Blockade of the renal tubular effects of vitamin D by cycloheximide in the rat

To further characterize the mechanisms by which 25(OH)vitamin D₃ (25(OH)D₃) and 1.25(OH)₂ vitamin D₃ (1,25(OH)₂D₃) suppress the phosphaturic action of parathyroid hormone (PTH) we have studied the effects of cycloheximide (cyclohex), a protein synthesis inhibitor, on the interaction between PTH and vitamin D metabolites in parathyroidectomized (PTX) rats, both in vivo and in vitro experiments. In clearance studies PTX PTH-infused rats were pretreated with cyclohex 2 h before the administration of vitamin D. In control, PTX PTH-infused rats not pretreated with cyclohex, the administration of 25(OH)D₃ and 1,25(OH)₂D₃ was associated with a fall in fractional excretion of phosphate (CP/CIN) from 0.30±0.05 to 0.16±0.02 and from 0.31±0.05 to 0.13±0.01 (P<0.005) respectively. Cyclohex-pretreated PTX PTH-infused rats failed to respond to both 25(OH)D₃ and 1,25(OH)₂D₃, and CP/CIN, which rose after PTH, remained 0.32±0.05 and 0.29±0.03 respectively. In vitro, both 25(OH)D₃ and 1,25(OH)₂D₃ inhibited the PTH-induced activation of adenylate cyclase in the renal isolated membrane fractions. Pretreatment with cyclohex abolished this effect of vitamin D metabolites. These results show that cyclohex blocks the antiphosphaturic effects of both 25(OH)D₃ and 1,25(OH)₂D₃ but does not alter the response to PTH. These findings are consistent with the possibility that the acute renal action of vitamin D depends on de novo synthesis of protein.An abstract of this work appeared in Clinical Research, 28 (2) A 387, 1980.