24,25(OH)2D3 enhances the calcemic effect of 1,25(OH)2D3 in PTX rats

The effect of 24,25(OH)2D3 on 1,25(OH)2D3-induced hypercalcemia was studied in parathyroidectomized (PTX) rats for 10 days. Serum (S) and urinary Ca excretion (UCaV) were measured in (a) control rats, (b) rats receiving a daily sc injection of 54 ng 1,25(OH)2D3, (c) rats receiving 24,25(OH)2D3 in the same dose and same manner, and (d) rats receiving 1,25(OH)2D3 + 24,25(OH)2D3. Our results show that (i) 24,25(OH)2D3 alone does not increase SCa2+ in PTX rats, (ii) combined administration of 1,25(OH)2D3 + 24,25(OH)2D3 enhances the hypercalcemic response to 1,25(OH)2D3 without a parallel increase in UCaV, (iii) combined administration of 1,25(OH)2D3 + 24,25(OH)2D3 reduces the rise in urinary excretion of Ca2+ compared with that of rats receiving 1,25(OH)2D3 alone for 10 days, and (iv) these alterations are independent of parathyroid hormone.