Effect of urethane on hydralazine-induced tachycardia in rats.

1. In conscious normotensive rats, hydralazine (5-10 mg kg-1 p.o.) produced a dose-related fall in systolic blood pressure, accompanied by a pronounced increase in heart rate. 2. The tachycardia induced by hydralazine (10 mg kg-1 p.o.) in conscious normotensive rats was strongly inhibited after anaesthesia with urethane (1.26 g kg-1 i.p.). 3. In anaesthetized normotensive rats, hydralazine (1 mg kg-1 i.v.) caused a fall in mean blood pressure, accompanied by irregular effects on the heart rate that consisted in a combination of initial tachycardia followed by bradycardia. 4. In pithed rats, hydralazine (1 mg kg-1 i.v.) did not affect mean arterial blood pressure but produced a significant decrease in heart rate. 5. In rat isolated atria, hydralazine (2 mM) produced a positive inotropic/negative chronotropic effect. 6. These results suggest that urethane inhibits the cardiovascular reflex that causes the tachycardia induced by hydralazine in conscious normotensive rats. For this reason, in anaesthetized normotensive rats appear the direct effect of the drug on the heart.