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Neuroendocrine Control of Follicle-Stimulating Hormone (FSH) Secretion: III. Is There a Gonadotropin-Releasing Hormone-Independent Component of Episodic FSH Secretion in Ovariectomized and Luteal Phase Ewes?

Departments of Pediatrics (V.P.), Physiology (F.J.K.), and Biostatistics (M.B.B., D.T.M.) and the Reproductive Sciences Program (V.P., G.E.D., N.P.E., F.J.K., J.V.C.), The University of Michigan, Ann Arbor, Michigan 48109; and Departments of Physiology and Biophysics (J.D.N.), University of Alabama-Birmingham, Birmingham, Alabama 35294

Address all correspondence and requests for reprints to: Vasantha Padmanabhan, University of Michigan, Reproductive Sciences Program, 300 North Ingalls Building, Room 1109 SW, Ann Arbor, Michigan 48109-0404. E-mail: vasantha{at}umich.edu.

Our previous studies in ovariectomized ewes have provided direct evidence that FSH secretion is comprised of basal and episodic modes. In those studies, each GnRH pulse coincided with an FSH pulse, but additional FSH pulses were noted. To determine whether non-GnRH-associated pulses of FSH represent a GnRH-independent component of FSH secretion, we determined whether episodic FSH secretion persists after blockade of GnRH action with a GnRH antagonist. Hypophyseal portal and jugular blood was collected from five ovariectomized and six luteal phase ewes at 5-min intervals for 6 h before and 6 h after a single iv injection of Nal-Glu (10 µg/kg body weight). Hypophyseal portal LH and FSH and jugular patterns of FSH were compared with patterns of GnRH. Before Nal-Glu, in both models, there was a one-to-one concordance between GnRH and portal LH pulses, and each GnRH pulse was associated with a FSH pulse. However, additional non-GnRH-associated pulses of FSH were present. Nal-Glu administration eliminated LH but not FSH pulsatility. Nal-Glu inhibited interaction of GnRH I with GnRH type I receptor but not interaction of GnRH II with type II receptor. These studies provide the first direct evidence of the existence of an acute GnRH I-independent component of episodic FSH secretion.

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