Effect of propranolol on the blood pressure of normotensive and pinealectomized hypertensive rats.

Abstract

The antihypertensive effect of propranolol on pinealectomy-induced hypertension was studied in male rats. Pinealectomized hypertensive and unoperated normotensive rats were treated with (±)-propranolol · HCl twice a day p.o. for 20 days, the total daily dose being 5 or 50 mg/kg. The blood pressure of unanaesthetized rats was measured by a tail cuff method about 3 h after the first half of the daily dose on the first, 6th and 16th day of treatment. In unoperated rats both doses of propranolol induced an elevation of blood pressure throughout the experimental period, the mean increase being 5–16 mm Hg. In pinealectomized rats propranolol did not affect the blood pressure on the first day of the treatment, but the higher dose lowered the blood pressure on the 6th day from 148±2 to 137±3 mm Hg, and on the 16th day from 144±2 to 127±3 mm Hg. The lower dose of propranolol lowered the blood pressure only on the 16th day of the treatment. In contrast to the delay in the onset of the antihypertensive action of propranolol, stress-induced tachycardia was antagonized as early as 1 h after the first administration of the drug. In pinealectomized rats plasma renin activity was slightly increased. On the 20th day of treatment plasma renin activity was not significantly affected by either dose of propranolol in unoperated or pinealectomized rats. In an acute experiment propranolol (50 mg/kg p.o.) clearly lowered plasma renin activity in both unoperated and pinealectomized rats within 1 h. A single dose of 5 mg/kg of propranolol influenced plasma renin activity in neither group of rats. Since there are beta-adrenoceptor antagonists which block cardiac beta-receptors and which lower plasma renin activity without exerting an antihypertensive effect, it is concluded that the antihypertensive effect of propranolol in pinealectomized rats is not attributable to a blockade of the cardiac beta-receptors or to a decrease of plasma renin activity.