EVIDENCE FOR 2 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.