The role of calcium in the control of vascular tone as assessed by the Ca2+ indicator aequorin.

An overview is given of the current understanding of the mechanism of contraction of vascular smooth muscle. The regulation of vascular tone by intracellular ionized calcium levels appears to occur primarily through calcium-dependent phosphorylation of the myosin light chains by the enzyme myosin light chain kinase. Evidence is presented that additional relatively calcium-independent processes also exist and contribute to the regulation of vascular tone. A scheme is presented whereby vasoconstriction may occur in the absence of any change in cytoplasmic ionized calcium levels. The multiplicity of excitation-contraction coupling pathways in vascular smooth muscle predicts a multitude of rational therapeutic approaches to vascular pathologies.