dtmax) without affecting the amplitude of the action potential (APA) or the maximum diastolic potential (MDP). All the effects of bPTH-(1-34) on sinus node cells were abolished by verapamil (5 X 10(-7) M), but not by pindolol (2 X 10(-7) M). In constantly driven guinea pig papillary muscles, bPTH-(1-34) caused a positive inotropic effect (+31%). The parameters of the action potential were not significantly affected. This inotropic effect of bPTH-(1-34) was inhibited by pretreatment with verapamil or a low calcium medium (0.12 mM), but was not affected by pindolol. In contrast, ryanodine (2 X 10(-6) M), an inhibitor of internal Ca2+ release, which decreased the contraction with a prolongation of the action potential duration augmented the inotropic effect of bPTH-(1-34). In papillary muscles depolarized by 26 mM [K+]O, bPTH-(1-34) enhanced the slow action potential. In voltage clamp experiments using a single sucrose-gap method, bPTH-(1-34) caused an increase in the peak amplitude of the slow inward current, while it did not affect the outward current.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypoxia with mild acidosis and reoxygenation on slow response action potentials in canine myocardial tissues

We studied slow response action potentials (SRAP) in canine ventricular muscle tissue during hypoxia (and mild acidosis) and reoxygenation. Slow responses were initiated by perfusing the tissue with a solution containing 27 mM KCl and 10(-6) M norepinephrine. The tissue was stimulated at 0.5 Hz, 20V and 4ms (duration of the pulse). Microelectrode techniques were used to record action potential and mechanical activity was recorded with a force displacement transducer. Hypoxia and mild acidosis progressively diminished the amplitude and duration of these SRAPs, whereas reperfusion/reoxygenation progressively increased their amplitude even more than that in the control (prehypoxic value). Action potential duration (APD) increased (at all levels) during reperfusion compared to that in hypoxia and mild acidosis but APD remained shorter than the control (prehypoxic level). The effects of hypoxia (and mild acidosis) and subsequent reoxygenation seem similar to that of elevating the extracellular calcium (increased inward current). From these experiments one cannot, however, distinguish the effects of hypoxia on inward current from those on the outward currents.

Adenosine inhibits the positive inotropic effect of 3‐isobutyl‐1‐methylxanthine in papillary muscles without effect on cyclic AMP or cyclic GMP

1 Adenosine and the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) produced a small positive and negative inotropic effect, respectively, in isolated electrically driven papillary muscles of guinea-pigs. 2 Adenosine (100 mumol l-1) had no effect on cyclic AMP or cyclic GMP content. PIA (100 mumol l-1) slightly increased cyclic AMP. 3 In the presence of 3-isobutyl-1-methylxanthine (IBMX; 60 mumol l-1), which increased force of contraction 2 fold, adenosine and PIA exerted strong negative inotropic effects. PIA was more potent than adenosine (mean IC25 2.1 and 168 mumol -1, respectively). 4 In contrast, the nucleosides did not affect the increase in force of contraction produced by elevating extracellular Ca2+ concentration. 5 The IBMX-antagonistic effects of adenosine and PIA were not accompanied by modification of the IBMX-induced increase in cyclic AMP and cyclic GMP. 6 The effects of adenosine and PIA on force of contraction were accompanied by a partial reversal of the IBMX-induced increase in the maximal rate of depolarization of slow action potentials. 7 It is concluded that adenosine and PIA are able to attenuate the positive inotropic effect of a phosphodiesterase inhibitor. This effect is unlikely to be due to a reduction of the IBMX-induced increase in cyclic AMP content. It is conceivably due to an inhibition of the stimulant action of cyclic AMP on slow Ca2+ channels leading to the reduction of the slow inward current which in turn reduces force of contraction.

