Studies on the testicular source of inhibin and its route of secretion in rams: Failure of the Leydig cell to secrete inhibin in response to a human chorionic gonadotropin/LH stimulus

To determine whether Leydig cells produce inhibin in the ram, Leydig cells were stimulated by administering human chorionic gonadotrophin (hCG) or raising the levels of endogenous LH by an injection of gonadotrophin releasing hormone (GnRH). Plasma concentrations of testosterone increased in the 72 h after either a single injection (P < 0.05) or two injections (P < 0.01) of hCG. Plasma concentrations of inhibin were not significantly influenced by either one or two injections of hCG. Administration of GnRH (1 μg) caused an 11-fold increase in plasma concentrations of LH but did not influence concentrations of inhibin in either the jugular or testicular veins (pampiniform plexus). In contrast, concentrations of testosterone were increased by about fourfold in both jugular (P < 0.01) and testicular (P < 0.05) veins. The concentrations of inhibin in the testicular vein were 1.3-fold higher than in the peripheral plasma (P < 0.05) both before and following treatment with GnRH whereas the concentrations of testosterone were 18- to 21-fold greater than in peripheral concentrations. In view of the difference in concentrations of inhibin between testicular and jugular veins, in a further experiment a sample was taken from the jugular vein, a vein located in the tunica vasculosa of the testis (testicular vein) and from a vein (spermatic vein) and lymph vessels located in the spermatic cord. The mean (± S.E.M.) concentrations of inhibin were highest in the testicular lymph (45.93 ± 4.21 μg/l; P < 0.001) compared with the peripheral (4.14 ± 0.31 μg/l), spermatic (8.0 ± 1.17 μg/l) or testicular (6.4 ± 0.82 μg/l) plasma. Plasma concentrations of inhibin were significantly higher in the spermatic vein than in the testicular vein (P < 0.05) and jugular vein (P < 0.01), and concentrations of inhibin in the testicular vein were significantly (P < 0.05) higher than in the jugular vein. There were no significant differences in the concentrations of testosterone in the spermatic vein, testicular vein or testicular lymph but the concentrations of testosterone in the peripheral plasma were significantly (P < 0.05) less than in the testicular plasma or lymph. These results suggest that, in the ram, the Leydig cell does not respond to hCG or endogenous LH by secreting inhibin or by influencing other cells within the testis to secrete inhibin within the time-frame of these experiments. The low testicular to jugular differences in the concentration of inhibin and the high concentrations of inhibin in the testicular lymph suggest that the lymph may be an important route of secretion of inhibin from the testis in the ram.