Blockade of cardiac and renal beta-receptors by low dose propranolol in normal subjects. Clues to its antihypertensive effect.

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Abstract
The effects of oral propranolol administered in 2 low dose schedules (5 or 20 mg 8 h) and for a short term (30-36 h) on heart rate, plasma renin activity and arterial pressure, both at rest and after an isoprenaline challenge (8 mg i.v. over 2 min) was studied in 6 normal subjects. Resting heart rate, plasma renin activity and systolic pressure fell significantly after the 5 mg dose; only small additional falls were seen when the dose was increased to 20 mg. The fall of diastolic pressure was not significant with 5 mg but was significant with 20 mg. That .beta.-blockade occurred with the low doses was confirmed by diminished isoprenaline responses. The isoprenaline-induced changes of heart rate, plasma renin activity and diastolic pressure were reduced by 77, 80 and 65%, respectively, by the 5 mg dose of propranolol and by 89, 94 and 87% by the 20 mg dose; cardiac and renal receptors are apparently equally sensitive to propranolol. A rapid blood pressure lowering action of low dose propranolol in normal subjects is indicated, and partly due to renal and/or cardiac .beta.-receptor blockade. Lower doses of propranolol than generally used may be clinically effective.