Effects of calcium on vascular smooth muscle tone. It is generally acknowledged that calcium plays a major role in the generation of vascular tone. However, in recent years it has become increasingly evident that relatively calcium-insensitive pathways of excitation-contraction coupling also exist in the vascular smooth muscle cell. Possible mechanisms of vascular smooth muscle contraction and their possible role in the pathophysiology of hypertension are reviewed. The rationale for the use of calcium channel blockers in the treatment of hypertension is discussed. 