Interaction of equine luteinizing hormone with binding sites for follicle-stimulating hormone in the rat seminiferous tubule.

Competitive binding assays using 125I-labeled equine LH confirm that this hormone binds with relatively high affinity to both LH- and FSH-binding sites on the separated Leydig cells and seminiferous tubules, respectively, of the 18-day-old rat testis. In contrast, ovine LH is potent only in Leydig cell LH-binding sites. However, despite its binding to FSH sites, equine LH, like all other LH preparations examined, lacks the ability to stimulate in vitro cAMP production from the rat seminiferous tubules, a characteristic action of FSH. Although all three species of LH examined (equine, ovine, and human) are similarly inactive in stimulating cAMP production when tested alone (all were <0.1% as active as FSH), there is a pronounced dichotomy in the nature of their interactions with FSH. The addition of ovine or human LH to incubation media either has no effect or slightly augments the cAMP response to FSH (ovine, equine, or human). In contrast, combination with equine LH markedly inhibits the cAMP response to...