London

Calcium is probably the most important, but not the only, second messenger in vascular smooth muscle cells Its entry into the cell is regulated by potential-sensitive ion channels in the membrane and by mechanisms brought into play upon activation of various recep-tors. Calcium is stored in large amounts in the cell, probably in the sarcoplasmic reticulum, and can be released from this site by receptor activation; the rise in free intracellular ionised calcium concentration, [Ca2+],, produces several effects including actin-myosin interaction and tension generation. Phosphorylation of the light chains of myosin by a calmodulin-regulated enzyme is an important prelude to contraction but other, actin-regulated, methods of contraction are likely to exist. However, the relationship between tension and [Ca2 +] , is not fixed but varies both with the nature of the stimulant and the duration of its application. [Ca2+], is reduced by storage and by extrusion of calcium across the cell membrane largely, it i