Activation of an ATP-dependent K~+ conductance in Xenopus oocytes by expression of adenylate kinase cloned from renal proximal tubules

摘要

In rabbit proximal convoluted tubules, an ATP-sensitive K~+ (K_(ATP)) channel has been shown to be involved in membrane cross-talk, i.e. the coupling (most likely mediated through intracellular ATP) between transepithelial Na~+ transport and basolateral K~+ conductance. This K~+ conductance is inhibited by taurine. We sought to isolate this K~+ channel by expression cloning in Xenopus oocytes. Injection of renal cortex mRNA into oocytes induced a K~+ conductance, largely inhibited by extracellular Ba~(2+) and intracellular taurine. Using this functional test, we isolated from our proximal tubule cDNA library a unique clone, which induced a large K~+ current which was Ba~(2+)-, taurine- and glibenclamide-sensitive. Surprisingly, this clone is not a K~+ channel but an adenylate kinase protein (AK3), known to convert NTP+AMP into NDP+ADP (N could be G, I or A). AK3 expression resulted in a large ATP decrease and activation of the whole-cell currents including a previously unknown, endogenous K~+ current. To verify whether ATP decrease was responsible for the current activation, we demonstrated that inhibition of glycolysis greatly reduces oocyte ATP levels and increases an inwardly rectifying K~+ current. The possible involvement of AK in the K_(ATP) channel's regulation provides a means of explaining their observed activity in cytosolic environments characterized by high ATP concentrations.