Levels of LH, FSH and T in rams in response to two small doses of LH-RH

THERIOGENOLOGY LEVELS OF LH,FSH AND T IN RAMS IN RESPONSE To TWO SMALL DOSES OF LH-RH T. Stelmasiak, D.B. Galloway and W.J. Bremner* University of Melbourne, Department of Veterinary Clinical Sciences, Werribee, 3030. *M.R.C., Prince Henry's Hospital, Melbourne, 3004. In rams, single large doses or prolonged infusions of LH-RHincluce a biphasic pattern of LH release (1,2). It has been suggested that this pattern may be due to two pools of LH in the pituitary (2). To further test this possibility, the priming effect index (3) was used. The results from 11 Merino and Border Leicester rams were pooled as no breed differences were evident. Blood was collected via indwelling catheters from jugular veins, plasma harvested and stored at -2OOC until assayed for FSH (4) LH and T (5). Three blood samples were collected before giving the first i.v. injection of 1 ug LH-RH (Lutal, Hoechst A-G.) in 1 ml saline. Sampling continued for one hour, at which time a second identical injection of IX-RH was given. Thereafter samples were collected for a further two hours. The mean peak (2 S.E.) LH response (ng NIH-LH-SlE/ml) to the second injection of LH-RH (16.1 + 2,3) was significantly (P C 0.005) greater than the response to the first injection (6.8 + 2.0). When the first injection of LH-RH was replaced by saline, response to LH-RH given one hour later was no greater than that produced by a single LH-RH injection. Plasma FSH levels did not vary significantly from the basal value (34.2 + 4.1 ng NIH-FSH-SG/ml). T levels began to increase 30 min. after the first IH peak, reached maximal values 20 min. later, and then gradually declined. It is interesting that the second, much larger release of LH did not evoke a further increase in T. The "priming effect" phenomen demonstrated here using two small doses of LH-RH one hour apart, simulated the biphasic pattern of LH release in rams (2) and suggests that the frequency and amount of LH-RH discharge may be important in determining pituitary secretion of LA. It is postulated that the first dose of LH-RH discharges some of the acutely releasable LH (equivalent to the first pool in the biphasic type of response) and "primes" the pituitary gland to further treatment with LH-PH, probably by mobilising the second pool of LH. The lack of T increase in response to the second peak suggests a different biological quality of the second pool of LH or a refractory stage at the level of the testis. REFERENCES (1) Arimura, A., Debeljuk, L., Matsuo, H. and Shally, A.V. Release of luteinising hormone by synthetic LH releasing hormone in the ewe and ram. Proc. Exp. Biol. Med. 139: 831-854 (1972). -. (2) Bremner, W.J., Findlay, J-K., Gumming, I-A., Hudson, B. and De Kretser, D.M. Pituitary-testicular responses in rams to prolonged infusions of LH-RH. Biol. Reprod. 15: 141-146 (1976). (3) Stelmasiak, T. and Galloway, D.B. Giming effect of LH-PH in rams. J. Reprod. Fert. in press (1977). (4) Salamonsen, L-A., Jonas, H.A., Burger, H.G., Buckmaster, J.=M., Charnley,W.A., Cumming, I-A., Findlay, J.K. and Goding, J.R. A heterologous radioimmunoassay for FSH: application to measurement of FSH in the ovine estrous cycle and several other species including man. Endocrinology 93: 610-615 (1973). (5) 'Stelmasiak, T., Galloway7D.B. and Winfield, C. An application of priming effect index (PEI) to the evaluation of serving capacity in rams. Theriogenology

OCTOBER 1977 VOL. 8 NO. 4