Effects of Guanethidine, Reserpine, and Methyldopa on Reflex Venous and Arterial Constriction in Man*

Abstract

The effects of guanethidine, reserpine, and methyldopa on the venous and arteriolar beds of the forearm were studied in 17 normal subjects by a plethsmo-graphic method. During the control period profound reflex veno-constriction was elicited by immersion of the opposite hand in ice water and by leg exercise. These reflex venopressor responses to ice water and exercise were abolished or almost completely inhibited in 10 subjects who received 30 to 50 mg/day of guanethidine orally for 26 to 43 days, in 7 subjects given 0.5 mg/day of reserpine for 35 to 132 days, and in 5 subjects given 2.0 to 5.0 g/day of methyldopa for 21 to 49 days. In addition, these drugs also decreased reflex arteriolar constriction. When the drugs were discontinued, the reflex constriction of the venous and arteriolar beds to cold immersion and exercise returned. Therefore, these antihypertensive drugs, in the usual clinical doses, are evidently capable of blocking reflex veno-constriction. Venous return is likely decreased by this action, and the resultant reduction of cardiac output contributes to the hypotensive effects observed in patients undergoing treatment with these drugs.