Enhanced Acute Antihypertensive Effect of Propranolol in the Absence of Circulating Epinephrine in the Rat

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Abstract
β-Adrenoceptor blocking agents might reduce blood pressure, in part, by blocking presynaptic β-adrenoceptors. Absence of circulating epinephrine should then reduce the antihypertensive effect of propranolol. Biadrenalectomized Wistar-Kyoto rats were made hypertensive with methylprednisolone (20 mg/kg s.c. weekly), given for 2 weeks, and supplemented with deoxycorticosterone pivalate (10 mg/kg weekly). Sham-operated controls received the same treatment. Baseline weight, mean intraarterial blood pressure, and heart rate of the groups were the same. After propranolol (5 mg/kg s.c) was administered to the unanesthetized rats, blood pressure fell within 90 min from 151 ± 4 by 23 ± 4 mm Hg (mean ± SEM) in the adrenalectomized animals and from 153 ± 4 by only 7 ± 3 mm Hg in the sham-operated controls (p < 0.001); heart rate fell by 91 ± 13 beats/min in the adrenalectomized animals and by 40 ± 11 beats/ min in the controls (p < 0.01). Propranolol's vehicle had no effect. At the end of the experiment, plasma epinephrine levels were <40 pg/ml for adrenalectomized rats, and 420 ± 60 pg/ml for controls. Norepinephrine levels were approximately equal in the two groups. Since blood pressure fell despite virtual absence of circulating epinephrine, these results suggest that propranolol reduces blood pressure, at least in part, by mechanisms other than presynaptic β-adrenoceptor blockade.