Changes in serum concentrations of inhibin in cyclic pigs

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Abstract
A sensitive and specific radioimmunoassay (RIA) system for porcine ovarian inhibin has been developed. Antisera to porcine inhibin of molecular weight 32 000 (32 kDa inhibin) were raised in male chickens. The recognition site of the antiserum used in the present study was the N-terminal region of the α-subunit of 32 kDa inhibin. The antiserum could recognize higher molecular weight forms of inhibin present in porcine follicular fluid as well as the 32 kDa form. The average effective dose and the least detectable amount of inhibin in this RIA were 643 and 30·7 pg/tube respectively. Non-specific effects of serum on the RIA could be overcome by including 100 μl serum from a castrated pig in the standards and by incubating at 30 °C. Serum concentrations of inhibin fluctuated between 0·6 and 2·5 μg/l during the oestrous cycles of the pigs. The amount of serum inhibin gradually increased from the late luteal phase to the early follicular phase and reached a maximum of 2·48 μg/l at day −4 (day 0 = day of ovulation). Concentrations then decreased rapidly to reach a minimum of 0·6 μg/l. Two small peaks were also observed during the luteal phase, although the concentration was relatively low during this phase. Changes in serum concentrations of oestradiol-17β did not parallel those of inhibin, especially during the luteal phase when serum concentrations of oestradiol-17β remained quite low. Serum concentrations of FSH were inversely related to those of inhibin rather than to those of oestradiol-17β, suggesting that the secretion of FSH during the oestrous cycles of pigs is mainly controlled by ovarian inhibin. J. Endocr. (1988) 118, 211–219