SUMMARY

calcium-binding protein (CaBP) and bone calcium mobilization in response to 1,24(R),25-(OH)3D3: Comparative effects of 1,25-(OH)2D3 and 24(R),25-(OH)2D3 in rats. Mol. Pharmacol. 17: 362-366 (1980). The response of intestinal calcium-binding protein (CaBP) synthesis and the rise in the serum calcium level to vitamin D metabolites were measured in vitamin D-deficient rats fed a low-calcium diet. Our results showed a linear relationship between the amount of intestinal CaBP and the logarithm of the dose for all the compounds. 1,24(R),25-(OH)3D3 was approximately four times less effective than 1,25-(OH)2D:3 in increasing the CaBP level and about eight times more effective than 24(R),25-(OH)2D3. The bone calcium mobilization response expressed as the rise in serum calcium was also dose related. The relative potencies of the metabolites in increasing the serum calcium level were approxi-mately the same as those elevating CaBP synthesis. Time course studies showed that 1,24(R),25-(OH)3D3 was as rapidly active as 1,25-(OH)2D3 in inducing intestinal CaBP synthesis and increasing serum calcium. The duration of its calcemic response was shorter and the magnitude less than that of 1,25-(OH)2D3. A longer time lag was necessary t