The relationships between the cardiovascular effects, alpha- and beta-adrenoceptor blocking actions and plasma concentration of labetalol in doca hypertensive rats.

The relationships between the cardiovascular effects, alpha- and beta-adrenoceptor blocking actions and plasma concentration of labetalol have been examined in conscious DOCA hypertensive rats. Labetalol (10, 30 and 100 mg/kg p.o.) reduced resting heart rate; blood pressure was reduced only by the two higher doses. The effects lasted 5-24 hours. There was a highly significant correlation between the plasma labetalol concentration and its cardiovascular effects. During the labetalol-induced hypotension and bradycardia the vasopressor responses to intra-arterial injections of phenylephrine were reduced; the tachycardia and vasodepressor responses produced by intra-arterial injections of isoprenaline were also reduced. It is concluded that alpha- and beta-adrenoceptor blockade probably account for the labetalol-induced decreases in resting blood pressure and heart rate respectively.