Differential Gonadotropin Secretion: Blockade of Periovulatory LH but Not FSH Secretion by a Potent LHRH Antagonist

Abstract

The dependence of periovulatory gonadotropin secretion on LHRH was assessed with the use of a potent LHRH antagonist [ALHRH; (Nac-L-Ala1,p-Cl-D-Phe2,D-Trp3,6)LHRH]. Blood samples were collected hourly from 1400 h proestrus (P) through 0900 h estrus (E) from intact cycling female rats. ALHRH was administered at 0900 or 1300 h P before the proestrous increases in gonadotropins had commenced or at 2300 h P after the LH [luteinizing hormone] and primary FSH surges had occurred but preceding the secondary FSH surge. Antagonist given at 0900 or 1300 h P completely blocked the LH release with levels remaining undetectable in most animals (< 30 ng/ml) throughout the sampling period. However, administration of antagonist at these times failed to block completely the primary FSH surge although peak values were reduced when compared with controls, which displayed normal gonadotropin surges. In addition, ALHRH administered at 2300 h failed to alter the magnitude or other characteristics of the secondary FSH surge when compared with controls. The estrous surge of FSH in the rat apparently is independent of acute hypothalamic release of LHRH. Although the proestrous release of FSH is to a large extent LHRH dependent, some other mechanism may also contribute to this primary FSH surge.