Calcium regulation in vascular smooth muscle contractility.

The contractile activity of vascular smooth muscle is regulated by control over the cytoplasmic calcium concentration. The intracellular calcium receptor is calmodulin, which, through stimulation of myosin light chain kinase, can activate 2 different contractile states. The calcium is supplied from the sarcoplasmic reticulum and the extracellular space; a minor component is supplied from the inner surface of the plasmalemma. The main intracellular messenger responsible for the transduction of receptor occupation and calcium release from the sarcoplasmic reticulum is IP3 and, to a lesser extent, calcium itself. The superficial location of the sarcoplasmic reticulum in vascular smooth muscle makes it the logical area for control of calcium entry due to calcium leak or through either or both types of calcium channel. The sarcoplasmic reticulum, therefore, acts as a "superficial calcium buffer barrier" and is probably the major system controlling free cytoplasmic calcium concentration in vascular smooth muscle.