Hydralazine and its metabolites: in vitro and in vivo activity in the rat

The vasodilator, hypotensive, and antihypertensive effects of hydralazine and its known and putative metabolites were compared in vitro, in the isolated, perfused mesenteric arterial bed of the rat, and in vivo, in the urethane-anesthetized normotensive rat (NR) and in the conscious renal hypertensive rat (RHR). In the mesenteric bed, hydralazine produced inhibition of noradrenaline (NA)-induced vasoconstriction (IC50-NA = 0.4 micrograms/ml). All the metabolites were five to greater than 250 times less potent than the parent compound. Hydralazine inhibited potassium-induced vasoconstriction at concentrations (IC50-K+ = 700 micrograms/ml) above those required to inhibit NA. Two metabolites, 9-hydroxy-3-methyl-s-triazolo-(3.4-a)phthalazine and the acetone hydrazone (HP-AH), were more potent (five- and 10-fold, respectively) than hydralazine in inhibiting potassium-induced vasoconstriction. The other metabolites produced less than 50% inhibition at the highest concentration tested. In in vivo studies, blood pressure in NR or RHR was reduced by hydralazine, following doses of 0.1 or 0.25 mg/kg i.v. and above. HP-AH was sixfold less active than hydralazine in NR and 10-fold less active in RHR, while the pyruvic acid hydrazone was 33- and 14-fold 1 s active, respectively. The other metabolites tested were practically inactive in concentrations up to the limits of solubility. Although several hydralazine metabolites show some vasodilator and blood pressure-lowering activity, it seems unlikely that the formation of metabolites is a major factor in the antihypertensive effect of hydralazine or is responsible for its long duration of action.