Role of calcium in the activation of smooth muscle.

Smooth muscle depends to a large extent on the extracellular calcium concentration for activation of its contractile proteins. Calcium influx via voltage-dependent or receptor-operated channels is thought to supply the cell with Ca2+ for contractile activation. If this is so, the inhibition of transmembranal calcium influx will prevent mechanical activation. A blockade of transmembranal calcium influx has been proposed as a mode of action for calcium antagonists. This suggestion seems to be supported by experiments in which tissue was incubated in 45Ca-containing solutions for 2-3 min and thereafter transferred for 40-60 min to ice-cold calcium-free or lanthanum-containing solutions. The amount of remaining 45Ca in the tissue at the end of the wash-out phase is taken as a measure of calcium influx during the labelling period. However, procedures involving rather long-lasting wash-out periods cannot yield information on Ca2+ influx, since the calcium store relevant for contractile activation adapts rapidly to changes in extracellular Ca2+ concentration.