Reversal of long-term LH deprivation on testosterone secretion and Leydig cell volume, number and proliferation in adult rats

The purpose of this study was to determine whether Leydig cell volume and function could recover fully from long-term LH deprivation upon restoration of endogenous LH secretion, and whether the restoration of LH would elicit a mitogenic response, i.e. stimulate Leydig cell proliferation or affect Leydig cell number per testis. LH secretion was inhibited by treating adult rats with testosterone and oestradiol-filled (TO) silicone elastomer implants (16 weeks), and was restored by removing the implants. Changes in serum concentrations of LH and FSH, LH-stimulated testosterone secretion by testes perfused in vitro, Leydig cell volume and number per testis, average Leydig cell volume and Leydig cell [³H]thymidine incorporation were measured at weekly intervals following implant removal. The TO implants inhibited (P < 0.01) LH secretion, but serum concentrations of FSH were not significantly different (P > 0.10) from control values. After implant removal, serum LH returned to control values within 1 week, whereas serum FSH increased twofold (P < 0.01) and returned to control values at 4 weeks. LH-stimulated in-vitro testosterone secretion was inhibited by more than 99% in TO-implanted rats, but increased (P < 0.01) to 80% of control values by 8 weeks after implant removal. The total volume of Leydig cells per testis and the volume of an average Leydig cell were 14 and 19% of control values respectively, after 16 weeks of TO implantation (P < 0.01), but returned to 83 and 86% of controls (P > 0.10) respectively, by 6 weeks after implant removal. Leydig cell proliferation ([³H]thymidine labelling index) was low (< 0.1%) in both control and TO-implanted rats, increased (P < 0.01) fivefold from 1 to 4 weeks after implant removal and then declined to control values at 6 weeks. The increase in Leydig cell [³H]thymidine incorporation was mimicked by treating TO-implanted rats with exogenous LH, but not FSH. Leydig cells were identified in both the interstitium and the lamina propria of the seminiferous epithelium. The proportion of Leydig cell nuclei in the lamina propria was 30-fold greater (P < 0.01) at 1 and 3 weeks after implant removal (3%) compared with that for control and TO-implanted rats (0.1%). Total Leydig cell number per testis was marginally but not significantly (P = 0.06) decreased in rats with TO implants for 16 weeks when compared with controls (18.4 ± 2.2 vs 25.4 ± 1.2 x 10⁶). Three weeks after implant removal, the numbers of Leydig cells per testis were identical (26.8 ± 2.8 x 10⁶) to those in control animals. These results not only demonstrate dramatic morphogenic effects of LH on mature rat Leydig cells, but also suggest that endogenous LH might be mitogenic at least to a subpopulation of Leydig cells.