Effect of dietary calcium stress on plasma vitamin D3 metabolites in the egg-laying Japanese quail.

Experiments were performed to ascertain whether circulating levels of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 varied in a cyclic manner during the 24-hr ovulatory period in Japanese quail hens and to assess the effects of dietary calcium restriction upon levels of the plasma vitamin D3 metabolites during that period. Birds fed a marginally-deficient calcium diet (1.7%) had significantly elevated plasma 1,25-dihydroxyvitamin D3 (.71 +/- .11 ng/ml, mean +/- SEM) compared with those fed high (3.5%) calcium (.35 +/- .04; P less than .01), although these values did not reveal any significant cyclic variation or correlation with known diurnal pulses in circulating 17 beta-estradiol. The magnitude of the plasma 1,25-dihydroxyvitamin D3 response to dietary calcium stress in some individuals was as great as 2.2 ng/ml, which are certainly among the highest values reported in any avian or mammalian species to date. These observations suggest that the total circulating level of plasma 1,25-dihydroxyvitamin D3 is not important in the direct cyclic regulation of laying hen calcium metabolism. The levels of free plasma hormone, target cell membrane permeability of the hormone, or regulation of cytosolic/nuclear 1,25-dihydroxyvitamin D3 receptor activity are potential aternate levels of the control of vitamin D3 mediated calcium metabolism in the quail hen.