SINGLE-CHANNEL PROPERTIES AND REGULATION OF PINACIDIL GLIBENCLAMIDE-SENSITIVE K+ CHANNELS IN FOLLICULAR CELLS FROM XENOPUS-OOCYTE

Follicular oocytes from Xenopus laevis contain K+ channels that are activated by members of the recently recognized class of vasorelaxants that includes the pinacidil derivative P1060. These channels are blocked by antidiabetic sulphonylureas such as glibenclamide. Opening of the glibenclamide-sensitive K+ channels with P1060 promotes follicular oocyte maturation. Whole-cell and single-channel patch-clamp configurations were used to monitor K+ channel activity in isolated follicular cells. In the presence of micromolar concentrations of intracellular Mg2+ ATP, P1060 activated a whole-cell K+ current that was blocked by glibenclamide. The P1060 response was depressed by millimolar concentrations of intracellular ATP and ATP[gammaS]. Single-channel recordings identified two different types of K+ channel. These channels differed in their unitary conductances (19 pS and 150 pS), in their sensitivities to internal Ca2+, to charybdotoxin and to pinacidil and glibenclamide. Only the Ca2+-independent K+ channel (19 pS) was activated by the pinacidil derivative and blocked by glibenclamide. Opening of the 19-pS glibenclamide-sensitive K+ channel by P1060 critically required the presence of a low concentration of Mg2+ ATP in the intracellular medium. The 19-pS K+ channel was opened by increasing intracellular cAMP. Similar effects were obtained by intracellular application of the catalytic subunit of protein kinase A in the presence of micromolar concentrations of Mg2+ ATP. Both acetylcholine and the phorbol ester phorbol 12-myristate 13-acetate blocked the 19-pS K+ channel after it was activated by P1060.