Risperidone-induced action potential prolongation is attenuated by increased repolarization reserve due to concomitant block of I(Ca,L).

The neuroleptic risperidone is an effective blocker of the rapidly activating component of the delayed rectifier current (I(Kr)) and hence is expected to prolong cardiac action potential duration (APD). However, unlike with other typical I(Kr) blockers we failed to demonstrate a marked prolongation of late repolarization with risperidone. It is hypothesized that the APD-prolonging effect of risperidone is masked by the high repolarization reserve due to the prominent delayed rectifier currents I(Kr) and I(Ks) in guinea pig papillary muscle. Action potentials and force of contraction were recorded in isolated guinea pig papillary muscles. L-type calcium current I(Ca,L) and I(Kr) were measured using the standard patch clamp technique in single ventricular cardiomyocytes. Reduction of the repolarization reserve by the blocking of I(Ks) with chromanol 239B augmented the effect of the selective I(Kr) blocker E-4031, but not of risperidone, although both drugs completely blocked I(Kr). In contrast to E-4031 risperidone markedly reduced the force of contraction due to the partial blocking of I(Ca,L) in the same concentration range as required for block of I(Kr). Reduction of the repolarization reserve by the blocking of I(Ks) cannot exacerbate the APD-prolonging effect of risperidone. However, even incomplete concomitant blocking of I(Ca,L) attenuates the APD-prolonging effect of the complete blocking of I(Kr). This behaviour may explain the small APD-prolonging effect of risperidone despite the drug's robust blocking of I(Kr).