The effect of upright tilt on nifedipine-induced natriuresis.

Calcium channel blockers are antihypertensive agents with diuretic actions. Yet edema occurs in some patients receiving long-term treatment with these drugs. As with other vasodilators, stimulation for fluid retention could result from systemic vasodilation. We speculated that the upright posture could enhance sodium retention. To test this hypothesis, we studied the effect of upright tilt in 10 patients before and after the oral administration of 20 mg nifedipine. Before nifedipine upright tilt caused a 41% drop in the sodium excretion rate, from 0.27 +/- 0.04 to 0.16 +/- 0.03 meq/min (p less than 0.05). Fractional sodium excretion decreased by 46%, from 2.4 +/- 0.5 to 1.3 +/- 0.3% (p less than 0.01). Urinary volume and renal plasma flow also decreased (p less than 0.05). Plasma renin activity (PRA) rose by 46% (p less than 0.005). With the patients in the supine posture nifedipine increased the sodium excretion rate to 0.49 +/- 0.09 meq/min (p less than 0.05). Fractional sodium excretion was 3.1 +/- 0.6 meq/min (p = 0.2). The natriuresis took place despite a fall in mean blood pressure and a significant rise in PRA (up 115% from prenifedipine supine values, p less than 0.005). Renal plasma flow also increased (p less than 0.01). The upright tilt caused a reversal of the nifedipine-induced natriuresis. The sodium excretion rate dropped to 0.23 +/- 0.05 meq/min and fractional sodium excretion to 1.3 +/- 0.2% (both not different from control). This drop in natriuresis occurred while mean blood pressure was at its lowest and PRA was 254% above the initial levels (p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)