Gonadotropin Releasing Hormone-Induced Change in Serum Luteinizing Hormone, Testosterone and Androstenedione in Bulls, Steers and Steers Given Testosterone

Abstract

Two experiments were conducted to determine the effect of gonadal hormones on LH release by successive injections of Gonadotropin Releasing Hormone (GnRH). In the first experiment 1 month-old Holstein bulls were used. Bulls were assigned to be: 1) left intact (n=5), 2) castrated (n=6), or 3) castrated and given 10 mg testosterone thrice daily for 15 days (n=4). All animals received three intramuscular injections of 20 µg GnRH administered at 12-hr intervals beginning 14 days post-castration. Average magnitude of LH release by GnRH, measured as area under the response curve was greatest (P<.01) in steers and least (P<.01) in steers given testosterone. Magnitude of LH release in response to the first GnRH challenge was greater (P<.01) than that of the second or third. Serum androstenedione was increased (P<.05) after GnRH treatment. These data indicate that testicular hormones affect the ability of the pituitary of bulls to respond to GnRH as early as 1 month of age. A second experiment utilized pubertal bulls and was conducted in two parts. In the first part, four bulls were left intact, four were castrated and four were castrated then given testosterone (20 mg) thrice daily from the day of castration (day 0) through day twenty-one. On day 21, each animal was given an intramuscular injection of 40 µg GnRH at 0900, 1300 and 1700 hour. Magnitude of increase in serum LH concentration averaged overall injections was less (P<.05) in steers given testosterone than in untreated steers or bulls. Similar to results with prepubertal bulls, the increase in serum LH that occurred after the first GnRH injection was greater (P<.001) than that after the second or third GnRH injection. These same animals were used in the second part of this experiment. Treatment of the two groups of steers was reversed from days 22 to 28. On day 28 each animal was given an intramuscular injection of 40 µg GnRH at 0900, 1300 and 1700 hour. Magnitude of increase in serum LH was greater (P<.01) in steers given testosterone from days 21 to 28 than in intact bulls and least (P<.01) in steers that had been given testosterone for 21 days post-castration. Following the first GnRH injection, magnitude of LH release was greater (P<.01) than the comparable average after the second or third injections. In pubertal bulls, serum testosterone and androstenedione was increased (P<.01) after each GnRH injection and the increases were quantitatively similar on day 21 and 28. These results suggest that a series of GnRH injections may be more useful than single injection in evaluating the capabilities of the pituitary to release luteinizing hormone. In addition, the degree to which the pituitary recovers its ability to respond to a second GnRH injection may be related to rate of LH biosynthesis. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.