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Clearance of Gonadotropin-Releasing Hormone in Beef Heifers after Intramuscular or Intravenous Administration¹

Colorado State University, Fort Collins 80523

Abstract

Ten heifers (170 to 200 kg) were administered 2.5 mg synthetic gonadotropin-releasing hormone (GnRH) intramuscularly (im) into the left semitendinosus muscle. Muscle biopsies from the injection site were taken from each of two heifers at 2, 8, 24, 48 and 72 hr after injection. Four heifers were administered 100 µCi (sp.act.1,600 µCi/µg)GnRH-^{1 2 5} I im and muscle biopsies were taken from each of two heifers at 2 hr and 8 hours. An additional four heifers received an im injection of saline and muscle biopsies were taken from each of two heifers at 2 hr and 8 hours. Four heifers were given intravenous (iv) injections of 2.5 mg gonadotropin-releasing hormone. Samples of jugular blood were collected at –30, –15, 0, 5, 10, 15, 30, 45, 60, 90, 120, 150, 180, 240, 300, 360, 420 and 480 min relative to the time of injection from all heifers. In addition, samples were collected at 4-hr intervals until 72 hr from those heifers in which biopsies were performed at 24, 48 and 72 hr after injection. Samples from all heifers receiving 2.5 mg GnRH or 5 ml saline were assayed for luteinizing hormone (LH), follicle-stimulating hormone (FSH) and gonadotropin-releasing hormone. Levels of GnRH were maximal at 5 min (186 ± 43 pg/ml) after iv injection and at 15 min (82 ± 37 pg/ml) after im administration. The GnRH-^{1 2 5}I also reached a maximal level in serum 15 min after the im injection. Levels of GnRH remained above baseline for 1.5 hr after iv administration compared to 4 to 8 hr after im administration. Concentrations of GnRH were elevated in muscle tissue obtained at 2 and 8 hr after injection but had decreased to baseline by 24 hours. Serum levels of LH and FSH were maximal at 2.5 hr after either iv or im injection of gonadotropin-releasing hormone. Increases of LH, FSH or GnRH were not observed in heifers administered saline.

¹ Supported in part by NIH grant HD-07841 and a contract from Parke-Davis Co., Ann Arbor, MI.