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 was reduced. Confocal microscopy during voltage-clamp depolarization revealed distinct local transients in the concentration of intracellular calcium ions (Ca2+i). When voltage was varied, the latency to occurrence and the relative probability of occurrence of local Ca2+i transients varied as predicted if Ca2+ release from the sarcoplasmic reticulum (SR) was linked tightly to Ca2+ flux through L-type Ca2+ channels 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 transients. Thus, the most efficacious "Ca2+ signal" for activating Ca2+ release from the SR may be a transient microdomain of high Ca2+i beneath an individual, open L-type Ca2+ channel.