Pituitary and testicular responses in sexually mature bulls after intravenous injections of graded doses of LH-releasing hormone.

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The capacity of the anterior pituitary gland and testes in mature bulls (705 +/- 9 (S.E.M.) kg body wt, n = 4) to respond to graded doses of LH-releasing hormone (LHRH) was assessed relative to endogenous profiles of LH and testosterone secretion. Endogenous hormone profiles were determined by bleeding bulls at 20-min intervals for 12 h. Responses to LHRH were assessed on successive days after single intravenous injections of 1, 5, 10, 50 or 100 ng LHRH/kg body wt. Blood samples were taken at -40, -20, 0, 10, 20, 30, 40, 60 and 120 min relative to LHRH injection. During a 12-h bleed bulls showed spontaneous pulses of LH and testosterone which had peak amplitudes of 2.6 +/- 0.5 micrograms/l and 44.5 +/- 7.1 nmol/l respectively. Respective peak LH (micrograms/l) and testosterone (nmol/l) responses to LHRH were as follows: 1 ng LHRH (3.0 +/- 0.7; 47.3 +/- 4.1); 5 ng LHRH (8.0 +/- 1.2; 52.8 +/- 6.2); 10 ng LHRH (11.1 +/- 2.3; 57.7 +/- 9.1); 50 ng LHRH (19.2 +/- 2.8; 47.9 +/- 8.6); 100 ng LHRH (19.1 +/- 4.7; 43.9 +/- 6.4). A dose of 1 ng LHRH/kg produced LH and testosterone responses which were comparable in amplitude to spontaneous peaks in the respective hormone. There was a linear (y = 0.28 X + 5.72; r = 0.81) increase in the LH response to doses of LHRH between 1 and 50 ng/kg; corresponding testosterone responses showed no relationship with the dose of LHRH. The capacity of the anterior pituitary gland to release amounts of LH eight to ten times in excess of those secreted during spontaneous peaks suggests that (1) there exists a large releasable store of LH in the anterior pituitary gland and (2) hypothalamic LHRH is a limiting factor in gonadotrophin secretion. In contrast to LH release, the androgenic response of the testes to acute gonadotrophic stimulation is determined largely by prevailing steroidogenic activity.