Ca2+ sparks and Ca2+ waves activate different Ca2+-dependent ion channels in single myocytes from rat portal vein

Ca2+ release from intracellular stores was examined with the use of a confocal microscope in single, voltage-clamped myocytes from rat portal vein loaded with both Fluo-3 and Fura-red. Spontaneous local increases in [Ca2+](i) from the sarcoplasmic reticulum, termed Ca2+ sparks, were observed in about 30% of the quiescent cells tested. Ca2+ sparks could be evoked by low concentrations of caffeine (1 mM) or ryanodine (1 mu M). Both spontaneous and caffeine-evoked Ca2+ sparks were insensitive to blockers of voltage-dependent Ca2+ channels. Caffeine (10 mM) triggered propagating Ca2+ waves of large amplitude which started from the same site than spontaneous Ca2+ sparks in 73% of the cells, as expected if Ca2+ sparks were the elementary events that could account for the initiation of Ca2+ waves. Spontaneous Ca2+ sparks activated both Ca2+-dependent K+ and non-selective cation currents, whereas Ca2+ waves were able to evoke Ca2+-dependent chloride current. These results suggest that both inward cation current and outward K+ current activated by Ca2+ sparks may exert a key role in controlling the basal activity of vascular myocytes.