Calcium transients during excitation-contraction coupling in mammalian heart: aequorin signals of canine Purkinje fibers.

Aequorin signals in mammalian heart muscle cells reveal the existence of two temporally distinct processes that increase cytoplasmic calcium ions after membrane excitation. The differential dependence of these processes on the pattern of stimulation suggests that the first process is, or is closely related to, calcium entry through the surface membrane and that the second is calcium release from intracellular storage sites.