The force of contraction of the heart muscle is largely determined by the amount of cytosolic Ca2+ ([Ca2+]) and the Ca2+ sensitivity of the myofilaments. Using patch-clamp techniques and fluorescent Ca2+ indicators (microfluorimetry) allowing the measurement of the cytosolic Ca2+ concentration under well-defined experimental conditions, we have analyzed the mechanisms underlying the [Ca2+]i transient in single cardiac myocytes. We conclude that in mammalian cardiac myocytes Ca2+ release from the sarcoplasmic reticulum (SR) is the main source of Ca2+ for activation of the myofilaments. Our results further show that the Ca2+ release channel of the SR is directly gated by Ca2+ influx through L-type Ca2+ channels that open during the action potential. Ca2+ influx via other pathways including the Na(+)-Ca2+ exchanger and T-type Ca2+ channels does not play a significant role in triggering Ca2+ release from the SR. Our results also clearly indicate that any change in the amplitude or time course of the [Ca2+]i transient leads to secondary changes in the duration of the action potential through specific actions on Ca2+ sensitive membrane currents. In particular, an increase in Ca2+ release will enhance Ca2+ efflux via the Na(+)-Ca2+ exchanger resulting in a depolarizing current and prolongation of the action potential. This prolongation of the action potential is to some extent compensated by pronounced inactivation of L-type Ca2+ channels and activation of a transient outward Cl- current.
