Comparison of in vitro follicle-stimulating hormone (FSH) activity of equine gonadotropins (luteinizing hormone, FSH, and chorionic gonadotropin) in male and female rats.

Not only equine FSH (eFSH) but also equine LH (eLH) and equine CG (eCG/PMSG) exhibit FSH activity in the rat. The concomitant loss of LH and FSH activities upon dissociation at acidic pH of eLH demonstrates that its FSH activity is intrinsic to the molecule and not due to contamination by FSH. Indeed, this latter hormone dissociates at a much higher pH. The binding activity as well as the in vitro biological activity of the equine gonadotropins were determined on rat gonadal cells from both male and female rats using the homologous hormone rat FSH as a reference. In the female, the biological activity of all of the gonadotropins is strictly related to their binding activity. In the male, this is true for all hormones except eFSH. Indeed, the biological activity of eFSH on rat Sertoli cells is higher than expected from its binding activity. Moreover, the FSH from other species (rat, ovine, and porcine) as well as eLH and eCG inhibit the response elicited by a submaximal dose of eFSH. These data indicate that eFSH acts as a superagonist of rat FSH in rat Sertoli cells, since it triggers cell activation at a much lower concentration than expected from its relative binding activity.