24,25(OH) 2 D 3 Enhances the Calcemic Effect of 1,25(OH) 2 D 3 in PTX Rats

Abstract

The effect of 24,25(OH)₂D₃ on l,25(OH)₂D₃-induced hypercalcemia was studied in parathyroidectomized (PTX) rats for 10 days. Serum (S) and urinary Ca excretion (U_CaV) 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 S_ca²⁺ 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 U_caV, (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.