Calcium channels in smooth muscle

The molecular and biophysical characterization of Ca2+ channels in smooth muscle has lagged behind that in cardiac and skeletal muscles. This is caused by the difficulties in isolating viable cells and applying electrophysiological techniques to smooth muscle cells. The heterogeneity of smooth muscle cells from different organs also adds to the complexity. In spite of these problems there has been an increasing number of reports on Ca2+ channels in smooth muscle. This review provides an overview of this area. As in skeletal muscles, L (long-acting) and T (transient) voltage-dependent Ca2+ channels are found in smooth muscle; the former is better characterized. L channel is regulated by voltage, Ca2+, cyclic nucleotides, and protein kinase C. Ca2+ also influxes through receptor-operated channels that are voltage insensitive. The receptor-operated and T channels are insensitive to dihydropyridine. Clinically, Ca2+ blockers are extensively used for diseases of the cardiovascular system, with more limited use in the gastrointestinal tract. Further understanding of the properties of Ca2+ channels could lead to development of selective Ca2+ channel blockers for the gastrointestinal tract.