Disappearance of LHRH and D-Leu6-Des Gly NH210 LHRM Ethylamide in Ewes

Abstract

The in vivo disappearance of biological activity of luteinizing hormone-releasing hormone/follicle stimulating hormone-releasing hormone (LHRH) and a structural analog, D-Leu6-des Gly NH210 LHRH ethylamide (D-Leu6) were compared in intact ewes. Nine ewes were in the luteal phase of an estrus cycle when randomly divided into three equal treatment groups. Ewes were infused with either saline (5 ml/hr), LHRH (1 mg/5 ml/hr), or D-Leu6 (.1 mg/5 ml/hr) for 16.5 hours. Jugular blood was drawn every 30 min during infusion and for 6 hr thereafter to determine serum luteinizing hormone (LH). One milliliter of each blood sample collected following cessation of infusion was injected (iv, within 5 to 10 sec of collection) into an estrogen-progesterone treated ovariectomized (EPO) rat. The EPO rats were bled at 0, 15, 30, 60, 120, 240, 360 min after injection of sheep blood. LH released from each rat was used as an index of biologically active LHRH or D-Leu6 remaining in sheep blood at the time of collection. Mean serum LH concentrations in ewes treated with D-Leu6 or LHRH were not different (P>.05) indicating similar bioactivities of the two different doses of peptide used. Ewes had serum LH concentrations of less than 5 ng/ml at the time their blood was injected into the bioassay rats. Serum LH was not increased in blood of rats compared to pretreatment levels when injected with blood from sheep given saline (P>.05). Regression lines were fit to the means of the disappearance curves of each treatment. Analysis of regression coefficients showed the LH-releasing activity of LHRH and the D-Leu6 treated ewes decreased at a similar rate (P>.05). Since the two treatments were not different, they were pooled and a half-life of 185 min or 3.07 hr was calculated. Biological activity of LHRH and D-Leu6 was not detectable after 4.0 hr post-infusion. These data indicate that the greater biological potency of the analog D-Leu6 relative to synthetic LHRH is not attributable to a longer circulating time in the blood.