Effect of propranolol on plasma norepinephrine during sodium nitroprusside–induced hypotension

Abstract

The possibility of an action of propranolol which reduces traffic in the sympathetic nervous system and/or reduces release of norepinephrine [NE] at sympathetic nerve endings was studied in normal subjects. Plasma NE was measured in resting supine subjects before and during infusions of sodium nitroprusside which reduced the systolic blood pressure by .apprx. 10 and later by 20 mm Hg. In 5 subjects the observations were repeated 2 h after a single 200 mg oral dose of propranolol and in 3 they were repeated after 2-8 wk of propranolol 320 mg twice daily. In the control experiments, plasma NE increased from 0.22 to 0.74 ng/ml when the blood pressure was reduced by 17/15 mm Hg. Corresponding figures 2 h after 200 mg propranolol were an increase of plasma NE from 0.30 to 0.79 ng/ml with a reduction of blood pressure of 19/15 mm Hg. After long-term treatment with 640 mg propranolol daily, a similar reduction of pressure with sodium nitroprusside increased plasma NE from 0.24 to 0.68 ng/ml. There was no significant change in the elevation of plasma NE after blood pressure reduction with nitroprusside after either short- or long-term treatment with propranolol. Apparently propranolol does not modify sympathetic activity significantly.