The purpose of this study was to assess the concentrations of LH that Leydig cells are exposed to upon in vivo stimulation of steroidogenesis. The concentrations of LH were measured in rats in testicular interstitial extracellular fluid, seminiferous tubular fluid and blood plasma from testicular veins from one testis before and from the other testis of the same rats after an intravenous injection of gonadotrophin-releasing hormone (GnRH) or saline, and compared with the concentrations in blood plasma from a peripheral vein. The concentrations of LH in interstitial fluid surrounding the Leydig cells before the injections were about 10% of the levels in blood plasma, and showed no significant rise at 15 min and a much smaller rise at later times in rats injected with GnRH than those seen in blood plasma from either of the two sources, which were similar. The concentrations of LH in tubular fluid were even lower and showed no change after GnRH. Testosterone concentrations in testicular cells, interstitial fluid and testicular venous blood plasma were significantly increased by 15 min after GnRH, when compared with saline-injected controls, with no change in the levels in tubular fluid. The rise in testosterone concentrations in testicular venous plasma after GnRH was smaller than those in the cells and interstitial fluid. In conclusion, the concentrations of LH reaching the testicular interstitial fluid were only about one-tenth of that measured in the circulation, presumably because the endothelial cells restrict access of the hormone to the interstitial fluid. This indicated that either the Leydig cells are extremely sensitive to LH stimulation or that testicular endothelial cells modulate the action of LH on the Leydig cells.

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