The pharmacology of calcium antagonists
Calcium antagonists constitute a chemically and pharmacologically diverse group of drugs. Chemically, at least four major types of calcium antagonists can be distinguished; they include phenylalkylamines, dihydropyridines, benzothiazepines and piperazines. As a group they share the common property of slowing the transmembrane entry of calcium ions through either voltage-activated, or (probably) receptor operated, ion selective channels. They differ from one another also in terms of their tissue specificity and selectivity. Different types (L, T, N) of calcium channels have been identified and their distribution in different tissues is specific. The molecular properties and the differential binding characteristics of the subgroups of calcium antagonists have been defined. At least one of calcium antagonists, diltiazem, has an intracellular action. All calcium antagonists share the ability to dilate blood vessels, but there is a marked difference in potency and selectivity, even within individual subgroups. The dihydropyridines are relatively selective antagonists of vascular smooth muscle cell calcium uptake. Their ability to inhibit myogenic vascular tone contributes to their predominant in vivo effect on vascular beds characterized by a relatively high myogenic tone, such as the coronary and skeletal muscle beds. Additional mechanisms contributing to the antihypertensive action of dihydropyridines include interference with neural control of circulation, as well as intrarenal actions to promote electrolyte and water excretion. In myocardium, the tissue protecting effects of calcium antagonists, especially the dihydropyridine-based group, in the presence of hypertension involve not only reduction of hypertrophy, but they also reduce ischemia-induced injury and the incidence of damage due to lipid peroxidation. Other possible therapeutic uses of calcium antagonist include prevention of atherogenic changes, treatment and prevention of acute asthma, prevention of consequences of cerebral ischemia and vasospasm, anticonvulsant therapy, prophylactic therapy of migraine, attenuation of acute renal failure and slowing of the progression of chronic renal failure, uterine relaxation, and treatment of some gastrointestinal disorders.