Frequency- and voltage-dependent depression of maximum upstroke velocity of action potentials by pirmenol in guinea pig ventricular muscles

Profiles of aprindine, cibenzoline, pilsicainide and pirmenol in the framework of the Sicilian Gambit. The Guideline Committee for Clinical Use of Antiarrhythmic Drugs in Japan (Working Group of Arrhythmias of the Japanese Society of Electrocardiology)

The Vaughan Williams classification has been used widely by clinicians, cardiologists and researchers engaged in antiarrhythmic drug development and testing in many countries throughout the world since its initial proposal in the early 1970s. However, a major criticism of the Vaughan Williams system arose from the extent to which the categorization of drugs into classes I-IV led to oversimplified views of both shared and divergent actions. The Sicilian Gambit proposed a two-dimensional tabular framework for display of drug actions to solve these problems. From April to December 1996, members of the Guideline Committee met to discuss pharmacologic profiles of 4 antiarrhythmic drugs (aprindine, cibenzoline, pilsicainide, and pirmenol) that were not included in the original spreadsheet but are used widely in clinical practice in Japan. The discussion aimed to fit the drug profiles into the Gambit framework based on all the important literature published to date regarding the actions of the 4 drugs. This report is a summary of that deliberation.

Effects of pirmenol on action potentials and membrane currents in single atrial myocytes

Electrophysiological effects of pirmenol hydrochloride (pirmenol) were investigated in single atrial myocytes obtained from rabbit and guinea-pig hearts by using a whole-cell clamp technique. Under current clamp conditions, pirmenol (2-30 microM) prolonged action potential duration in a concentration-dependent manner without affecting resting membrane potential in rabbit atrial myocytes. However, in the presence of 4-aminopyridine (4 mM), pirmenol (10 microM) failed to prolong the action potential duration further. Pirmenol also suppressed acetylcholine-induced hyperpolarization and action potential duration shortening, resulting in a significant prolongation of the action potential duration in the presence of acetylcholine. Under voltage clamp conditions, pirmenol (1-1000 microM) inhibited transient outward current (I(to)) in a concentration-dependent manner. The concentration for half-maximal inhibition (IC50) of pirmenol on I(to) was about 18 microM. Pirmenol did not show the use and frequency dependent inhibition of I(to). The voltage dependence of the steady-state inactivation of I(to) and the recovery from inactivation were not significantly affected by pirmenol. Pirmenol accelerated the inactivation of I(to) and blocked I(to) as an exponential function of time, consistent with a time-dependent open channel blockade. Pirmenol (30 microM) did not affect the inwardly rectifying K+ current significantly, but it decreased the voltage-dependent L-type Ca2+ current by about 20%. In guinea-pig atrial myocytes, both acetylcholine and adenosine induced a specific K+ current activated by GTP-binding proteins. Pirmenol suppressed both the acetylcholine- and adenosine-induced K+ current effectively. The IC50 of pirmenol for acetylcholine- and adenosine-induced current was about 1 and 8 microM, respectively. The present results suggest that pirmenol prolongs the action potential duration by primarily inhibiting the transient outward current in atrial myocytes. In addition, since pirmenol inhibits acetylcholine- and adenosine-induced K+ current, pirmenol may effectively prolong the action potential duration in atrial myocytes under various physiological conditions as in the whole heart or ischemia.

Pirmenol inhibits muscarinic acetylcholine receptor-operated K+ current in the guinea pig heart

We examined the effects of pirmenol and disopyramide on the muscarinic acetylcholine receptor-operated K+ current (I[K.ACh]) in atrial cells and on experimental atrial fibrillation in isolated guinea-pig hearts. In isolated atrial myocytes, both pirmenol and disopyramide concentration-dependently inhibited the I(K.ACh) induced by carbachol or intracellular loading of GTPgammaS. Their inhibitory effects on the carbachol-induced current were more potent than those on GTPgammaS-induced current, suggesting that these drugs inhibit I(K.ACh) mainly by blocking muscarinic receptors. In Langendorff-perfused hearts these drugs reversed the carbachol-induced decreases in effective refractory periods and atrial fibrillation threshold. These drugs may be useful for the prevention of vagally induced atrial fibrillation.

