The antihypertensive effect of prazosin on mild to moderate hypertension, changes in plasma volume, extracellular volume and glomerular filtration rate.

Changes in blood pressure, plasma volume (PV) (125I-albumin space), extracellular volume (ECV) (82Br-space) and glomerular filtration rate (GFR) (51Cr-EDTA clearance) were measured in 12 patients with mild to moderate essential hypertension on placebo and during long-term treatment with prazosin. During the study, BP decreased from an average of 172/107 to 166/102 mmHg (n.s.). PV increased from 3278 to 3324 ml (n.s.) and ECV from 18360 to 18639 ml (n.s.). GFR was almost unchanged, 95 and 93 ml/min, prespectively. An inverse significant correlation was found between the changes in mean BP and changes in ECV, i.e. fluid retention was demonstrated in patients with the smallest BP reduction. It is concluded that inadequate BP response during treatment with prazosin may in part be due to fluid retention. It is therefor suggested that prazosin should in principle be used together with a diuretic in order to prevent fluid retention.