GNRH IN THE MALE DOG - DOSE-RESPONSE RELATIONSHIPS WITH LH AND TESTOSTERONE

Dose-response relationships between GnRH and LH, and between GnRH and testosterone, were investigated in six male dogs by intravenous admini stration of a GnRH analogue at different doses. Each dose of GnRH anal ogue induced an immediate rise in the plasma concentration of LH and t hen a rise in plasma testosterone concentration. Irrespective of the d ose used, the rise in testosterone began 10 min after the GnRH injecti on. Administration of GnRH at doses of 0.01, 0.1, 1, 10 and 100 mug kg -1 resulted in maximum LH concentrations in plasma (mean +/- SEM; n = 6) of 22 +/- 7, 27 +/- 6, 40 +/- 7, 57 +/- 13 and 56 +/- 10 mug l-1, r espectively. These doses induced maximum concentrations of testosteron e in plasma (mean +/- SEM; n = 6) of 16 +/- 4, 20 +/- 4, 22 +/- 3, 22 +/- 4 and 24 +/- 7 nmol l-1, respectively. The lag time between peak c oncentrations of LH and testosterone varied from 35 to 55 min. The cal culated maximum response of testosterone to LH, secreted by the anteri or pituitary after GnRH injection, was 1.8 times higher than to GnRH. It was concluded that intravenous administration of GnRH induced marke d and dose-dependent increases in plasma concentrations of LH and test osterone, and that there does not appear to be a direct effect of GnRH on Leydig cells in male dogs.