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

Parathyroid hormone (PTH) receptor bindings were examined in the membrane fraction of the calvaria and the kidney of the hen by the use of [125I]PTH-related protein (PTHrP) binding assays. The binding specificity, reversibility, and saturation of the receptor were demonstrated. The equilibrium dissociation constant (Kd) and the maximum binding capacity (Bmax) were obtained by Scatchard analyses. In both calvaria and kidney, Kd and Bmax values decreased at 3 h before oviposition in egg-laying hens, but not in nonlaying hens. Administration of 17 beta-estradiol or progesterone in vivo caused a decrease in the Kd and Bmax 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.