mercaptopurine and carsatrin

We examined the effects of the cardiotonic agent RWJ 24517 (Carsatrin, racemate) and its (S)- and (R)-enantiomers on action potential duration, Na(+) current (I(Na)), and delayed rectifier K(+) current (I(K)) of guinea pig ventricular myocytes. RWJ 24517 (0. 1 and 1 microM) prolongation of action potential duration could not be accounted for by suppression of either the rapid (I(Kr)) or slow (I(Ks),) component of I(K), although RWJ 24517 did reduce I(Kr) at concentrations of 1 microM. A more dramatic effect of RWJ 24517 (0.1-1 microM) and the (S)-enantiomer of RWJ 24517 (0.1-3 microM) was an increase in peak I(Na) and slowing of the rate of I(Na) decay, eliciting a large steady-state current. Neither RWJ 24517 nor the (S)-enantiomer affected the fast time constant for I(Na) decay, but both significantly increased the slow time constant, in addition to increasing the proportion of I(Na) decaying at the slow rate. Both agents elicited a use-dependent decrease of peak I(Na) (3-10 microM), which probably resulted from a slowing of both fast and slow rates of recovery from inactivation. In contrast, the (R)-enantiomer of RWJ 24517 did not induce a steady-state component I(Na) or increase peak I(Na) up to 10 microM, but it decreased peak I(Na) at 30 microM. The (R)-enantiomer displayed little use-dependent reduction of I(Na) during trains of repetitive pulses and had no effect on rates of inactivation or recovery from inactivation. These actions of the racemate and the (S)-stereoisomer to slow inactivation and to prolong both Na(+) influx and action potential duration may contribute to the positive inotropic actions of these agents because the resulting accumulation of intracellular Na(+) would increase intracellular Ca(2+) via Na(+)/Ca(2+) exchange.

Topics: Action Potentials; Animals; Cardiotonic Agents; Dose-Response Relationship, Drug; Guinea Pigs; Heart Ventricles; In Vitro Techniques; Mercaptopurine; Muscle Contraction; Patch-Clamp Techniques; Piperazines; Potassium; Sodium; Stereoisomerism; Time Factors

Pharmacological modulation of human sodium current was examined in Xenopus oocytes expressing human heart Na+ channels. Na+ currents activated near -50 mV with maximum current amplitudes observed at -20 mV. Steady-state inactivation was characterized by a V1/2 value of -57 +/- 0.5 mV and a slope factor (k) of 7.3 +/- 0.3 mV. Sodium currents were blocked by tetrodotoxin with an IC50 value of 1.8 microM. These properties are consistent with those of Na+ channels expressed in mammalian myocardial cells. We have investigated the effects of several pharmacological agents which, with the exception of lidocaine, have not been characterized against cRNA-derived Na+ channels expressed in Xenopus oocytes. Lidocaine, quinidine and flecainide blocked resting Na+ channels with IC50 values of 521 microM, 198 microM, and 41 microM, respectively. Use-dependent block was also observed for all three agents, but concentrations necessary to induce block were higher than expected for quinidine and flecainide. This may reflect differences arising due to expression in the Xenopus oocyte system or could be a true difference in the interaction between human cardiac Na+ channels and these drugs compared to other mammalian Na+ channels. Importantly, however, this result would not have been predicted based upon previous studies of mammalian cardiac Na+ channels. The effects of DPI 201-106, RWJ 24517, and BDF 9148 were also tested and all three agents slowed and/or removed Na+ current inactivation, reduced peak current amplitudes, and induced use-dependent block. These data suggest that the alpha-subunit is the site of interaction between cardiac Na+ channels and Class I antiarrhythmic drugs as well as inactivation modifiers such as DPI 201-106.

Topics: Animals; Azetidines; Cardiotonic Agents; Electrophysiology; Female; Flecainide; Humans; Lidocaine; Mercaptopurine; Myocardium; Oocytes; Piperazines; Quinidine; RNA, Complementary; Sodium Channels; Tetrodotoxin; Transcription, Genetic; Xenopus laevis

RWJ-24517 is a positive inotropic agent whose mechanism of action is under investigation. We examined the effects of RWJ-24517 on guinea pig papillary muscle action potentials and myofilament response to Ca2+. RWJ-24517 increased the fast action potential duration (APD) in a dose-dependent fashion but had no effect on the myofilament response. Tetraethylammonium (TEA 10 mM), which in itself slightly prolonged the control AP, completely blocked the increase in APD75 (APD at 75% of repolarization) and suppressed the increase in APD25 induced by RWJ-24517 (10 microM). Verapamil (5 microM) had little effect on control APs but did decrease the prolongation of the AP induced by RWJ-24517. The increase in APD25 induced by RWJ-24517 was not completely blocked by TEA even with addition of verapamil or 4-aminopyridine (2 mM). RWJ-24517 enhanced Ca(2+)-dependent slow APs elicited by 0.1 microM isoproterenol in preparations depolarized by high extracellular K+ (25 mM), but had no effects on slow APs elicited by 10 mM TEA. These results suggest that the primary electrophysiologic effect of RWJ-24517 is a substantial AP prolongation, which appears to occur largely through a mechanism which is likely to involve inhibition of Ca(2+)-dependent K+ channels. It also appears to have additional effects on some other channels (possibly Na+ channels), which may contribute to the positive inotropic action of RWJ-24517.

Topics: Actin Cytoskeleton; Action Potentials; Adenosine Triphosphatases; Animals; Cardiotonic Agents; Dose-Response Relationship, Drug; Electrophysiology; Female; Guinea Pigs; Male; Mercaptopurine; Papillary Muscles; Piperazines; Potassium Channel Blockers; Sodium Channels; Tetraethylammonium; Tetraethylammonium Compounds; Verapamil

A series of purine derivatives was prepared and examined for selective inotropic activity in vitro and in vivo. Thioether-linked derivatives were superior to their oxygen and nitrogen isosteres. Substitution of electron-withdrawing groups on the benzhydryl moiety of these agents increased potency. The best compound of the study, 17 (carsatrin), was examined further and demonstrated selective oral activity as a positive inotrope. These compounds are presumed to act by affecting the kinetics of the cardiac sodium channel by analogy to the prototypic agent DPI 201106 (1). Their high selectivity for increasing contractile force and dP/dt without affecting blood pressure or heart rate is consistent with this mechanism. Carsatrin (17) was selected as a potential development candidate.

Topics: Animals; Blood Pressure; Cardiotonic Agents; Dogs; Ferrets; Heart Rate; Male; Mercaptopurine; Molecular Structure; Myocardial Contraction; Papillary Muscles; Piperazines; Purines; Sodium Channels; Stimulation, Chemical; Structure-Activity Relationship