Ca2+ channels, ca2+ sparks, and regulation of arterial smooth muscle function

In cardiac, skeletal, and arterial muscle, transient, spatially localized elevations in [Ca2+](i), termed 'Ca2+ sparks', have been observed using confocal laser scanning microscopy. Ca2+ sparks are thought to represent 'elementary' Ca2+ release events, which arise from one or more ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). In striated muscle, Ca2+ sparks are thought to be key elements of excitation- contraction coupling. In arterial smooth muscle, Ca2+ sparks have been suggested to oppose myogenic vasoconstriction and to influence vasorelaxation. Using a developmental model, we have investigated whether RyRs causing Ca2+ sparks and activation of Ca2+-activated K+ (Kca) channels (STOCs) function as 'elementary' Ca2+ release units that regulate arterial myogenic tone. Whereas increases in the global [Ca2+](i) induce sustained constriction of arterial smooth muscle, Ca2+ sparks induce vasodilation through the local activation of K(Ca) channels. In cerebral arteries, the global bulk [Ca2+](i) and a Ca2+ spark frequency <10-2 Hz/cell do not cause sufficient K(Ca) channel activity to regulate membrane potential of smooth muscle cells and myogenic tone. The frequency of Ca2+ sparks and STOCs is regulated by agents that modulate protein kinase G and protein kinase A activity. Our findings suggest that 'elementary' Ca2+ release units may represent novel, important therapeutic targets for regulating function of the intact arterial smooth muscle tissue.