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 (Ca$^{2+}$) channels was reduced. Confocal microscopy during voltage-clamp depolarization revealed distinct local transients in the concentration of intracellular calcium ions ([Ca$^{2+}$]$_i$). When voltage was varied, the latency to occurrence and the relative probability of occurrence of local [Ca$^{2+}$]$_i$ transients varied as predicted if Ca$^{2+}$ release from the sarcoplasmic reticulum (SR) was linked tightly to Ca$^{2+}$ flux through L-type Ca$^{2+}$ channels but not to that through the Na-Ca exchanger or to average [Ca$^{2+}$]$_i$. Voltage had no effect on the amplitude of local [Ca$^{2+}$]$_i$ transients. Thus, the most efficacious "Ca$^{2+}$ signal" for activating. Ca$^{2+}$ release from the SR may be a transient microdomain of high [Ca$^{2+}$]$_i$ beneath an individual, open L-type Ca$^{2+}$ channel.