Effects of quinine and quinidine on the transient outward and on the L-type Ca(2+) current in rat ventricular cardiomyocytes.

The effects of the enantiomers quinine and quinidine on the transient outward current (I(to)) and on the L-type Ca(2+) current (I(ca)) were investigated in rat ventricular cardiomyocytes using the patch-clamp technique. At a stimulation frequency of 2 Hz, both quinine and quinidine depressed the magnitude of I(to) and I(Ca); the half-maximal effects on I(to) were achieved at 11 and 15 micromol/l, respectively, and those on I(Ca) at 14 and 10 micromol/l, respectively. At 0.2 Hz, both drugs depressed the magnitude of I(to), but not that of I(Ca). A change in extracellular pH from 7.3 to 8.3 did not significantly influence the effects of the drugs(which are protonated to 98% at pH 7.3) on I(to) or I(Ca). It is concluded that neither the different chemical structure nor the amount of protonation of quinine and quinidine controls their effects on I(to) or I(Ca).