Enterocyte expression of calbindin, calbindin mRNA and calcium transport increases in jejunal tissue during onset of egg production in the fowl (Gallus domesticus).

1. Quantitative measurements of calbindin mRNA, calbindin protein and calcium uptake have been made in sectioned intestinal villi to determine the location and cellular characteristics of their expression in immature, point of lay and laying chickens. 2. Trace amounts of calbindin mRNA were detected by in situ hybridization in enterocytes located around the crypt-villus junction in jejunal tissue taken from immature and point of lay chickens. Large amounts of calbindin mRNA were detected in upper crypt and all villus enterocytes in tissue taken from laying chickens. Maximal levels of calbindin mRNA occurred in the basal third of the villus in laying chickens. 3. No calbindin was detected immunocytochemically in tissue taken from immature and point of lay chickens. Large amounts of calbindin were expressed in tissue taken from laying chickens. Maximal expression of calbindin in this case occurred in villus tip enterocytes. 4. Rapid uptake of calcium by tissue taken from laying chickens was twice that found in immature and point of lay birds. Calcium uptake in tissue taken from laying hens was also shown by quantitative autoradiography to take place maximally in villus tip enterocytes. 5. Regulation of calbindin gene expression and the cellular characteristics of calcium transport in laying chickens are discussed in terms of an adaptive response taking place in birds undergoing a daily loss of egg shell calcium.