CA2-DEPENDENT ION CHANNELS IN SINGLE MYOCYTES FROM RAT PORTAL-VEIN( SPARKS AND CA2+ WAVES ACTIVATE DIFFERENT CA2+)

Ca2+ release from intracellular stores was examined with the use of a confocal microscope in single, voltage-clamped myocytes from rat porta l vein loaded with both Fluo-3 and Fura-red. Spontaneous local increas es in [Ca2+](i) from the sarcoplasmic reticulum, termed Ca2+ sparks, w ere observed in about 30% of the quiescent cells tested. Ca2+ sparks c ould be evoked by low concentrations of caffeine (1 mM) or ryanodine ( 1 mu M). Both spontaneous and caffeine-evoked Ca2+ sparks were insensi tive 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 ex pected if Ca2+ sparks were the elementary events that could account fo r the initiation of Ca2+ waves. Spontaneous Ca2+ sparks activated both Ca2+-dependent K+ and non-selective cation currents, whereas Ca2+ wav es were able to evoke Ca2+-dependent chloride current. These results s uggest that both inward cation current and outward K+ current activate d by Ca2+ sparks may exert a key role in controlling the basal activit y of vascular myocytes.