Effects of bovine 35 kDa FSH-suppressing protein on FSH and LH in rat pituitary cells in vitro: comparison with bovine 31 kDa inhibin.

The mode of action of a recently isolated gonadal protein, termed FSH-suppressing protein (FSP) or follistatin, on basal and gonadotrophin-releasing hormone (GnRH)-stimulated release of FSH and LH and on pituitary cell content of FSH and LH was examined in rat pituitary cell cultures and compared with the previously reported effects of inhibin. Pituitary cells were cultured for 3-9 days in the presence of graded doses of FSP and the basal release rates and changes in cell contents of FSH and LH determined during this period. FSP suppressed both the basal release rate and the cell content of FSH with median inhibitory concentrations (IC50) of 135 and 161 pmol/l respectively. The corresponding effects of FSP on LH basal release rate and LH cell content (IC50 = 200 pmol/l) were limited compared with the effects on FSH. The effect of FSP on GnRH-stimulated release of FSH and LH during 4 h was determined in cells which had been preincubated with FSP for 3 days, and the GnRH-stimulated release of FSH and LH analysed as a percentage of the respective gonadotrophin available for release. FSP antagonized GnRH action with dose-related increases in the GnRH median effective (stimulatory) concentrations for FSH and LH release (EC50 values = 56 and 400 pmol/l respectively) and a suppression in the maximum release of FSH and LH by excess GnRH (IC50 values = 142 and 150 pmol/l respectively). The effect of FSP on FSH cell content after 3 days in culture was insensitive to the neutralizing effects of an inhibin antiserum.(ABSTRACT TRUNCATED AT 250 WORDS)