Abstract
The in vivo biological activities of various luteinizing hormone releasing hormone (LHRH) analogues in immature 60-day-old cockerels were compared, based on their capacity to increase circulating luteinizing hormone (LH) concentrations. Intravenous injections of 50, 100, or 500 ng of [Gln 8]LHRH (cGnRH-I) and [His 5,Trp 7,Tyr 8]LHRH(cGnRH-II) induced significant increases in the plasma LH level. The maximum response was always observed 2 min after the injection. Mammalian LHRH(mGnRH) significantly increased plasma LH, but only at the 500 ng dosage level. [Ile 8]LHRH, [D-Ala 6]LHRH, [Ala 4]LHRH, [Leu 8]LHRH, and [Ile 5]LHRH increased plasma LH at 50–500 ng. [Ile 7]LHRH, [Phe 3]LHRH, and [Phe 2]LHRH did not significantly increase plasma LH. Relative in vivo biological activities, calculated from dose (30–100 ng)-response curves of mGnRH-I, cGnRH-I, cGnRH-II, [Ile 8]LHRH, [D-Ala 6]LHRH, [Ala 4]LHRH, [Leu 8]LHRH, [Ile 5]LHRH, [Ile 7]LHRH, [Phe 3]LHRH, and [Phe 2]LHRH were 1.00, 1.40, 1.81, 1.50, 4.59, 1.90, 1.28, 1.56, 0.57, 0.67, and 0.57, respectively. These results demonstrate that avian LHRHs have higher biological activities than mammalian LHRH in the chicken. Further-more, cGnRH-II is more potent than cGnRH-I, and [D-Ala 6]LHRH has a very high activity while [Ile 7]LHRH and [Phe 2]LHRH have very low activities.
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