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 receptors. 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, [Ca 2+ ] 1 produces several effects including actin-myosin interaction and tension generation. Phosphorylation the 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 [Ca 2+ ] 1 is not fixed but varies both with the nature of the stimulant and the duration of its application. [Ca 2+ ] 1 is reduced by storage and by extrusion of calcium across the cell membrane largely, it is believed by the calcium-pump and to a lesser extent by a sodium-calcium exchange mechanism.