Electrophysiologic and anticholinergic effects of pirmenol enantiomers in guinea-pig myocardium

Since it has been reported that several class I drugs stereoselectively block sodium channels, potassium channels and muscarinic receptors in cardiac tissues, electrophysiologic and anticholinergic effects of enantiomers of pirmenol, a class I antiarrhythmic drug, were examined. Both (+) and (-) pirmenol depressed the maximum upstroke velocity (Vmax) of the action potential in a concentration-dependent manner in guinea-pig papillary muscles driven at 1.0 Hz, and there was no significant difference in the potency of the class I effect between the enantiomers. The onset rates of use-dependent block (UDB) of Vmax at 2.0 Hz for 10 mumol/l (+) and (-) pirmenol were 0.30 +/- 0.03 and 0.29 +/- 0.01 per action potential, and the recovery time constants from UDB for (+) and (-) pirmenol were 27.0 +/- 2.7 and 27.7 +/- 1.9 s, respectively, indicating no difference in the binding and unbinding kinetics to the sodium channel between the enantiomers. Both (+) pirmenol and (-) pirmenol prolonged action potential duration (APD) at low concentrations (1-10 mumol/l) and shortened it at high concentrations (30-100 mumol/l). Again, there was little difference with respect to the effects on APD between the enantiomers. However, in the isolated guinea-pig left atria (-) pirmenol more potently antagonized the negative inotropic effect of carbachol than (+) pirmenol, and the pA2 values for (+) and (-) pirmenol were 6.41 and 6.71, respectively. The functional study was supported by the radioligand binding experiments using [3H]N-methylscopolamine ([3H]NMS) in guinea-pig left atrial membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the effects of pirmenol and lidocaine on intraventricular conduction in canine myocardial infarction

Effects of pirmenol hydrochloride and lidocaine hydrochloride on intraventricular conduction were examined in the infarcted heart of anesthetized dogs. The effects of the drugs on the excitation induced by ventricular stimulations were determined with coupling intervals between 150 and 1,000 ms. Effects of the drugs on the His bundle electrocardiograms were also examined. Pirmenol in doses of 1-5 mg/kg prolonged the conduction time in the infarcted zones over a wide range of coupling intervals. Pirmenol at 5 mg/kg blocked the delayed conduction at a short coupling interval in the infarcted zones. The effect of pirmenol on the conduction time in the normal zone was slight. Lidocaine in doses of 3 and 10 mg/kg prolonged the conduction time in the infarcted zone at a short coupling interval. Pirmenol did not significantly prolong PQ or AH interval. In conclusion, pirmenol selectively depressed the delayed conduction in the infarcted zone, which could result in a reentrant pathway. The effect of pirmenol on delayed conduction was dependent on the coupling interval and was quite different from that of lidocaine.

Use-dependent effects of pirmenol on Vmax and conduction in guinea-pig ventricular myocardium

1. The use-dependent effects of pirmenol, a new antiarrhythmic drug, on the maximal rate of rise (Vmax) of the action potential, conduction velocity, and their corresponding recovery kinetics were studied in isolated papillary muscles of guinea-pig. Standard microelectrode techniques were used to monitor the conduction and action potential characteristics of the muscles. 2. Pirmenol decreased Vmax and the overshoot of action potentials in a dose-dependent fashion. Also, doses of pirmenol greater than 1 mM abolished the generation of action potentials. Low concentrations of pirmenol (3 and 10 microM) prolonged the action potential duration, while concentrations greater than 0.1 mM shortened it markedly. 3. The resting block of Vmax in the presence of 10 and 30 microM pirmenol was 9.48 +/- 3.12 and 20.36 +/- 3.61%, and that of conduction velocity 2.87 +/- 1.52 and 6.58 +/- 2.09%, respectively. 4. The degree of use-dependent block induced by 10 and 30 microM pirmenol during 0.2, 1, 2 and 3 Hz stimulations was dose- and rate-dependent. 5. In the presence of 30 microM pirmenol, mean values of time constants for the onset of the use-dependent inhibition of Vmax and conduction velocity during a 2 Hz stimulation were 1.32 +/- 0.15 and 1.28 +/- 0.09 s, respectively. The recovery time constants averaged 15.83 +/- 2.14 (for Vmax) and 27.80 +/- 8.74 (for conduction velocity) s in the presence of 30 microM pirmenol. 6. These results showed that the characteristics of the use-dependent inhibition of Vmax and conduction velocity induced by pirmenol are similar to those of slow kinetic drugs such as disopyramide rather than of fast ones.