[Contraction and relaxation mechanisms of the smooth vascular musculature]

In this short review some aspects of our current knowledge of the cellular processes which are involved in the regulation of vascular tone are discussed. We describe at first the structural components of the contractile apparatus in the vascular smooth muscle and discuss then the calcium-dependent mechanisms that regulate contractile protein activity. Phosphorylation of the 20.000-Dalton-myosin light chain is described as one of the key events in the regulation of the smooth muscle activity. However, mechanisms complementary to the primary effect of myosin phosphorylation are also considered. The differences in calcium modulation of contractile processes between striated and smooth muscle are emphasized. The activation of vascular smooth muscle through graded depolarization and/or action potential and the excitation-contraction coupling as a result of an increase of myoplasmic free calcium concentration is described. The concept of calcium channels which open by agonists acting at the membrane receptors (receptor-operated channels) without any change in membrane potential is presented. Finally, we discuss the different ways by which the level of intracellular calcium as the main factor controlling the contractility of vascular smooth muscle can be reduced. However, other mechanisms, e.g. changes in the sensitivity of the contractile proteins at a given calcium concentration must also be considered as a mechanism of relaxation.