Calmodulin and calcium channel blockers.

IONIZED calcium regulates many cellular processes, such as cell motility, muscle contraction, axonal flow, cytoplasmic streaming, neurotransmitter release, endocytosis, andexocytosis. Intracellular Ca concentrations under steady state conditions are low, usually below 10" M. Upon stimulation, the Ca concentration increases transiently and apparently signals an appropriate response, which is often mediated by Ca-binding proteins, of which the most ubiquitous is calmodulin. Calmodulin binds Ca in the presence of physiological concentrations of Mg (10~ M), resulting in a conformational change that allows it to activate any of a number of enzymes. Many drugs, including some organic Ca channel blockers, bind to the active form of calmodulin and antagonize its activity in vitro.