Circulating 1α,25-dihydroxyvitamin D in the chicken: Enhancement by injection of prolactin and during egg laying

In order to investigate possible modulation of vitamin D metabolism by prolactin, circulating 1α,25-dihydroxyvitamin D (1α,25-(OH)₂D) was measured by radioreceptor assay in chicks given injections of prolactin for five days. At a dose of 100 μg/day, the lactogenic hormone elicited a two-fold increase in plasma 1α,25-(OH)₂D. This effect may explain the known action of prolactin in producing hypercalcemia and could be physiologically important in birds. The laying hen represents a physiologic state in which calcuim absorption is known to be stimulated and prolactin has been reported to be elevated. Assay of serum 1α,25-(OH)₂D in the laying hen demonstrates a nine-fold enhancement over non-laying controls. Since this marked increase during egg laying is at least partially mimicked by injecting prolactin, a possible causative relationship between elevated prolactin and 1α,25-(OH)₂D is suggested.