STUDY ON REGULATING MECHANISM FOR CLACIUM METABOLISM OF BONE CELLS VIA PARATHYROID HORMONE RECEPTORS

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Laying White Leghorn hens, 18 to 20 months of age, were killed at nine different times, and nonlaying hens were sacrificed at the corresponding hours. Additionally, 17beta-estradiol, progesterone, 5alpha-dihydrotestosterone were intramuscularly administrated into nonlaying hens. These hens were killed at 1,2, and 4 hours after administration.Plasma membrane fractions were prepared from the periosteum of the calvaria and kidney. Samples were incubated at 4ﾟC for 1 h with various concentrations of chicken [^<125>I] PTHrP in the absence or presence of 1 muM unlabeledl PTHrP in a total volume of 300 mul. Free and bound ligands were separated by centrifugation, and radioactivity was determined by a gamma-counter. Specific binding was obtained by subtracting the nonospecific binding from the total binding. The equilibrium dissociation constant (K_d) and the maximum binding capacity (B_<max>) were determined by Scatchard's method.The K_d value of PTH receptor of the calvaria and bone in the laying hens showed decreases at 3 h before and the time of oviposition. But, no changes in K_d values were observed in the nonlaying hens. The time of the decrease in K_d value accords with the time of increased secretion of the ovarian steroid hormones. In vivo administration of 17beta-estradiol or progesterone induced a decrease in K_d value. The decrease in K_d value is suggested to be caused by either or both 17beta-estradiol and progesterone. Decrease of B_<max> values in the calvarial and renal PTH receptors were noted 6 h before oviposition in the laying hen, and the decrease preceded the changes in K_d values. Plasma PTH level is elevated between 15 and 9 h before oviposition. The smaller B_<max> values PTH receptors, therefore, may be consequence from the increase of the binding PTH to the receptors in vivo by an increase of the affinity.