Follicle-stimulating hormone stimulates circulating gonadotropin surge- attenuating/inhibiting factor bioactivity in cows

Abstract

This study aimed to determine whether superovulation in cattle stimulates gonadotropin surge-attenuating/inhibiting factor (GnSAF/IF) bioactivity, as it does in humans. Blood samples were collected from cows (n = 7 per treatment) at -4, 8, 20, 32, 44, 56, and 68 h after injections of saline, eCG, or FSH. Equal volumes of plasma at each treatment and time point were pooled, and GnSAF/IF and inhibin bioactivities were measured using an established rat pituitary cell culture bioassay. Plasma from saline- and eCG- treated cows had little effect on GnRH-induced LH secretion (116.3 ± 8.3%- 81.6 ± 6.0% of control), while plasma from FSH-treated cows produced a time- dependent suppression of GnRH-induced LH secretion, falling to 64.6 ± 4.0% of the control value at 56 h after first FSH injection (p < 0.001). The GnSAF/IF bioactivity from the 56-h plasma eluted at pH 5.73 by pseudochromatofocusing-similar to the Gn-SAF/IF isoelectric point value of 5.81 determined using serum from superovulated women. Plasma from FSH- treated cows reduced basal FSH secretion more than plasma from eCG-treated cows (to 55.5 ± 5.7% and 63.2 ± 6.6% of the control value, respectively, p < 0.01) although immunoreactive inhibin concentrations were similar between the two groups. We conclude that FSH, but not eCG, treatment causes a time- dependent production of circulating GnSAF/IF bioactivity in cattle.