EXPRESSION OF SPERM CA2-ACTIVATED K+ CHANNELS IN XENOPUS OOCYTES AND THEIR MODULATION BY EXTRACELLULAR ATP()

Ionic fluxes across the sperm membrane have been shown to be important in the initiating process of sperm activation and gamete interaction; however, electrophysiological investigation of the ion channels invol ved has been precluded by the small size of the sperm, especially in m ammalian species, In the present study sperm ion channels were express ed in Xenopus oocytes by injection of RNAs of spermatogenic cells isol ated from the rat testes. The RNA-injected oocytes responded to ATP, a factor known to regulate sperm activation, with the activation of an outwardly rectifying whole-cell current which was dependent on K+ conc entrations and inhibitable by K+ channel blockers, charybdotoxin (CTX) and tetraethylammonium (TEA), The ATP-induced current could be mimick ed by a Ca2+ ionophore but suppressed by a Ca2+ chelator applied intra cellularly, indicating a Ca2+ dependence of the current. Single-channe l measurements on RNA-injected oocytes revealed channels of large cond uctance which could be blocked by CTX and TEA, Go-injection of germ ce ll RNAs with the antisense RNA for a mouse gene encoding slowpoke 'Max i' Ca2+-activated K+ channels resulted in significant reduction of the ATP- and ionomycin-induced current. The expression of the 'Maxi' Ca2- activated K+ channels in sperm collected from the rat epididymis was also confirmed by Western blot analysis. These results suggest that s perm possess Ca2+-activated KC channels which may be involved in the p rocess of sperm activation. (C) 1998 Federation of European Biochemica l Societies.