Decrease in serum immunoreactive parathyroid hormone in rats and in parathyroid hormone secretion in vitro by 1,25-dihydroxycholecalciferol

fects of 1,25-dihydroxycholecalciferol on serum im-munoreactive parathyroid hormone and on parathyroid hormone secretion in vitro. Rats injected i.p. with 1,25-dihydroxycholecalciferol, 130 pmol (2 U)/140 g body wt, which is probably a physiologic dose, had a sig-nificant 43 % decrease in serum immunoreactive para-thyroid hormone at 4 h. In addition, this dose of 1,25-dihydroxycholecalciferol inhibited the serum immuno-reactive parathyroid hormone response to hypocalcemia induced by phosphate injection. Because the increment in serum immunoreactive parathyroid hormone was less but the decrement in serum calcium more in phosphate plus 1,25-dihydroxycholecalciferol-treated than in phos-phate plus vehicle-treated rats, the impaired serum immunoreactive parathyroid hormone response to 1,25-dihydroxycholecalciferol could not be attributed to the change in serum calcium. In studies of parathyroid hor-mone secretion from bovine parathyroid tissue in vitro, the concentration of 1,25-dihydroxycholecalciferol used for most experiments was 1 nM, which is in the range found in rat serum. 1,25-Dihydroxycholecalciferol at 1 or 100 nM significantly inhibited parathyroid hor-mone secretion when medium calcium concentration was normal (1.5 mM), high (3.0 mM), and low (1.0 mM). Maximum inhibition ranged from 19 to 74%; Dr. Baylink is recipient of Research Career Development Award DE 19108. Received for publication 31 December 1974 and in revise