Evidence for a parathyroid hormone-independent calcium modulation of phosphate transport along the nephron.

Abstract

A B S T R A C T To disclose a parathyroid-independent calcium modulation of phosphate transport along the nephron, the effect of increasing plasma calcium concentration to subnormal levels in rats 6 days after parathyroidectomy (chronic PTX) was studied. Fractional phosphate reabsorption was significantly increased. The whole kidney response to calcium infusion was similar whether or not the thyroid gland was removed, which suggests that calcitonin is not involved. The micropuncture study indicated an increase in the reabsorptive capacity for phosphate (absolute reabsorption/absolute delivered phosphate per nephron segment) in the proximal tubule, the loop, and the terminal nephron when calcium was infused. Thus, the level of plasma calcium or some related factor affects the phosphate transport by the tubule independently of parathyroid hormone.With calcium infusion, the profile of phosphate reabsorption along the nephron became close to that of acutely parathyroidectomized rats, but with persisting differences. The level of plasma calcium concentration may partly account for the differences between the acute and the chronic steps of parathyroidectomy.The role of possible interferences between alterations of extracellular calcium concentration or some related factor and the adenylate cyclase-cyclic AMP system in such an action of calcium was evaluated. Cyclic AMP was infused so as to achieve a 10 M plasma concentration. Combined infusions of calcium and cyclic AMP were also performed. The results are compatible with calcium inhibition of adenylate cyclase, although they do not rule out a direct action of calcium.

Mechanisms whereby extracellular adenosine 3',5'-monophosphate inhibits phosphate transport in cultured opossum kidney cells and in rat kidney. Physiological implication.

“…Experiments were performed on 24 male SpragueDawley rats, of average weight 198±8 g. Feeding, anesthesia, infusion (saline, [3H ] inulin, 32p), surgical preparation and clearances were as previously described (9). Seven groups of rats were studied.…”

“…Interestingly, other renal effects of PTH, such as those exerted on the loop of Henle or on the distal tubule, were not reproduced by cAMP infusion (8,9). Moreover, infused cAMP did not mimick the renal effects of other peptidic hormones, such as antidiuretic hormone (8,9).…”

“…The question of whether luminal cyclic AMP per se may influence Pi reabsorption has not been adequately addressed. However, it has been clearly established that the phosphaturic effect of PTH could be mimicked, in vivo, by systemic infusion of cAMP to APTX animals (8,9) when plasma concentration of cAMP was increased to the micromolar range. Interestingly, other renal effects of PTH, such as those exerted on the loop of Henle or on the distal tubule, were not reproduced by cAMP infusion (8,9).…”

“…However, it has been clearly established that the phosphaturic effect of PTH could be mimicked, in vivo, by systemic infusion of cAMP to APTX animals (8,9) when plasma concentration of cAMP was increased to the micromolar range. Interestingly, other renal effects of PTH, such as those exerted on the loop of Henle or on the distal tubule, were not reproduced by cAMP infusion (8,9). Moreover, infused cAMP did not mimick the renal effects of other peptidic hormones, such as antidiuretic hormone (8,9).…”

Abnormal sulfate metabolism in vitamin D-deficient rats.

“…[ H]inulin were performed as previously described (29,30 (43 Ci/mg; ICN Biomedicals) was suspended in 0.15 M NaCl and administered by intravenous infusion at a rate of 50 l/min per 100 g body wt until a total dose of 100 Ci/200 g body wt had been administered. Inulin and sulfate clearances were measured as previously reported (29,30).…”