MECHANISM OF THE ALTERED BLOOD PRESSURE RESPONSIVENESS PRODUCED BY CHLOROTHIAZIDE *

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Abstract
Chlorothiazide lowered basal blood pressure by 15% in hypertensive patients but not in normo-tensive subjects. However, blood pressure responsiveness to levarterenol infusion was significantly reduced in normotensive individuals after chlorothiazide. The change seemed to be due in large measure to reduction in the plasma volume since replacement with the salt-free dextran resulted in a significant restoration of pressor responsiveness to levarterenol. It is suggested that basal blood pressure is lowered by chlorothiazide specifically in hypertensive patients because the reduction in plasma volume produced by the drug diminishes blood pressure responsiveness to the unknown pressor mechanisms operative in that disorder.