Interaction of hydralazine with tension development and mechanisms of calcium accumulation in K+-stimulated rabbit aortic strips.

A study was made of the effects of hydralazine on K+ contractures in rabbit aortic strips. Measurements were made of effects on tension generation, Ca++ accumulation measured using 45Ca++ (lanthanum method) and 45Ca++ efflux. Hydralazine relaxed established K+ contractures and inhibited tension development when tissue exposure to hydralazine preceded K+ depolarization. Both the threshold and the maximum tension responses of strips to Ca++ (K+-depolarized tissues) were altered by hydralazine in a dose-dependent manner. The Ca++ uptake associated with K+ depolarization was inhibited by relaxant doses of hydralazine; however, hydralazine did not significantly affect the rate of 45Ca++ efflux. Hydralazine also raised the threshold to the inhibitory (relaxant) effects of higher (greater than 2.5 mM) bath concentrations of Ca++. It is suggested that hydralazine inhibits K+ contractures in aortic strips by interference with the entry of Ca++ into the cell, reflecting an effect of hydralazine at the cell surface membrane.