1. In two separate studies the lithium clearance method was used to evaluate the influence of acute and long-term nifedipine treatment on renal tubular sodium reabsorption.

2. In the acute study, after a 4 week placebo period two doses of 20 mg of nifedipine decreased supine blood pressure from 155/101 (20.6/13.5) ± 11/4 (1.5/0.5) to 139/88 (18.5/11.7) ± 16/9 (2.1/1.2) mmHg (kPa) (means ± sd; P < 0.01). Lithium clearance, glomerular filtration rate and sodium clearance did not change. Therefore the calculated values of absolute proximal and absolute distal sodium reabsorption rates were also unchanged, as were potassium clearance, urine flow and body weight.

3. In the long-term study, lithium clearance, glomerular filtration rate, sodium clearance, potassium clearance, urine flow and body fluid volumes were measured after a 4 weeks placebo period and after 6 and 12 weeks of nifedipine treatment. As compared with placebo, mean supine blood pressure decreased significantly. The glomerular filtration rate did not change but lithium clearance fell by 30%. Consequently, the absolute and the fractional proximal sodium reabsorption increased significantly. The fractional distal sodium reabsorption did not change. Sodium clearance, fractional sodium excretion, potassium clearance, plasma volume and extracellular fluid volume were also unchanged.

4. In conclusion, we found no changes of renal tubular sodium reabsorption during acute nifedipine treatment, whereas long-term nifedipine treatment caused a redistribution of tubular sodium reabsorption without a change in overall sodium excretion or body fluid compartments.

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