Because in vitro adenylate cyclase activity studies suggest that parathyroid hormone (PTH) and calcitonin (CT) increase renal tubular calcium and magnesium reabsorption by stimulating the same transport mechanism, the separate and combined effects of these hormones on calcium and magnesium transport was assessed in the rat. 2. Fractional excretion of calcium was 9.5 +/- 0.5% in calcium-infused thyroparathyroidectomized (TPTX) rats and was reduced to 5.1 +/- 0.6 and 4.1 +/- 0.6% by maximal hypocalciuric concentrations of PTH and CT, respectively. 3. Combined administration of maximal PTH and CT produced an effect on fractional and absolute calcium excretion similar to that recorded with maximal CT alone. The combined administration of half-maximal concentrations of PTH and CT also produced a comparable fall in the fractional excretion of calcium to 3.0 +/- 0.7%, which was similar to that observed in the 'maximal CT' and 'maximal PTH plus CT' groups. The magnesium reabsorption data were comparable. 4. These results support biochemical data suggesting that PTH and CT act upon the same transport site, presumably within the thick ascending limb of Henle's loop, to facilitate calcium and magnesium reabsorption.
