Ca2+ spark as a regulator of ion channel activity

Ca2+ spark is a local and transient Ca2+ release from sarcoplasmic reticulu m (SR) through the ryanodine receptor Ca2+-releasing channel (RyR). In card iac myocytes, Ca2+ spark is an elementary unit of Ca2+-induced Ca2+ release (CICR) by opening of RyR(s) in junctional SR (jSR), which is triggered by Ca2+-influx through L-type Ca2+ channels to the narrow space between a tran sverse tubule and jSR. Ca2+ spark has, therefore, been des crib ed as the e vidence for "the local control of excitation-contraction coupling". In cont rast, Ca2+ sparks in smooth muscle have been reported in relation to Ca2+-d ependent K+ (K-Ca) channel activation and muscle relaxation. A spontaneous Ca2+ spark in a superficial area activates 10-100 K-Ca channels nearby and induces membrane hyperpolarization, which reduces Ca2+ channel activity. In several types of smooth muscle cells, which have relatively high membrane excitability, an action potential (AP) elicits 5-20 Ca2+ hot spots (evoked sparks with long life) in the early stage via CICR in discrete superficial SR elements and activates K-Ca-channel current highly responsible for AP re polarization and afterhyperpolarization. CICR available for contraction may occur more slowly by the propagation of CICR from superficial SR to deeper ones. The regulatory mechanism of ion channel activity on plasma membrane by superficial SR via Ca2+ spark generation in smooth muscle cells may be a nalogously common in several types of cells including neurons.