Binding characteristics of gonadotropin-releasing hormone receptors throughout the estrous cycle of the hamster.

Abstract

The magnitude of LH secretion in response to exogenous gonadotropin-releasing hormone (GnRH) varies regularly through the stages of the estrous and menstrual cycles. To examine the postulate that cyclic changes in the binding characteristics of GnRH-specific receptor sites underlie the fluctuation in pituitary responsiveness, we determined the concentration and affinity of binding of GnRH receptors in anterior pituitary tissue of the hamster. The interaction of D-Ala6,des-Gly10-[125I]iodo-GnRH ethylamide (GnRH analog) with binding sites in hamster tissue was examined by Scatchard analysis. Anterior pituitary tissue from ovariectomized hamsters contained a single class of high affinity receptors specific for the GnRH analog [affinity constant (Ka) equals 3.47 ± 0.14 × 109 M-1; receptor concentration (R) equals 13.42 ± 0.40 fmol/mg fresh tissue equivalent (FTE)]. The association (k1) and dissociation (k-1) rate constants were 6.8 × 105 M-1 sec-1 and 5.5 × 10-4 sec-1, respectively. Specific binding of the radiolabeled GnRH analog to anterior pituitary tissue was blocked by coincubation with a 1000-fold excess of the unlabeled analog. Specific receptors for GnRH analog were not demonstrable in liver, lung, spleen, renal cortex, or cerebral cortex. Only GnRH and related peptides could compete with [125I]iodo-GnRH analog for high affinity binding sites in anterior pituitary tissue. Dopamine, TRH, ovine LH, and ovine PRL were ineffective in that regard. The relation between binding characteristics of pituitary receptors for the GnRH analog and stage of the estrous cycle was determined at hourly intervals during the afternoon of proestrus (P) and at less frequent intervals throughout the remainder of the cycle. At all time points examined, anterior pituitary tissue contained a single population of binding sites with uniform affinity (Ka equals 3.42 ± 0.10 × 109 M-1). The concentration of receptor approached a maximum near the onset of the critical period [1200 h on P (1200 P); R equals 15.45 fmol/mg FTE]. The receptor concentration declined rapidly during the afternoon of proestrus, coincident with the preovulatory gonadotropin surge, to a nadir at 1600 P (R equals 5.05 fmol/mg FTE). The receptor concentration was unchanged from 1600–1900 P, but a second significant decline was noted at 2200 P (R equals 3.45 fmol/mg FTE) immediately antecedent to the estrous release of FSH. A progressive increase in the receptor concentration was demonstrated during the afternoon of estrus (E; 1300 h on E, R equals 4.85 fmol/ mg FTE; 1900 E, R equals 10.30 fmol/mg FTE). The receptor concentration remained stable through diestrous I and diestrous II but increased to maximal levels by 0700 P (R equals 17.10 fmol/mg FTE). Receptor levels declined during the morning of proestrus (0900 P, R equals 9.90 fmol/mg FTE; 1100 P, R equals 10.31 fmol/mg FTE) and rose to approach maximal levels again by 1200 P (R equals 15.45 fmol/mg FTE). Anterior pituitary responsiveness is not precisely correlated with GnRH receptor concentration during all stages of the reproductive cycle. Therefore, although changes in the concentration of GnRH receptor may play a critical role in determining gonadotroph responsiveness, other cellular mechanisms must also contribute in the expression of this phenomenon.