Electrophysiological effects of bimoclomol in canine ventricular myocytes

Abstract

Concentration-dependent effects of bimoclomol, a novel heat shock protein (HSP) coinducer, were studied on the parameters of action potential and transmembrane ionic currents in enzymatically dispersed canine ventricular cardiomyocytes using conventional microelectrode and whole cell voltage clamp techniques. Bimoclomol (10-100 microM) decreased the maximum velocity of depolarization (Vmax) and amplitude of action potentials in a concentration-dependent manner. These effects were fully reversible after a 5-min period of washout in drug-free medium. Action potential duration measured at 50% or 90% level of repolarization (APD-50 and APD-90, respectively) was markedly shortened by bimoclomol. Both APD-50 and APD-90 were decreased, but the reduction in APD-50 was more pronounced. The APD-shortening effect of bimoclomol was significantly reduced in the presence of 20 nM charybdotoxin (inhibitor of the Ca-dependent K current) or 0.5 mM anthracene-9-carboxylic acid (inhibitor of the Ca-dependent Cl current) or 1 microM glibenclamide (inhibitor of the ATP-sensitive K current). In the presence of anthracene-9-carboxylic acid, APD-90 was lengthened by bimoclomol. The APD-shortening effect of bimoclomol was also partially antagonized by chelation of intracellular Ca2+ by application of the cell permeant form of BAPTA, or when using 10 mM EGTA-containing patch pipettes to record action potentials. The Vmax-depressant effect of bimoclomol was not affected by charybdotoxin, anthracene-9-carboxylic acid, glibenclamide, or BAPTA load. In voltage clamped cardiomyocytes bimoclomol (100 microM) had no effect on the amplitude of I(Ca), but decreased significantly the inactivation time constant of I(Ca) (from 19.8+/-1.6 ms to 16.8+/-1.2 ms at 0 mV). Bimoclomol also decreased significantly the amplitude of I(K1) (from -20.5+/-1.1 pA/pF to -16.6+/-0.8 pA/pF at -135 mV), causing reduction in slope of the negative branch of the I-V curve. At positive potentials, however, bimoclomol increased outward current. The bimoclomol-induced current, therefore, was studied in the presence of BaCl2, when I(K1) current was blocked. The bimoclomol-induced current had a reversal potential close to -90 mV. Bimoclomol (100 microM) had no effect on the amplitude or kinetic properties of the transient outward K current (I(to)) and the delayed rectifier K current (I(K)). It is concluded that bimoclomol exerts both Ca-independent (inhibition of I(Na) and I(K1), activation of the ATP-sensitive K current) and Ca-dependent effects (mediated by Ca-activated Cl and probably K currents) in canine ventricular myocytes.