Evidence for Two Types of Calcium Currents in Frog Cardiac Sinus Venosus Cells

Two types of calcium currents were recorded in single sinus venosus cells of the frog heart, using the whole-cell patch-clamp technique. The threshold potentials were approximately -65 mV for T-type current and -40 mV for L-type current. The amplitude and time course of T-type current were unaffected by exchanging calcium for barium, while the amplitude of L-type current was increased and its decay slowed. T-type current was neither modified by 10(-6) M nifedipine nor by 10(-7) M isoprenaline in contrast with the effects of these agents on L-type current. T-type current began to inactivate at -90 mV and was fully inactivated at -45 mV. Its steady-state inactivation curve was approximately 35 mV negative to that of L-type current. Overlap of activation and inactivation relationships was present for both T- and L-type currents and was maximal at -57 and -30 mV, respectively. It was concluded that T- and L-type currents can easily be separated by their voltage, kinetic and pharmacological differences. The presence of a high density of T-type current may be correlated to its contribution to the pacemaking function of the sinus venosus cells.