Plasma LH Changes in Intact Adult, Castrated Adult and Pubertal Male Pigs Following Various Doses of Synthetic Luteinizing Hormone-Releasing Hormone (LH-RH)1

Abstract

The hypothesis that castration increases the sensitivity of the male pituitary to luteinizing hormone-releasing hormone (LH-RH) was tested. Five adult intact, 6 adult castrate and 4 pubertal intact male Gottingen Miniature Pigs were provided with chronic venous cannulae for injection and blood sampling. A factorial design was utilized where all individuals receivedall treatments. Adult animals received 0.05 μg, 0.5 μg, or 2.0 μg synthetic LH-RH⁄kg body wt. Control treatment was saline or sham injection. Pubertal boars received 0.2 μg and 2.0 μg synthetic LH-RH⁄kg body wt. Control treatment was saline injection. Blood samples weredrawn shortlybefore and at regular intervals up to 4 hr following LH-RH administration. Plasma LH content was estimated by radioimmunoassay. Resting plasma LH concentration (± S E) inthe castrated male was 3.0 ng (R 786-3)⁄ml ± 0.09, whereas, pubertal and adult intact animals demonstrated 0.6 ng⁄ ml ± 0.05 and 0.7 ng⁄ml ± 0.03,respectively. These resting values fluctuated with time. All groups treated with LH-RH showed significant (p < 0.025) elevations in plasma LH at the first (5 min) post-treatment sample. Peak values were noted between 5 and 15 min post-injection. The slope of the logarithmic dose-response curves for the adults were not different, therefore, the sensitivity hypothesis was rejected. The decay constant for plasma LH after releasing hormone administration was estimated to be 76 min, 29 min and 25 min for the adult intact, adult castrated and pubertal groups, respectively, and wasnot influenced by LH-RH dosage. It was concluded that the greater response of the castrate to LH-RH was due to factors other than alterations in pituitary sensitivity, perhaps pituitary content prior to treatment. It is proposed that the presence of the mature testis prolongs the biologic alf-life of LH-RH. (Endocrinology94: 330, 1974)