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

We studied the effects of oral propranolol administered in 2 low dose schedules (5 or 20 mg 8 hourly) and for a short term (30 to 36 hours) on heart rate, plasma renin activity, and arterial pressure, both at rest and after an isoprenaline challenge (8 mg i.v. over 2 min) in 6 normal subjects. Resting heart rate, plasma renin activity, and systolic pressure all fell significantly after the 5 mg dose and only small additional falls were observed 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 these low doses was confirmed by diminished responses to isoprenaline. The isoprenaline-induced changes of heart rate, plasma renin activity, and diastolic pressure were reduced by 77, 80, and 65 per cent, respectively, by the 5 mg dose of propranolol, and by 89, 94, and 87 per cent by the 20 mg dose, suggesting that cardiac and renal receptors are equally sensitive to propranolol. These data indicate a rapid blood pressure lowering action of low dose propranolol in normal subjects and suggest that this effect is in part at least a consequence of blockade of renal and/or cardiac betareceptors. Our results also raise the possibility that much lower doses of propranolol than generally used may be clinically effective.