The effect of enalapril on the renal response to tilting in humans.

1. Seventeen normal subjects were tilted on a tilt table and changes in blood pressure, heart rate and renal function were monitored in the absence of drug treatment; the same measurements were repeated during treatment with enalapril in eight of these subjects. 2. In the absence of drug, tilting of the whole body to the 45 degree head up position caused an antidiuresis, antinatriuresis and antikaliuresis; the antinatriuresis resulted from a transient fall in glomerular filtration and from a sustained increase in the tubular reabsorption of sodium, as reflected by a prolonged fall in fractional sodium excretion. 3. Enalapril caused a reduction in blood pressure in the upright position; there was no effect on heart rate. The antinatriuresis of tilt was significantly blunted by enalapril. This resulted from blunting of the transient fall in creatinine clearance and of the increase in tubular sodium reabsorption. Urinary potassium excretion during tilting was not altered by enalapril. 4. These findings are consistent with a direct intrarenal action of angiotensin II, mediating some of the alterations in renal function apparent on assumption of the upright position.