A POSSIBLE MECHANISM OF ACTION FOR FERTILIZATION PROMOTING PEPTIDE, ATRH-RELATED TRIPEPTIDE THAT PROMOTES CAPACITATION AND FERTILIZING ABILITY IN MAMMALIAN SPERMATOZOA

Fertilization promoting peptide (FPP), a tripeptide structurally relat ed to thyrotrophin releasing hormone (TRH), has been shown to stimulat e capacitation and fertilizing ability in both mouse and human spermat ozoa, but the mechanisms of action involved in these responses are cur rently unknown. In the present study utilizing epididymal mouse sperma tozoa, we have compared the ability of FPP, TRH, and pyroglutamylpheny lalanineprolineamide (an uncharged structurally related tripeptide fou nd in seminal plasma) to stimulate capacitation. At 50 nM, the mean co ncentration of FPP found in human seminal plasma, only FPP produced a significant response. This suggests that if a receptor is involved, it is one distinct from the TRH receptor. A significant response to FPP required the presence of extracellular Ca2+, with 90 mu m Ca2+ being s ufficient to support a stimulation of capacitation. The addition of FP P to suspensions at later stages of capacitation indicated that the na ture of the response changed, such that addition of FPP to capacitated suspensions inhibited spontaneous acrosome reactions; however, FPP-tr eated cells were still able to undergo acrosomal exocytosis in respons e to progesterone, a physiological agonist of acrosomal exocytosis. Be cause earlier studies had identified a similar capacitation-related ch ange in response to adenosine, being stimulatory early in capacitation and inhibitory later in capacitation, we investigated the possibility that FPP and adenosine might be acting via the same pathway. The comb ination of FPP plus adenosine, whether used at low, non-stimulatory co ncentrations or high, maximally-stimulatory concentrations, was more e ffective in promoting capacitation than either compound used individua lly. As observed with FPP, addition of adenosine to capacitated cells inhibited spontaneous acrosome loss but did not inhibit exocytosis in response to progesterone. This suggests that the two molecules are aff ecting a common pathway. Since adenosine, acting via specific cell sur face receptors, can stimulate fertilizing ability and adenylate cyclas e activity in uncapacitated cells and then inhibit enzyme activity in capacitated cells, we propose that FPP may act by modulating the adeny late cyclase/cyclic AMP signal transduction pathway. In vivo, FPP, whi ch would contact spermatozoa at ejaculation and probably remain bound to cells for some time, could stimulate capacitation as the spermatozo a ascend the female tract; adenosine, present in seminal plasma and th e female tract, could either augment FPP's action or replace it if FPP is lost from the cell surface. We therefore suggest that FPP and aden osine, by modulating adenylate cyclase activity to promote capacitatio n but inhibit spontaneous acrosomal exocytosis, may provide an endogen ous mechanism that helps to optimize the fertilizing potential of the few sperm cells that reach the site of fertilization in vivo. (C) 1996 Wiley-Liss, Inc.