Existence of two calcium currents recorded at normal calcium concentrations in single frog atrial cells

Abstract

A low threshold calcium current (ICaLT) was found in Cs +-loaded frog atrial cells in addition to the classical (high threshold) calcium current (ICaHT), and was investigated at physiological Ca 2+ concentrations using the whole-cell patch-clamp technique. The threshold potentials were approximately −60 mV for ICaLT and −40 mV for ICaHT. The amplitude and time course of ICaLT were almost unaffected by exchanging Ca 2+ for Ba 2+ or Sr 2+, while those of ICaLT were modified. ICaLT was inhibited by Ni 2+ (40 × 10 −6 M) but was not affected by Cd 2+ (20 × 10 −6 M) while ICaHT was inhibited by Cd 2+ and only slightly reduced by Ni 2+ at the same concentrations. Co 2+ (10 −3 M) inhibited both types of calcium currents while La 3+ (5 × 10 −6 M) had a greater blocking effect on ICaHT. ICaLT was neither modified by dihydropyridines (nisoldipine, Bay K) nor by adrenergic agonists (adrenaline, noradrenaline, isoprenaline), in contrast with the effects of these agents on ICaHT. Angiotensin II (40 × 10 −9 M) increased and atrial natriuretic factor (0.1 × 10 −6 M) decreased ICaLT while ICaHT, was not modified by these two substances.