LOCAL CALCIUM TRANSIENTS TRIGGERED BY SINGLE L-TYPE CALCIUM-CHANNEL CURRENTS IN CARDIAC-CELLS

Excitation-contraction coupling was studied in mammalian cardiac cells in which the opening probability of L-type calcium (Ca2+) channels wa s reduced. Confocal microscopy during voltage-clamp depolarization rev ealed distinct local transients in the concentration of intracellular calcium ions ([Ca2+](i)). When voltage was varied, the latency to occu rrence and the relative probability of occurrence of local [Ca2+](i) t ransients varied as predicted if Ca2+ release from the sarcoplasmic re ticulum (SR) was linked tightly to Ca2+ flux through L-type Ca2+ chann els but not to that through the Na-Ca exchanger or to average [Ca2+](i ). Voltage had no effect on the amplitude of local [Ca2+](i) transient s. Thus, the most efficacious ''Ca2+ signal'' for activating Ca2+ rele ase from the SR may be a transient microdomain of high [Ca2+](i) benea th an individual, open L-type Ca2+ channel.