Acute actions of 1,25-dihydroxy-vitamin D3 in normal man: effect on calcium and parathyroid status

The present study was undertaken to evaluate the acute effect of 1,25-dihydroxy-vitamin D3 (1,25 (OH)2D3) on serum Ca, P and immunoreactive parathyroid (iPTH) and urinary Ca, P. and cyclic AMP. In 8 normal subjects, samples were collected over intervals of 30 to 60 min during a control day and on a treatment day following oral ingestion of 1,25(OH)2D3, 2.7 microgram. For the entire group there were no significant changes in serum Ca. P, iPTH or urinary P. Urinary Ca increased significantly 7 h after administration of 1,25(OH)2D3, and urinary cAMP decreased at 12 h. In 4 patients (group A). showing an increase in serum Ca by 0;2 to 0.4 mg/dl, serum iPTH decreased in 3, and the decrease in urinary cAMP appeared sooner. Among 4 patients showing no change in serum Ca after 1,25(HO)2D3 (group B), 3 showed an increase in iPTH. These data document the early onset of action of 1,25(OH)2D3 following its administration to normal man; increments in urinary Ca provide the most sensitive index of its action. The data provide no support for the view that 1,25(OH)2D3 exerts any direct inhibitory effect on the secretion of parathyroid hormone.