The interaction between dietary calcium and gonadal hormones in their effect on plasma calcium, bone, 25-hydroxycholecalciferol-1-hydroxylase, and duodenal calcium-binding protein, measured by a radioimmunoassay in chicks.

Male chicks, fed low, normal, or high calcium- and cholecalciferol-containing diets for 14 days, were given three combined injections of 17 beta-estradiol and testosterone (7 and 2.4 mg/kg/dose, respectively) or the vehicle alone, at 3-day intervals. The hormonal treatment resulted in increased plasma calcium and medullary bone calcium concentrations, independently of the dietary calcium intake. Kidney 25-hydroxycholecalciferol-1-hydroxylase and duodenal calcium-binding protein were increased in response to gonadal hormones. The magnitude of this response markedly diminished with increased calcium intake and almost completely disappeared in chicks fed the high calcium diet. The results suggest that the increases in plasma calcium and medullary bone formation due to gonadal hormones are independent of calcium intake while the effect of hormones on duodenal calcium-binding protein and the 25-hydroxycholecalciferol-1-hydroxylase activity appears to be mediated through the change in calcium needs due to medullary bone formation.