INHIBIN-B IN SEMINAL PLASMA - TESTICULAR ORIGIN AND RELATIONSHIP TO SPERMATOGENESIS

In men, inhibin B is the circulating isoform involved in the regulatio n of follicle stimulating hormone (FSH) secretion. Within the testis, inhibin B may have a role in Sertoli and germ cell interactions, thus secretion into seminal plasma may reflect seminiferous tubule function . Using specific immunoassays, inhibin B was present in seminal plasma in fertile men (n = 105) and in unselected men attending an infertili ty clinic (n = 174) with a wide range in concentration from undetectab le (<15 pg/ml) up to 54 100 pg/ml (geometric mean 280 pg/ml), There wa s a highly significant correlation between seminal plasma inhibin B co ncentration and sperm concentration (r = 0.46, P < 0.001), but no corr elation with percentages of spermatozoa with progressive motility or n ormal morphology, Inhibin h and isoforms containing pro and alpha C im munoreactivity were not detectable. In post-vasectomy seminal plasma s amples (18 of 20) inhibin B was undetectable, indicating that the test is is the predominant source. In unselected men attending an infertili ty clinic, inhibin B was undetectable in 17% (present in remainder; ma ximum concentration 26 200 pg/ml; mean 263 pg/ml), with a highly signi ficant correlation between seminal plasma inhibin B and sperm concentr ation (r = 0.55, P < 0.0001). In men with oligo/azoospermia (sperm con centration <20 X 10(6)/ml), seminal plasma inhibin B concentrations we re lower in those with elevated plasma FSH concentrations (mean values 42 and 205 pg/ml, P < 0.05). Inhibin alpha and beta B subunits were l ocalized predominantly in Sertoli and Leydig cells, using immunohistoc hemistry. We conclude that inhibin B of testicular origin is present i n normal human seminal plasma, but with a very wide range in concentra tion, and may reflect the functional state of the seminiferous epithel ium.