SECRETION PATTERN OF IMMUNOREACTIVE INHIBIN IN MEN

Chronological changes in serum concentrations of inhibin, a gonadal glycoprotein hormone, were studied in healthy male Volunteers (age 24-27 years). Secretion profiles of immunoreactive inhibin (ir-inhibin) were compared with those of luteinizing hormone (LH), follicle-stimulating hormone (FSK) and testosterone. Blood samples were collected every 15 min for 24 h. Serum inhibin concentrations were measured by a two-site immunoenzymatic assay with antibodies raised against distinct epitopes of the recombinant 1-32 amino acids of the cr-subunit of human inhibin, The normal range for men was 0.79-3.1 U/l x 10(-3), the sensitivity of the assay was 0.1 U/l x 10(-3) (cv: within-assay, 6.8%; between-assay, 8.2%). Luteinizing hormone and FSH were measured by immunoradiometric assay and testosterone by radioimmunoassay. Secretion profiles of inhibin and testosterone were tested for diurnal Variations by cosinor rhythmometry. Highest ir-inhibin concentrations were observed in the morning at 08.00 h, with peak values of 2.45-3.20 U/l x 10(-3). During the evening and the night, ir-inhibin levels were relatively low; lowest concentrations were observed between 01.00 h and 02.00 h at night: 1.20-1.86 U/l x 10(-3). Highest testosterone levels were observed in the morning (20.5-36.6 pmol/l), lowest concentrations were detected at night (7.35-12.6 pmol/l). Cosinor rhythmometry supported the suggestion that there is a clear circadian secretion of ir-inhibin and testosterone, respectively. The secretion pattern of ir-inhibin was analyzed by the Cluster pulse analysis computed algorithm, which identified four to seven inhibin pulses per day, depending on the person under observation, A significant correlation could be observed between median testosterone and ir-inhibin concentrations (r = 0.449, p < 0.001). In conclusion, ir-inhibin and testosterone in healthy male volunteers follow a clear diurnal rhythm. Moment to moment changes of ir-inhibin can be observed in al secretion profiles investigated. A probable physiological role for pulsatile inhibin secretion is not yet clarified.