Effects of vitamin D3 analogues on parathyroid hormone secretion and calcium metabolism in vitamin D-deficient rats

22-Oxa-1α, 25-dihydroxyvitamin D₃ (OCT) and 2β-(3-Hydroxypropoxy)-1α, 25-dihydroxyvitamin D₃ (ED-71) are novel synthetic vitamin D₃ analogues. In order to examine their calcemic actions on intestine and bone, we have investigated the effects of OCT and ED-71 on intestinal Ca transport, bone mobilization and plasma parathyroid hormone (PTH) level in vitamin D-deficient rats. The vitamin D-deficient rats were intravenously given either 6.25μg/kg or 0.2μg/kg of 1,25-D₃, OCT or ED-71 and theirplasma Ca levels and intestinal Ca transport were measured periodically. At a high dose, 1,25(OH)₂D₃ and ED-71 showed a strong biphasic stimulation of intestinal Ca transport and bone mobilization, and reduced the plasma PTH levels to the normal level completely. On the other hand, OCT failed to suppress the PTH secretion although it exerted first phase action on the both intestinal Ca transport and bone mobilization in vitamin D-deficient rats. The reason why OCT failed to suppress the PTH secretion even at a high dose, has not yet been clarified, but it may be at least in part due to its weak calcemic action and short half-life in plasma.