Renal adenylate cyclase and the interrelationship between parathyroid hormone and vitamin D in the regulation of urinary phosphate and adenosine cyclic 3',5'-monophosphate excretion

Abstract

This study examined the role of cyclic AMP in the phosphaturic response to parathyroid hormone in vitamin D-deficient rats. Infusion of purified bovine parathyroid hormone (13.3 mug/h) into control, D-fed, or D-deficient, thyroparathyroidectomized rats produced a sixfold increase in renal phosphate and cyclic AMP excretion in D-fed rats, but only a two- to threefold increase in both parameters in D-deficient animals. Intravenous injection of parathyroid hormone over the dosage range from 1-50 mug/kg resulted in a dose-dependent increase in phosphate and cyclic AMP excretion with both D-fed and D-deficient thyroparathyroidectomized rats. However, the D-deficient rats responded to these injections of parathyroid hormone with a two- to threefold increase in both renal phosphate and cyclic AMP excretion at the highest dose of 50 mug/kg, whereas the D-fed animals' response was 35-fold and 11-fold over control excretion levels of phosphate and cyclic AMP, respectively. To directly examine the role of the renal cortical adenylate cyclase system in the blunted phosphaturic and urinary cyclic AMP responses to parathyroid hormone in D-deficient rats, we prepared a plasma membrane fraction enriched in this enzyme activity from the renal cortex of D-fed and D-deficient thyroparathyroidectomized rats. The renal cortical adenylate cyclase of D-deficient rats showed significantly (P less than 0.001) less activation by parathyroid hormone over the hormone concentration range from 0.3 to 7.0 mug/ml than was observed with the enzyme prepared from D-fed animals. Basal adenylate cyclase activity and the fluoride-stimulated enzyme activity were not altered by the state of D-deficiency. These experiments demonstrate that the blunted phosphaturic response to parathyroid hormone observed in D-deficient rats is associated with the reduced responsiveness of the renal cortical adenylate cyclase to the hormone. Moreover, the defect in the renal membrane adenylate cyclase system appears to be localized at the level of PTH binding to membrane receptors or, alternatively, at the level of transmission of the hormone-receptor binding signal to the catalytic moiety of this membrane enzyme.