Analysis of the ionic basis for cocaine's biphasic effect on action potential duration in guinea-pig ventricular myocytes.

The effects of cocaine on the duration of the cardiac action potential were investigated in isolated guinea-pig ventricular myocytes at 37 degrees C. Following a 10-min exposure of cells to 3 microM cocaine, APD90 increased significantly by +22 +/- 5% (n = 6). In contrast, following a ten minute exposure to 30 or 100 microM cocaine, APD90 was reduced by -24 +/- 6% (n = 5) and -53 +/- 2% (n = 8), respectively. The ionic basis for cocaine's effects on the APD was investigated using the whole cell voltage-clamp technique at 37 degrees C. Cocaine produced a concentration-dependent reduction in the amplitude of IK tail currents with an estimated IC50 of 4 microM. The kinetics and voltage dependence of the cocaine-sensitive current indicate that cocaine selectively blocks a current identical to the E-4031 sensitive current IKr. No significant reduction of the slow component of IK (IKs) was observed during exposure to 30 or 100 microM cocaine. High (30 and 100 microM) concentrations of cocaine also produced a significant reduction of both the L-type calcium current and the TTX-sensitive plateau current. Pre-treatment of cells with 10 microM TTX also converted the APD-shortening effect of 30 microM cocaine to one of APD-prolonging. This implies that cocaine block of a TTX-sensitive window current contributes to the APD-shortening effects produced by high concentrations of cocaine. We conclude that: (1) cocaine produces a biphasic concentration-dependent effect on repolarization in guinea-pig ventricular myocytes; and (2) this biphasic effect on repolarization results from differences in the sensitivity of inward and outward currents to the blocking effects of cocaine.