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 administration of a GnRH analogue at different doses. Each dose of GnRH analogue induced an immediate rise in the plasma concentration of LH and then a rise in plasma testosterone concentration. Irrespective of the dose used, the rise in testosterone began 10 min after the GnRH injection. Administration of GnRH at doses of 0.01, 0.1, 1, 10 and 100 micrograms 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 micrograms l-1, respectively. These doses induced maximum concentrations of testosterone 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 concentrations of LH and testosterone varied from 35 to 55 min. The calculated maximum response of testosterone to LH, secreted by the anterior pituitary after GnRH injection, was 1.8 times higher than to GnRH. It was concluded that intravenous administration of GnRH induced marked and dose-dependent increases in plasma concentrations of LH and testosterone, and that there does not appear to be a direct effect of GnRH on Leydig cells in male dogs.