Inhibitory Effects of Active Vitamin D Preparations on PTH Secretion in Rats

Abstract

To study the effects of various vitamin D preparations on PTH secretion, serum calcium and urinary excretion of cAMP were monitored in conscious perfused rats, and the influences of a bolus iv injection of the preparations on these parameters were examined. Three hours after the administration of 0.25 μg/kg (0.6 nmol/kg) of 1α, 24 (R)-dihydroxycholecalciferol [1α, 24 (OH) ₂D₃], the urinary excretion of cAMP decreased to a level compatible with that of parathyroidectomized (PTX) rats (50% of initial value; p<0.05) with no change in the concentration of serum calcium (total and ionized). In PTX rats supplemented with bovine PTH (1 U/h), the vitamin D preparation showed no significant effects either on the urinary excretion of cAMP or on serum calcium. These effects were rather specific for active vitamin D preparations, i. e. 1α, 25 (OH) ₂D₃ (0.25μg/kg) and 1αOHD₃ (1.25-6.25μg/kg). However, 24, 25 (OH) ₂D₃ (up to 25μg/kg) had no significant effect on these parameters. These results suggest that, in rats, active vitamin D preparations specifically inhibit PTH secretion without causing a significant increase in the serum calcium concentration, reflecting a direct feedback mechanism between active vitamin D metabolite and the parathyroid glands.