Pituitary involvement in renal adaptation to phosphate deprivation

The present study was undertaken to examine 1) the effect of phosphate restriction on growth hormone (GH) secretory dynamics in freely moving, chronically cannulated rats and 2) the effect of hypophysectomy on the renal adaptive responses to phosphate deprivation. Phosphate restriction led to an increase in renal brush-border membrane Na+-dependent phosphate transport (2,511 +/- 283 vs. 1,006 +/- 122 pmol.mg protein-1.15 s-1, P less than 0.001) and in the plasma concentration of 1 alpha,25-dihydroxyvitamin D [1,25(OH)2D] (127 +/- 10 vs. 63 +/- 4 pg/ml, P less than 0.001). In contrast, phosphate deprivation had no effect on either amplitude or frequency of spontaneous GH secretory bursts and did not alter pituitary GH concentration. Hypophysectomy led to a decrease in brush-border membrane Na+-dependent phosphate transport (669 +/- 78 vs. 1,006 +/- 122 pmol.mg protein-1. 15 s-1, P less than 0.003) and to a fall in plasma 1,25(OH)2D (42 +/- 9 vs. 63 +/- 4 pg/ml, P less than 0.02). Phosphate restriction of hypophysectomized rats elicited a twofold increase in Na+-dependent phosphate transport (1,312 +/- 106 vs. 669 +/- 78 pmol.mg protein-1.15 s-1, P less than 0.001) but no rise in plasma 1,25(OH)2D. We conclude that the renal adaptive responses to phosphate deprivation are not mediated by specific alterations in pulsatile GH secretion. Moreover, we demonstrate that the adaptive increase in brush-border membrane phosphate transport occurs after hypophysectomy, is not dependent on increased vitamin D hormone production, and is most likely subject to a different regulatory mechanism.