[The excitation-contraction coupling of the vascular smooth muscle cells]

Two phenomena may lead to an increase in intracellular calcium concentration of vascular smooth muscle cells: an increase in the permeability of the cell membrane to Ca2+ ions; liberation of Ca2+ ions from the intracellular reservoirs. The calcium channels of smooth muscle are varied. There are two types of voltage operated calcium channels: the fast (T) and the slow (L) channels. The calcium channels activated by extracellular membrane receptors are not voltage dependent. Only the L calcium channels are sensitive to dihydropyridines. The liberation of Ca2+ from the sarcoplasmic reticulum which is the intracellular reservoir of calcium can be controlled by two different mechanisms: a direct mechanism by the influx of Ca2+ into the cell through the voltage-operated channels; by the intermediary of a second intracellular messenger. High conductance calcium channels controlled by cytosolic Ca2+ and by IP2 have been demonstrated on the membrane of the sarcoplasmic reticulum. The contraction of smooth muscle may therefore be regulated directly through control of the phosphorylation of the contractile proteins by the intermediary of the systems of adenylate and guanylate cyclase.