Comparison of acute effects of 1.25- and 24.25-dihydroxy-vitamin D3 in normal subjects

Effects of small iv doses of 1.25-dihydroxy- and 24.25-dihydroxy-vitamin D3 (a microgram) were studied in 10 normal subjects. Injection of 1.25 (OH)2D3 was associated with small but significant increases in plasma calcium and phosphate but plasma levels of immunoreactive parathyroid hormone (iPTH) and calcitonin (iCT) did not change. The administration of 24.25 (OH)2D3 was associated with comparable decreases in plasma calcium and a small and transient decrease in plasma iPTH. Plasma levels of iCT did not change. 24.25 (OH)2D3 also significantly increased glomerular filtration rate and decreased the urinary excretion of noradrenaline, in contrast to 1.25 (OH)2D3 which had no effect on these variables. The rapid infusion of calcium significantly decreased levels of iPTH. We conclude that small doses of 1.25 (OH)2D3 and 24.25 (OH)2D3 have little, if any, direct effect on the secretion of PTH and CT in man.