Trafficking between the testicular compartments

Steroids such as pregnenolone, progesterone and testosterone not only rapidly pass the Leydig cell membranes but they can also equilibrate rapidly between different testicular compartments (van Doorn etal. 1974). The secretion pattern in the testis is thus most likely determined by amounts that are produced inside the tissue, the permeability characteristics of the membranes and the binding proteins in various testicular fluids. The Leydig cells in the testis are surrounded by an interstitial fluid that is rich in plasma proteins and the cells are also in close contact with blood vessels. The preferential direction of secretion in the testis is mainly determined by the concentration gradient and flow rates ofthe various fluids. Since theblood flow is much higher than the flow ofthe interstitial fluid, most ofthe unconjugated steroids diffuse from the interstitial space to the blood and leave the testis via the venous blood (Maddocksand Sharpe 1989). The porcine testis is an exception that supports this view. In this species steroid sulphates, which cannot readily diffuse through the walls surrounding the interstitial space, were 13 to 35 fold more concentrated in interstitial fluid than in venous blood (Setchell et al. 1983). During the passage of venous blood through the pampiniform plexus the primary venous blood is diluted approximately 2 fold by incoming arterial blood. This occurs through anastomoses present in this network of interacting blood vessels (Noordhuizen-Stassen et al. 1985). The presence of relatively high levels of dihydrotestosterone in human spermatic venous blood has been taken as evidence that dihydrotestosterone is produced in the testis (Hammond et al. 1977). However, other studies have shown virtually no 5a-reductase in human testis (Miautani et al. 1977). Although these data are contradictory, dihydrotestosterone is most likely an epididymal steroid and it is conceivable that dihydrotestosterone produced in the epididymis is transported to testicular venous blood during the dilution process that occurs in the pampiniform plexus. Although dihydrotestosterone is not produced in the testis from testosterone, another derivative from testosterone, 17^-estradiol, is produced by Leydig cells. The testicular contribution to total estrogen production, however, is small (in the order of 20%) as compared to peripheral aromatisation. The local production of 17^-estradiol may be of great importance for regulating Leydig cell functions, for instance in the previously mentioned development of steroidogenic lesions.

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