The present findings demonstrate that interference with the feedback effects of T, as a result of active immunization against this steroid, results in major changes in Leydig cell function in rats.These changes include a trebling in the number of LH receptors per Leydig cell and in the capacity of these cells to secrete T in vitro, while there was a smaller increase in the number of LHRH receptors per Leydig cell. Therefore, while confirming previous observations that suggested Leydig cell function was stimulated in T-immunized animals,in this study we have detailed precisely what changes have occurred.
The etiology of these changes is of considerable interest as it may provide important information on the basic mechanisms involved in the regulation of the Leydig cell. In the original studies in T-immunized rats and rabbits, it was considered that the observed changes in Leydig cell structure and function were simply a consequence of the chronically raised serum level of LH, which itself resulted from diminution of negative feedback to the hypothalamic-pituitary axis. We have also observed a massive increase in blood levels of LH in T-immunized rats, and the associated increase in testosterone responsiveness of the Leydig cells which we have demonstrated is consistent with
the known effects of LH or hCG treatment. On the other hand, the changes in receptor numbers in T-immunized rats are the exact opposite of what would be expected to result from raised LH levels. This may mean that factors other than LH are involved, such as interference with androgen feedback mechanisms within the testis. With these possibilities in mind, it is pertinent to recount the known actions of LH and to contrast these to the changes observed in T-immunized rats.
Continued treatment of rats with hCG leads to hypertrophy and hyperplasia of the Leydig cells (Schoen and Samuels, 1965; Chemes et al, 1976;Christensen and Peacock, 1980) and increased T responsiveness in vitro (Risbridger et al, 1982). Comparable changes in Leydig cell size and number have been reported in T-immunized rabbits (Nieschlag et al, 1973, 1975), and the increased testicular weight in T-immunized rats in the present study is perhaps also indicative of such a change. Certainly, the enormously increased basal output of T in vitro by Leydig cells from T-immunized rats is suggestive of increased expo-sure to LH in vivo, while the enhanced capacity to respond to maximal hCG stimulation in vitro is consistent with findings in other situations, such as bilaterally-induced cryptorchidism, in which Leydig cell hypertrophy and hyperplasia occur in the presence of raised serum levels of LH (deKretser et al, 1979; Schanbacher, 1980; Sharpe,1982a). These reports provide support for the idea that LH mediates the changes in Leydig cell func-
tion observed in T-immunized rats.