A TETRAPEPTIDE WITHIN A RECEPTOR-BINDING REGION OF HUMAN FOLLICLE-STIMULATING-HORMONE BETA-SUBUNIT, HFSH-BETA-(34-37), REGULATES SODIUM-CALCIUM EXCHANGE IN SERTOLI CELLS

In a previous study, we showed that binding of FSH by cultured rat Ser toli cells significantly inhibited basal levels of Na+/Ca2+ exchange. Similar inhibition was observed when proteoliposomes enriched with bov ine calf testis follicle-stimulating hormone (FSH) receptors were stim ulated with FSH. In the present study, we screened a series of overlap ping synthetic peptide amides, representing the entire primary structu re of the beta-subunit of hFSH, for their effects on sodium-dependent calcium uptake (as Ca-45(2+)) by monolayer cultures of Sertoli cells f rom immature rats. hFSH-beta-(33-53), previously identified as a recep tor binding region of hFSH-beta-subunit, significantly (P < 0.05) inhi bited Na+/Ca2+ exchange. A tetrapeptide [TRDL, hFSH-beta-(34-37)] cont ained within this sequence, was observed to be equally as active as hF SH-beta-(33-53) at 200 muM, suggesting that the regulatory effect of h FSH-beta-(33-53) on sodium-dependent Ca-45(2+) influx was due to resid ues 34-37. hFSH-beta-(81-95) also inhibited Na+-dependent calcium infl ux, although to a lesser extent than hFSH-beta-(33-53) or hFSH-beta-(3 4-37). Sodium-dependent Ca-45(2+) entry into Sertoli cells was enhance d in a concentration-related manner when extracellular sodium was repl aced by equimolar concentrations (up to 135 mM) of choline chloride. h FSH-beta-(34-37) significantly reduced basal uptake of Ca-45(2+) in ch oline-containing buffer, but was without effect in buffer containing 1 35 mM NaCl. These results were similar to those obtained with FSH. Att enuation of basal sodium-dependent Ca-45(2+) influx by hFSH-beta-(34-3 7) was also evident in the presence of ouabain, a specific inhibitor o f Na+-K+-ATPase in Sertoli cells. hFSH-beta-(34-37) reduced basal leve ls of Na+/Ca2+ exchange in FSH receptor-enriched proteoliposomes. Our data provide evidence that the recently reported effect of FSH on sodi um-dependent calcium uptake in Sertoli cells may be due, at least in p art, to the tetrapeptide TRDL, hFSH-beta-(34-37), present within a rec eptor-binding region of FSH-beta-subunit. Our studies suggest that var ious domains of FSH-beta-subunit may exert unique physiologic regulato ry functions associated with FSH binding and Sertoli cell responsivene ss to hormonal stimulation.