Metabolic support of Na+ pump in apically permeabilized A6 kidney cell epithelia: role of creatine kinase.

Abstract

The contribution of ATP-generating systems to Na+ pump (Na+-K+-ATPase) function was studied in Xenopus laevis A6 kidney epithelia apically permeabilized with digitonin. The ouabain-inhibitable Na+ pump current (I(P)) was measured in the presence of otherwise impermeant inhibitors and/or substrates at Na+ and K+ concentrations that allowed near-maximal pump function. Confocal fluorescence microscopy after apical addition of sulfosuccinimidobiotin (molecular weight of 443) showed that all cells were permeabilized. Less than 15% of the endogenous lactate dehydrogenase and creatine kinase (CK) were released into the apical medium. The I(P) was approximately 5 microA/cm2 in the presence of D-glucose. Blocking glycolysis with 2-deoxy-D-glucose or oxidative phosphorylation with antimycin A decreased it by > or = 50%. Exogenously added ATP prevented these decreases fully or partially, respectively. Two CK isoforms were detected, one likely being mitochondrial and the other corresponding to mammalian B isoform of CK. Phosphocreatine partially restored Na+ pump activity during inhibition of either ATP synthesis pathway. In conclusion, the ATP used by Na+ pumps of apically digitonin-permeabilized A6 epithelia is generated to a similar extent by glycolysis and oxidative phosphorylation. The CK system can partially support the ATP supply to the Na+ pumps.