PHARMACOLOGY OF CALCIUM CHANNELS IN CARDIAC-MUSCLE, VASCULAR MUSCLE, AND NEURONS

The properties of voltage-dependent calcium channels in muscle cells and neurons have been studied using the whole-cell patch clamp technique. Cardiac muscle cells, vascular muscle cells, and neurons all have multiple types of calcium channels, and the different types have different pharmacological properties. The predominant calcium current in most cardiac and vascular muscle cells is carried through L-type calcium channels, which are potently and completely blocked by dihydropyridine drugs; the block is voltage-dependent, and is more potent at depolarized holding potentials where the calcium current is partly inactivated. In neurons, much less of the high-threshold calcium channel current is blocked by dihydropyridines, because much of it is carried through N-type calcium channels, which are not blocked by dihydropyridines but are blocked by the peptide toxin omega-conotoxin GVIA. In addition to L-type and N-type calcium channels, rat neurons have a third type of high-threshold calcium channel that is not blocked by either omega-conotoxin or dihydropyridines. Some neurons also have a fourth type of calcium current, a low threshold current carried by T-type calcium channels.