Changes in inhibin-A (alpha-beta A dimer) and total alpha inhibin in the peripheral circulation and ovaries of rats after gonadotrophin-induced follicular development and during the normal oestrous cycle.

Abstract

Recent modifications to a previously reported two-site IRMA have permitted the measurement of serum/plasma concentrations and ovarian contents of inhibin-A (alpha-beta A dimer) in pregnant mare serum gonadotrophin (PMSG)-treated immature female rats and adult rats throughout the 4-day oestrous cycle. For comparison, total alpha inhibin levels were also measured by alpha subunit-directed inhibin RIA and found to be at least tenfold higher (relative to the same 32 kDa bovine inhibin standard used to calibrate both assays). In immature female rats, serum levels of inhibin-A dimer and total alpha inhibin increased within 3 h of PMSG injection and rose in parallel over the next 48 h to values four- to fivefold higher than pretreatment levels. Ovariectomy led to a rapid and parallel fall in both inhibin-A dimer and total alpha inhibin; initial half-lives (+/- 95% confidence intervals) were 22 +/- 4 and 20 +/- 5 min respectively. In adult rats, marked fluctuations in plasma concentrations and ovarian contents of inhibin-A dimer and total alpha inhibin occurred during the 4-day oestrous cycle, most notably between the morning of pro-oestrus and the morning of oestrus. Plasma levels of inhibin-A dimer and total alpha inhibin peaked on the afternoon of pro-oestrus just before the preovulatory gonadotrophin surge. After ovulation, both inhibin-A dimer and total alpha inhibin fell abruptly (two- to threefold by 0200 h on oestrus; P < 0.001), while FSH showed a secondary rise which peaked at 0700 h on oestrus. Although IRMA- and RIA-derived inhibin values generally followed a similar pattern across the 4-day cycle (plasma: r = 0.52, P < 0.001; ovary: r = 0.41, P < 0.001), a transient rise in plasma and ovarian inhibin-A dimer was detected at 0700 h on oestrus (P < 0.01) which was unaccompanied by a rise in total alpha inhibin. This rise in plasma inhibin-A dimer was probably responsible for terminating the post-ovulatory FSH surge since FSH levels declined steadily over the next 15 h. Overall, plasma inhibin-A dimer and FSH concentrations across the whole cycle were negatively correlated (r = -0.22, P < 0.01) whereas no correlation existed between total alpha inhibin and FSH (r = -0.11, P = 0.12).