Changes in parathyroid hormone receptor binding affinity during egg laying: implications for calcium homeostasis in chicken.

Parathyroid hormone (PTH) receptor bindings were examined in the membrane fraction of the calvaria and the kidney of the hen by the use of [ 125 I]PTH-related protein (PTHrP) binding assays. The binding specificity, reversibility, and saturation of the receptor were demonstrated. The equilibrium dissociation constant (K d ) and the maximum binding capacity (B max ) were obtained by Scatchard analyses. In both calvaria and kidney, K d and B max values decreased at 3 h before oviposition in egg-laying hens, but not in nonlaying hens. Administration of 17β-estradiol or progesterone in vivo caused a decrease in the K d and B max values. Ionized calcium concentrations in the blood plasma showed a decrease at 13 h before oviposition. The results suggest that the PTH receptor binding in the calvaria and the kidney is affected by ovarian steroid hormones and may play a role in maintaining the calcium homeostasis in the egg-laying hen.