Norepinephrine-induced contractions of the rat aorta in the absence of extracellular calcium--II. Effects of calcium antagonists.

The rat aorta responds biphasically to norepinephrine (NE) in calcium-free medium, with the initial phasic and subsequent tonic components of the contraction being apparently mediated through mobilization of calcium from different intracellular pools. Membrane calcium channel blockers (verapamil, nifedipine, and SKF24260) did not affect the biphasic aortic contractions induced by NE in calcium-free medium, but reduced the response to NE in calcium-containing medium. Intracellular calcium antagonists (tertiary propyl and butyl methylenedioxyindenes) inhibited both phases of the NE-induced aortic contractions in calcium-free medium at concentrations which were similar to those required to inhibit NE-induced contractions in calcium-containing medium. These results support the concept that both components of the biphasic response of the rat aorta to NE are mediated through the mobilization of intracellular calcium when evoked in a calcium-free medium.