Evidence that calcitonin stimulates 1,25-dihydroxyvitamin D production and intestinal absorption of calcium in vivo

Although it is well established that parathyroid hormone and phosphate are important regulators of 1,25-dihydroxyvitamin D [1,25(OH)₂D] production, it remains unclear whether calcitonin affects vitamin D metabolism in vivo. Experiments were performed in the rat to determine the effect of chronic calcitonin infusion (0.2 U · h^-1) on plasma levels of vitamin D metabolites and on calcium metabolism. Thyroparathyroidectomized animals fed a calcium-replete or calcium-free diet were studied for as long as 2 wk before they were killed. In control rats, a calcium-free diet alone for 12 d resulted in an increase in 1,25(OH)₂D levels from 24 ± 5 to 139 ± 37 pg · ml^-1, P = 0.025. The infusion of calcitonin also stimulated 1,25(OH)₂D levels compared with controls on a regular diet (80 ± 17 vs. 38 ± 6 pg · ml^-1, P < 0.05) and on a calcium-free diet (460 ± 50 vs. 139 ± 37 pg · ml^-1, P < 0.001). In addition, calcitonin increased plasma calcium levels in animals on a regular diet by 50%; this effect was most likely due to increased intestinal absorption of calcium, because removal of calcium from the diet markedly blunted this effect. In contrast, calcitonin administration did not significantly affect 1,25(OH)D plasma levels. Collectively, these data suggest that calcitonin and calcium are independent regulators of 1,25(OH)₂D production and that calcitonin stimulates intestinal absorption of calcium, by increasing circulating levels of 1,25(OH)₂D.