THE ROLE OF GONADAL-HORMONES IN GENE-EXPRESSION OF CALBINDIN (M(R)28,000) IN THE LAYING HEN

Acute and chronic changes in calbindin (M(r) 28,000) mRNA and calbindi n concentrations were determined to assess the roles of steroid hormon es in calbindin mRNA and calbindin synthesis in the eggshell gland (ES G). The results support an earlier suggestion that calbindin gene expr ession in the ESG is associated with Ca2+ flux through the ESG. The ev idence includes wide oscillation of the mRNA during the diurnal egg cy cle, in close temporal association with egg shell calcification. Proge sterone (single im injection of 1 mg/kg body weight, BW) prolonged the period of egg formation and reduced the rate of Ca2+ transport and th e concentration of calbindin mRNA in the ESG. Dexamethasone (single im injection of 5 mg/kg BW) prolonged the period of egg formation, incre ased shell Ca2+, and reduced calbindin mRNA in the ESG and intestine. Testosterone (single im injection of 2 mg/kg BW) did not affect calbin din mRNA synthesis in the ESG. The effects of estradiol on the synthes is of calbindin mRNA in the ESG of sexually immature or laying birds w ere minor, while it affected plasma Ca in the same birds. The antiestr ogen Tamoxifen (60 mg/kg BW, given orally) reduced plasma Ca, but did not affect the synthesis of calbindin mRNA in the ESG. The antiprogest erone RU-38486 (20 mg/kg BW, orally) increased shell Ca2+ but had no e ffect on plasma Ca or the synthesis of calbindin mRNA. It appears that estrogens alone cannot account for the markedly elevated synthesis of calbindin mRNA in the ESG of the laying bird. The hypothesis that the regulatory mechanism for the synthesis of calbindin mRNA in the ESG m ay involve a stimulator(s), associated with the onset of production, a nd an oscillating depressor(s) is supported and both appear to be clos ely linked to the reproductive cycle. The specific in vivo effect of p rogesterone on calbindin mRNA in the ESG, together with its already kn own changes during the ovulatory cycle in birds, supports the idea tha t it is a depressor. (C) 1996 Academic Press. Inc.