CIRCADIAN-RHYTHM OF 1-ALPHA,25-DIHYDROXYVITAMIN D3 PRODUCTION IN EGG-LAYING HENS

The activity of renal 25-hydroxyvitamin D3(25-OH-D3)-1α- and 24-hydroxylase and the plasma concentrations of vitamin D metabolites were investigated in relation to the ovulatory cycle in egg-laying hens. The time after ovulation was estimated from the position of the egg in the oviduct and the dry weight of the egg-shell. The in vitro renal 25-OH-D3-1α-hydroxylase activity was significantly enhanced 14-16 hr after ovulation, whereas 25-OH-D3-24-hydroxylase activity remained unchanged. The plasma level of 1α,25-dihydroxyvitamin D [1α,25-(OH)2-D] was also increased 14-16 hr after ovulation in accord with the enhancement of the renal 1α-hydroxylase activity. The plasma level of 24,25-dihydroxyvitamin D did not change during the ovulatory cycle. These results strongly suggest that 1α,25-(OH)2-D3 production in the kidney varies in a circadian rhythm during the ovulatory cycle in egg-laying hens. © 1979.