Responsiveness of the pituitary gland to androgens and of the tests to gonadotrophins following damage to spermatogenesis in rats.

To test the interaction of androgen and inhibit feedback on the pituitary gland, inhibin-type feedback from the testes was reduced when they were made aspermatogenetic by bilateral ligation of the efferent ducts or local heating (43 degrees C for 30 min). There were only minor effects on the subsequent response of the pituitary gland to the removal of androgen feedback by the administration of antiserum against testosterone or by castration. However, in the antiserum-injected animals the steroidogenic response of the testis to the increased serum concentrations of LH was less in aspermatogenic than in control rats. Furthermore, unilateral aspermatogenesis was associated with reduced testosterone output by the treated testis and with compensatory increased output by the contralateral control testis, despite the absence of significant changes in serum LH and normal peripheral levels of testosterone. This suggests that the tubules can regulate the responsiveness of the Leydig cells to LH.

Effects of passive immunization against testosterone on serum and pituitary gonadotrophins: implications for the relative importance of androgens and inhibin in the control of gonadotrophin secretion in the male rat.

The acute effects of a specific reduction in androgen feedback to the hypothalamus and pituitary gland have been investigated in male rats by passive immunization against testosterone. An ovine antiserum raised against testosterone which had been conjugated through position 3 to bovine serum albumin was employed. Negative feedback control by androgens was effectively reduced by administration of the antiserum, as shown by an increase in levels of LH in the circulation. Immunized animals had a high concentration of testosterone in the circulation of which virtually all was tightly bound to antibody. In normal animals specific increases of serum LH concentration were obtained at all ages using a low dose of antiserum. At higher doses, serum FSH concentration was also increased. The LH response was reduced by anaesthesia and sham-operation. In sham-operated rats given a high dose of antiserum for 3 days the serum concentrations of LH and FSH could not be distinguished from those which followed castration while differences were found in the pituitary contents. It was concluded that testicular androgen provides an important inhibitory feedback control of secretion of FSH as well as that of LH in the adult male rat. Some of the data can best be explained by the action of inhibin as a minor or alternative inhibitor of FSH secretion.

The effects of long-term administration of either a crude inhibin preparation or an antiserum to FSH on serum hormone levels, testicular function and fertility of adult male rats.

Adult male rats were given either daily injections of ram rete testis fluid for periods of up to 70 days or injections of an antiserum against FSH every 3 days for 90 days. Compared with the control groups, the rats injected with ram rete testis fluid had lowered serum FSH levels, but only at treatment periods of 30 days and less. The levels of LH and testosterone in serum, testicular fluid secretion, sperm counts, testis weights and fertility were not affected by rete testis fluid treatment. The rats injected with anti-FSH serum exhibited an impairment of fertility which was never complete and evident only after 49 days of treatment. After 90 days of anti-FSH treatment, testis weight and free serum FSH were reduced, but sperm counts, testicular fluid secretion and serum levels of LH and testosterone were not affected.

The evidence that inhibin must exist.

The historical development of the 'inhibin' hypothesis is restated and the recent evidence reviewed. The circumstantial evidence for inhibin now seems very strong, and it probably acts in both sexes. Active materials have been isolated from several sources but it is not clear which is the circulating form of inhibin and a reciprocal relationship between circulating inhibin and FSH levels has not yet been demonstrated. Nevertheless, it appears that we may be optimistic about the existence of inhibin though the importance of its role as a physiological feedback inhibitor is still controversial.

Feedback control by the testis of gonadotrophin secretion: an examination of the inhibin hypothesis.

Spermatogenesis in rats was interrupted by local X-irradiation, heat or ligation of the testicular efferent ducts. A significant and specific rise in the serum level of FSH occurred 5--8 days after ligation of the efferent ducts, reaching twice the value observed in sham-operated controls by 21 days after the operation. After the testes were heated to 43 degrees C for 30 min, the serum levels of both LH and FSH were raised within 3 days and remained so up to 50 days after treatment. After X-irradiation, no changes in the concentration of FSH were observed in the first 21 days after treatment, but the serum levels of both gonadotrophins were increased at 49 days. By comparing the relative increases in the concentrations of FSH and LH after germ cell damage with those occurring after castration, it was evident that testicular androgens could account for only part of the normal feedback control of FSH secretion; at least one third of the inhibition of FSH secretion appeared to be due to non-androgenic sources, presumably 'inhibin'.

Drugs and the blood-testis barrier.

The functional and morphological evidence for the blood-testis barrier is discussed, together with evidence for the various processes (simple diffusion and facilitated diffusion) by which various substances enter the seminiferous tubule. Data are presented to show that methylmethane-sulfonate (MMS) and dimethylnitrosamine (DMNA) both enter the seminiferous tubules rapidly, although from the published rates of methylation of testicular DNA, by these two compounds, it might be expected that the entry of DMNA would be slower than that of MMS. It appears, however, that DMNA in blood is gradually converted to some nonpermeant compound. The possibility, as yet unsubstantiated, is discussed that a nontoxic permeant precursor may be converted into a nonpermeant toxic substance inside the tubules, thereby effectively concentrating the toxic compound inside the tubules.

The blood-testis barrier and temperature damage to the testis of the rat.

The integrity of the blood-testis barrier was investigated during and after local heating of rat testes sufficient to produce a temporary cessation of spermatogenesis. The flow, ionic composition and protein content of rete testis fluid (RTF) collected from testes maintained at 33 or 41 degrees C were unaffected either at the time of treatment or up to 2 days later when the major cytological consequences of heating occurred. The normally low rate of transfer of albumin from blood to RTF was unaffected during and after heating. Transfer constants for radioactive K, Rb, Na and lysine consistently increased during heating although there were time-dependent differences between the patterns of response for each molecule. The normally rapid transfer of testosterone was unaffected by heating, but the entry rates of radioactivity into RTF after the infusion of more slowly diffusing steroids were enhanced at 41 degrees C. The clearest effects of heating were an approximate doubling in the uptake of oxygen and decrease in the net synthesis of protein by the testis. It is concluded that heating sufficient to damage spermatogenesis was not associated with dramatic alterations in the integrity of the blood-testis barrier but more with changes in testicular metabolism.