Characterization of the Luteinizing Hormone Response to Continuous Infusions of Gonadotropin Releasing Hormone Using Perifused Pituitaries From Intact, Ovariectomized and Steroid-Treated Rats 1

Abstract

Anterior pituitary glands obtained from rats at various stages of the estrous cycle and from short-term ovariectomized (OVX) rats with or without physiological replacement of estradiol-17.beta. (E2) and/or progesterone (P), were perifused in-vitro with continuous infusions (4 h) of gonadotropin releasing hormone (GnRH) (12 ng/h). In all treatment groups the in-vitro pattern of luteinizing hormone (LH) release in response to GnRH was characterized by an initial low rate of LH release (initial phase; 20-70 min) followed by a significant augmented rate of secretion during the late phase (120-240 min) with the exception of the estrous and OVX + P groups in which LH was released at a constant rate. The total amount of LH released in response to GnRH was similar for glands removed on esturs, diestrus-I (D-I) and D-II, but increased significantly for glands removed on 0900 h proestrus with a further increase at 1400 h proestrus. Ovariectomy reduced the total LH released in vitro by 50-60% and 85-90% compared with estrous, D-I, D-II and proestrous groups, respectively. In-vivo treatment of OVX rats with E2 restored the in-vitro LH response to levels comparable to those in the 0900 h proestrous group while treatment with P + E2, further increased the rate of LH release in the initial phase to levels similar to those observed in the 1400 h proestrous group. In all treatment groups, addition of 5 .mu.M cycloheximide to the perifusion media significantly inhibited GnRH-stimulated LH release during the late phase without significantly altering the initial LH response except in the OVX + E2P group in which it was partially inhibited. Perifused pituitaries maintain their characteristic responsiveness to GnRH in vitro. LH secretion in response to continual GnRH stimulation involves protein synthesis-independent and -dependent components. E2, in vivo, enhances the magnitude of both components in response to GnRH in vitro, while E2 + P may further enhance the initial in-vitro response to GnRH through a protein synthesis-dependent mechanism.