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Testicular Steroid Secretion in Response to GnRH-Mediated LH and FSH Release in Bulls¹

U. S. Meat Animal Research Center, U. S. Department of Agriculture², Clay Center, NE 68933

Abstract

Assay of frequently collected blood samples in four mature Hereford bulls indicated the existence of a tonic mechanism for the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH), i.e., their episodic release was not observed. Increased plasma concentrations of LH and FSH, however, were obtained in response to an intravenous injection of 500 µg of gonadotropin releasing hormone (GnRH). These gonadotropins showed similar secretory profiles after GnRH, but the relative magnitude of the LH response (30-fold) was considerably greater than that of the FSH response (sevenfold).

Concentrations of testosterone in jugular and spermatic vein blood were increased sevenfold after the administration of GnRH; whereas, concentrations of estrogen increased only twofold. Both steroids reached maximum concentrations 3 hr after GnRH (approximately 1 hr after the LH and FSH peak) then returned to preinjection levels. Estrogen profiles showed considerable between animal variation as compared to the testosterone secretory profiles. Concentrations of testosterone and estrogen in control animals were markedly higher in plasma from the spermatic vein (40 ng/ml and 14.4 pg/ml) than in plasma from the systemic circulation (1 ng/ml and 4.8 pg/ml).

It is concluded from these data that the bovine testis secretes both testosterone and estrogen and that their secretion is regulated to a certain extent by hypothalamic and hypophyseal hormones. The possibility that these steroids are secreted by separate compartments within the testes is discussed.

¹ The authors are indebted to Dr. K. Folkers, University of Texas at Austin, for synthetic gonadotropin releasing hormone; Dr. L. E. Reichert, Emory University, Atlanta, GA, for purified LH (LER-1065-C2); Dr. D. J. Bolt, USDA-ARS, Beltsville, MD, for LH antisera (DJB 3-12/11); Dr. K. W. Cheng, University of Manitoba, Winnipeg, for purified FSH and FSH antisera; and the Endocrine Study Section, NIH, Bethesda, MD, for bovine LH (NIH-LH-B8) and bovine FSH (NIH-FSH-B1) used in this study. The technical assistance of Ms. Marti Brown, Donna Taubenheim and Marilyn Bierman and cooperation of the Nebraska Agricultural Experiment Station, University of Nebraska, Lincoln, is acknowledged.

² Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U. S. Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.