Absence of direct effects of GnRH on testicular steroid secretion in the ram

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Abstract
Effects of GnRH, administered via the testicular artery, on testicular steroidogenesis were studied in rams during the non-breeding season. Concentrations of testosterone and 17-hydroxyprogesterone in testicular venous blood showed similar profiles which were identical for GnRH-treated (0.5 ng infused over 60 min or 25 ng injected) and control testes. Increases of testicular venous concentrations of both hormones were only marginally reflected in peripheral venous concentrations. Peripheral administration of cCG (200 i.u., i.v.) stimulated testosterone secretion to a larger extent than 17-hydroxyprogesterone secretion in 10/11 rams, GnRH-treated and controls testes showing identical responses. Higher testicular venous concentrations of both hormones after administration of GnRH were paralleled by increased concentrations of endogenous LH. These LH peaks were evoked by 25 ng GnRH in 7/8 rams. The observed effects of GnRH treatment on testicular steroid secretion thus cannot be considered to be the result of direct stimulation of steroidogenesis by GnRH.