Effect of chlorothiazide on renal calcium and magnesium handling in the hamster

Recollection micropuncture and clearance studies were performed on 21 thyroparathyroidectomized hamsters to characterize the effect of chlorothiazide on tubular sodium, calcium, and magnesium transport. Acute administration of chlorothiazide resulted in a marked natriuresis while urinary calcium excretion fell and magnesium remained unchanged. The fraction of sodium, calcium, and magnesium remaining at the late proximal tubule increased modestly from 65 +/- 4 to 75 +/- 3, 68 +/- 3 to 75 +/- 4, and 78 +/- 4 to 85 +/- 2%, respectively. Distal tubular fluid to ultrafilterable plasma (TF/UF) sodium concentration rose from 0.24 +/- 0.03 to 0.44 +/- 0.04 whereas distal TF/UFCa concentration fell from 0.58 +/- 0.05 to 0.38 +/- 0.06. The fraction of sodium remaining at the distal tubule rose from 4.0 +/- 1.4 to 10.0 +/- 1.4% while that of calcium decreased from 10.2 +/- 1.1 to 7.6 +/- 1.2% following administration of chlorothiazide. No change was observed in distal delivery of magnesium. Thus chlorothiazide acted in the distal tubule to decrease sodium reabsorption, enhance calcium transport, and had little effect on distal magnesium reabsorption. These data are consistent with the distal tubular action of chlorothiazide which is independent of parathyroid hormone.