Regulation of biologically active dimeric inhibin A and B from infancy to adulthood in the male

Inhibins are glycoprotein members of the transforming growth factor-β family that have been implicated in the control of spermatogenesis by exerting a negative feedback on FSH secretion. In addition, locally produced inhibins may play a role in paracrine regulation of testicular function. Immunoassays were used to measure the two biologically active dimeric forms of inhibin (inhibin A and B) in serum, seminal plasma, and urine. To better define their actions, inhibins were measured in the male during infancy, sexual maturation, and senescence. Inhibin B but not A was measurable in the serum of male newborns, infants, children, and adults. In adult males, measurable levels of inhibin B were detected in the seminal plasma but not the urine. The circulating levels of inhibin B increased shortly after birth and peaked at 4-12 months of age (210 ± 31 pg/mL). The concentration measured in the serum then decreased to a low of 81 ± 12 pg/mL of inhibin B from 3-9 yr of age followed by a gradual increase beginning with the onset of puberty and reaching another peak of 167 ± 20 pg/mL in males who were 20-30 yr of age. Inhibin B levels then gradually declined with increasing age up through 90 yr of age. Serum levels of gonadotropins and total testosterone production were also measured in these same males. There was a brief increase in the gonadotropins (FSH and LH) during the few months of postnatal development, followed by a decrease to basal levels until the onset of puberty at 10-14 yr of age. Testosterone was also increased in the serum of infants from day 1 through 12 months of age, which decreased in young children but increased again following the elevation of gonadotropins during puberty. In adults aged 20-90 yr, serum levels of inhibin B were inversely proportional to levels of FSH but not LH or testosterone. In males in which a semen analysis was performed, those males with normal semen analysis had a significantly higher inhibin B levels, sperm production, and lower FSH levels than males with either oligospermia or nonobstructive azoospermia. The levels of Inhibin B found in circulation were a good marker for testicular function and could be useful in the diagnosis of patients with semen abnormalities or a complete absence of spermatogenesis. Because this glycoprotein is secreted in high amounts in the prepubertal testis up to 3 yr of age, inhibin B could potentially be used as a marker in the diagnosis of cryptorchidism and precocious puberty.