Suppression of PTH and decreased action on bone are partially responsible for the low calcemic activity of 22-oxacalcitriol relative to 1,25-(OH)2D3.

We previously showed that OCT, an analog of 1,25-(OH)2D3 with little calcemic activity, can decrease PTH mRNA levels in normal rats and inhibit PTH secretion in cultured bovine parathyroid cells with the same potency as 1,25-(OH)2D3 and that in normal rats fed a normal calcium diet, administration of OCT (500 ng) for 5 days did not increase plasma Ca. Thus, to determine if PTH suppression by OCT contributes to its lack of calcemic activity and to further characterize the effects of OCT on Ca metabolism, we performed several studies in parathyroidectomized (PTX) rats. PTX rats, maintained on a normal diet (0.9% Ca), received daily injections of vehicle, 1,25-(OH)2D3 (200 ng/day), or OCT (200 ng/day) for 6 days. Plasma Ca was measured daily. Plasma Ca in control rats stayed between 6.60 and 7.40 mg/dl, whereas Ca increased to 12.9 +/- 0.42 mg/dl in 1,25-(OH)2D3-treated rats and to 9.53 +/- 0.35 mg/dl in OCT-treated rats after 6 days. With a Ca-deficient diet, control rats maintained a plasma Ca between 4.25 and 4.60 mg/dl, but Ca increased to 13.7 +/- 0.24 mg/dl with 1,25-(OH)2D3 and to 7.29 +/- 0.17 mg/dl with OCT. Since the elevation in Ca by OCT was similar with both diets, OCT appears to act primarily on bone. PTX rats were infused with PTH (1.84 micrograms/kg/day) via an Alzet pump to achieve normal plasma Ca and then treated daily with either vehicle or OCT (200 ng/day). After 6 days, OCT increased serum Ca to 10.7 +/- 0.21 mg/dl over a control value of 8.58 +/- 0.29 mg/dl.(ABSTRACT TRUNCATED AT 250 WORDS)