The physiological role of calcium-dependent channels.

Calcium (Ca2+)-dependent channels may be classified in three broad categories, which are, respectively, selective for potassium ions, for chloride ions, and for monovalent cations. The usual action of Ca2+ is to increase the probability of opening of the channels, but examples of the reverse, Ca2+-induced inhibition of ion channels, have recently been found. Ca2+-dependent channels help to shape the action potentials of excitable cells as well as the synaptic currents of muscular and neuronal preparations. They are involved in several aspects of electrolyte transport including regulation of osmolarity in animal cells and of turgor in plant cells, electrolyte secretion in exocrine glands, fluid absorption and secretion in epithelial tissues.