[Calcium-calmodulin and vasomotor activity].

Calmodulin is a small protein (16.7 KDa) calcium receptor which plays a fundamental part in vasomotricity. When intracellular Ca2+ concentrations increase (from 0.1 to 10 M), calmodulin fixes four Ca2+, changes its conformation and interacts with its target proteins. In vascular smooth muscle it activates the kinase of the myosine light chain and interacts with caldesmone to allow phosphorylation of myosine and the actine-myosine interaction. These two processes lead to vascular smooth muscle contraction. Calmodulin also activates enzymes involved in the regulation of the cyclic nucleotides: adenylate cyclase which synthesises cyclic AMP and the calmodulin dependent phophodiesterase which preferentially hydrollyses cyclic GMP. Cyclic AMP and GMP contribute to the relaxation of smooth muscle by inhibiting, the kinase of the myosine light chain and stimulating the Ca2+ ATPase responsible for the extrusion of Ca2+. The contractile action of calmodulin is counterbalanced by the relaxing effects of cyclic AMP and GMP. In addition, caldomodulin participates in the control of vasomotricity by regulating the phosphorylation and dephosphorylation of proteins influencing intracellular Ca2+ concentrations and the activation of contractile proteins. Caldomodulin activates the enzyme responsible for the synthesis of nitric oxide in the endothelium (endothelium derived relaxing factor) and thereby participates in the endothelium dependent relaxation process.