Interaction of the parathyroid and 1,25-dihydroxyvitamin D3 in the control of renal 25-hydroxyvitamin D3 metabolism.

Parathyroid extract and 1,25 dihydroxyvitamin D3 (1,25-(OH)2D3) were found to antagonize each other's action to regulate the kidney hydroxylation of 25-hydroxyvitamin D3. Parathyroid extract prompted an increase in serum 1,25-(OH)2D3 in thyroparathyroidectomized rats whereas the administration of exogenous 1,25 (OH)2D3 resulted in stimulated 24,25-dihydroxyvitamin D3 and suppressed 1,25-(OH)2D3 serum levels. The actions of parathyroid and 1,25-(OH)2D3 to modulate kidney 25-hydroxyvitamin D3 hydroxylase enzyme activities were not additive; rather, the hormone and metabolite acted to suppress each other's in vivo regulatory action. The specificity and rate of 25-hydroxyvitamin D3 hydroxylation was determined by the interdependent modulatory actions of parathyroid extract and 1,25-(OH)2D3. Such results suggest that the kidney 25-hydroxyvitamin D3-1- and 24-hydroxylase enzyme systems are regulated in response to the relative modulatory activities of parathyroid hormone and 1,25-(OH)2D3.