Uterine calcium-binding protein activity of nonlaying hens and hens laying hard-shelled or shell-less eggs

Uterine calcium-binding protein (CaBP) activity was compared at oviposition and 14 h postoviposition in nonlaying (NL) hens and hens laying a high incidence of shell-less (SL) or hard-shelled (HS) eggs fed a commercial laying diet. Epithelial cells were removed from uteri of NL, SL, and HS hens and homogenized in Tris buffer, pH 7.4. Proteins in homogenate were fractionated by the addition of ammonium sulfate and centrifugation, separated by Sephadex chromatography, and measured by ultraviolet spectrophotometry at A280. Calcium-binding protein activity was determined using a 45Ca-chelex binding assay. Two peaks of CaBP activity were detected and designated CaBP-a (28,000 Da) and CaBP-b (16,000 Da). Uterine CaBP-b may differ from CaBP-a only in Ca content. The activities of CaBP (a and b) were higher (P less than or equal to .05) in HS and SL hens than in NL hens at 0 h. There was no significant difference (0 h) between CaBP activity (a versus b) of either HS or SL hens. There was also no difference (0 h) of CaBP-a or CaBP-b activity when SL and HS hens were compared. Uterine CaBP-a activity was lower (P less than or equal to .05) in SL hens than HS hens at 14 h. Under normal dietary conditions, the level of CaBP activity at 14 h may be associated with SL egg production.