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 [Ca ²⁺]_i transients varied as predicted if Ca²⁺ release from the sarcoplasmic reticulum (SR) was linked tightly to Ca ²⁺ 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 Ca ²⁺ channel.