A peptide in gastric secretion with inhibin-like properties.

The presence of inhibin like peptide which is biologically active in reducing FSH secretion in castrated rats and suppressing hCG-induced increase in ovarian weights of immature mice, is reported for the first time in gastric aspirates of normal men. Immunological characteristics of this peptide were similar to inhibin extracted from human seminal plasma. The concentration of inhibin-like peptide measured by RIA developed for seminal inhibin is more than thirty-fold higher in gastric aspirates compared with serum.

Biological studies with low and high molecular weight inhibin preparations.

The effects of both high and low molecular weight inhibin preparations on testicular and pituitary receptors were studied. Both these preparations were able to inhibit the binding of labelled hFSH to testicular receptor in a dose related manner, but were unable to affect the binding of labelled hCG to its receptor, suggesting that the observed inhibition of FSH binding was specific to inhibin. In addition, the binding of LHRH to pituitary receptors was affected by inhibin preparations. Interestingly, the antiserum raised against high molecular weight inhibin was able to neutralize, totally, the biological effect of high molecular weight inhibin, and only, partially, the biological effect of low molecular weight inhibin.

Inhibin in the human prostate.

"Inhibin"-like protein, partially purified from the human prostate, was similar to that obtained from human seminal plasma under identical conditions. Concentration of inhibin in human prostate was tenfold higher as compared to human testis and was active biologically and immunologically. The presence of a high amount of inhibin in prostate seems to be unique in humans. Further inhibin concentration in benign prostatic hypertrophy tissue increased by tenfold in comparison to normal or cancerous tissue. Hence determination of serum level of inhibin may be used as a therapeutic tool to monitor prostate diseases.

Measurement of inhibin.

The bioassays developed so far, although good for detecting inhibin from different sources and for monitoring their purification, are not really practical for physiological studies. Moreover, they are influenced by steroidal hormones. In this regard radioimmunoassays, with their high capacity and greater sensitivity, are of immense value in studies involving the physiology of inhibin. More information on this aspect will probably lead to the development of more rational methods of assay. However, the need for an international reference preparation that will provide a common basis for comparing data from different laboratories cannot but be emphasized.

Negative feedback of secretion of follicle-stimulating hormone by the pituitary gland of developing male rats.

Rabbit antiserum to human seminal plasma inhibin (hSPI) was administered subcutaneously to developing male rats of 5, 10, 14, 17 and 24 days of age and the size of the endogenous FSH rise in serum was measured. The FSH levels were threefold higher on day 9 and 1.5-fold higher on days 14 and 18 when compared with levels in control rats treated with normal rabbit serum. Furthermore, the in-vitro binding capacity of pituitary plasma membrane to 125I-labelled hSPI declined with increase in age of the rats. Thus, the results of the present study suggest that the sensitivity of the testicular inhibin-FSH feedback relationship is related to age-dependent changes in pituitary binding of inhibin.

Levels of inhibin in human semen and accessory reproductive organs.

Levels of inhibin in the seminal plasma of oligozoospermic and normozoospermic semen and normal human accessory reproductive organs were estimated using specific and sensitive radioimmunoassay (RIA). Seminal inhibin content showed a positive correlation (r = 0.827) with the sperm concentration. Further, in the human accessory reproductive organs, prostate showed maximum content of inhibin as compared to the seminal vesicle and the epididymis.

Development of a radioimmunoassay for human seminal plasma inhibin.

Using a homogeneous inhibin preparation from human seminal plasma with a molecular weight of about 19 000, a sensitive and specific radioimmunoassay (RIA) for inhibin has been developed. None of the purified hormones tested, such as LH, FSH and prolactin from different species, showed any cross-reaction in this RIA. Steroid hormones such as testosterone, dihydrotestosterone, oestradiol-17 beta and progesterone did not interfere with the assay. The antiserum had an affinity constant (Ka) of 2.379 X 10(9). The assay sensitivity was 10-15 ng per tube and the intra- and inter-assay coefficients of variation were 5-7% (n = 6) and 15% (n = 10) respectively. The recovery for inhibin added to the serum of a castrated man was 95-110%. Using this RIA, inhibin levels in various biological fluids and tissues were measured. Normo-spermic semen contained significantly higher levels of inhibin than did oligospermic semen. Human prostate contained a substantial quantity of inhibin. Monkey semen, rat serum, and bovine, ovine and porcine follicular fluids cross-reacted in the RIA, while ram testicular inhibin and bull semen did not do so. In developing (9-28 days of age) male rats, circulating inhibin levels showed an inverse relationship with serum FSH levels. In female rats of this age endogenous inhibin concentrations changed in parallel with those of serum FSH.