Intrinsic sympathomimetic activity of labetalol

In intact cats, cumulative doses (0.1-10 mg/kg i.v.) of labetalol produced dose-dependent decreases in heart rate and arterial blood pressure and dose-dependently reduced i.v. phenylephrine induced pressor responses. In spinal cats devoid of resting sympathetic tone, labetalol (1 mg/kg i.v.) produced a sustained elevation of heart rate and a transient fall in arterial blood pressure. In reserpine-pretreated, adrenalectomized cats, labetalol produced quantitatively the same effects as in spinal cats, indicating that the cardiovascular effects observed in cats with no resting sympathetic tone are due to a direct action of labetalol rather than via catecholamine release. The elevated heart rate due to labetalol in spinal cats was reduced by subsequent administration of the beta-adrenergic receptor antagonist, propranolol. Further, pretreatment with propranolol prevented the tachycardic and depressor effects of labetalol in spinal cats. In a separate group of spinal cats, labetalol administered in cumulative doses of up to 1 mg/kg i.v., produced graded increases in heart rate and also dose dependently reduced i.v. isoproterenol-induced tachycardic responses. Pindolol, a beta-adrenergic receptor antagonist with partial beta-agonist activity, produced similar effects in spinal cats at cumulative doses of 1-30 micrograms/kg. These results indicate that the alpha- and beta-adrenergic receptor antagonist, labetalol possesses partial beta-adrenergic receptor agonist activity. This intrinsic sympathomimetic action of labetalol appears to be more sustained on cardiac than on vascular beta-adrenergic receptors.