Effects of Halothane and Isoflurane on Isolated Human Ventricular Myocardium

Segments of viable human left ventricular trabeculae were obtained at the time of endocardial resection for intractable ventricular ectopy. Muscle segments which showed suitable and reproducible contractions in 26 mM K Tyrode solution with 1 microM isoproterenol were electrically stimulated after rest, and at frequencies of 0.1, 0.25, 0.5, and 1 Hz. Effects of 0.75% halothane and 1.3% isoflurane on peak tension, maximum rate of tension development (dT/dt-max), and on slow (calcium dependent) action potential (AP) characteristics were studied. Halothane depressed peak tension, dT/dt-max, and slow AP maximum rate of depolarization (Vmax) at all frequencies, and caused a significantly greater depression of peak tension and dT/dt-max at 0.5-1 Hz than after rest and at 0.1-0.25 Hz. Isoflurane did not significantly depress slow AP Vmax, showed no frequency dependent contractile depression, and depressed dT/dt-max less than halothane at 0.5 and 1 Hz. Halothane and isoflurane caused differing depression in the pattern of developed tension. The differential depression by halothane and isoflurane of human ventricular myocardium was similar to that previously observed in isolated animal ventricular tissue.

Intracellular N-methyl-d-glucamine modifies the kinetics and voltage-dependence of the calcium current in guinea pig ventricular heart cells

The effects of internal substitution of the impermeant cation N-methyl-D-glucamine (NMG) for Cs ion on the properties of the Ca-current (L-type channel) were examined in single guinea pig cardiac myocytes with the whole-cell clamp technique. The properties of the cobalt-sensitive Ca current recorded in the presence of internal NMG or Cs were compared and the results were as follows. (1) The overall duration of the Ca-dependent slow action potential was markedly increased in the presence of internal NMG (6-fold at 0 mV) when compared to action potentials recorded with internal Cs. (2) The cobalt-sensitive Ca currents recorded with internal NMG or Cs had similar reversal potentials. However, in the presence of internal NMG, the maximum current density of the cobalt-sensitive Ca current was decreased and both the threshold and potential at which maximum current occurred were negatively shifted. (3) Voltage-dependence of steady-state activation, but not inactivation, of the cobalt-sensitive Ca current was shifted by -11.8 mV with internal NMG. (4) NMG increased the half-time of activation and inactivation of the cobalt-sensitive Ca current. The voltage-dependence of the half-time of inactivation was shifted by about -30 mV between 0 and +60 mV. Time constants measurements showed that NMG affected more the slow phase of inactivation of the Ca current. (5) When Ba was the charge carrier, NMG removed most of the inactivation of the current, suggesting a slowing of the voltage-dependent process of inactivation. (6) The results are consistent with a modification of the properties of the Ca channel by internal NMG.

Effects of the novel antiarrhythmic compound TR 2985 (ropitoin) on action potentials of different mammalian cardiac tissues.

Ropitoin (TR 2985) is a novel antiarrhythmic drug. In the present work we have found that the main effect of the compound on the normal action potential of different cardiac tissues, guinea-pig atrial and ventricular muscle (1-3 mumol/l), and dog Purkinje fibres (0.5-1.0 mumol/l) was a depression of the maximum upstroke velocity. This effect was dependent on the frequency of stimulation, being stronger at higher frequencies. In the presence of the drug (3 mumol/l), we observed a shift of 9 mV of the resting membrane potential-maximum upstroke velocity relationship to more negative potentials. Under control conditions recovery from inactivation of maximum upstroke velocity was complete in less than 200 ms. Ropitoin induced a very slow component of recovery of the maximum upstroke velocity, explaining the frequency-dependent effects of the drug. The slow recovery of the maximum upstroke velocity induced by ropitoin was dependent on the membrane potential being faster at a more hyperpolarized membrane potential. The time course of the recovery was also dependent on pH; acidosis slowed it considerably. Ropitoin increased action potential duration of atrial muscle at 20% and 90% of repolarization. In contrast, the compound shortened action potential duration of ventricular muscle at 20% and 90% of repolarization, and dog Purkinje fibres at 50% and 90% of repolarization. In addition, ropitoin (1-3 mumol/l) depressed guinea-pig ventricular slow action potentials. This effect was the stronger the higher the stimulation frequency.