ACTIVATION OF ATP-DEPENDENT K+ CURRENTS IN INTACT SKELETAL-MUSCLE FIBERS BY REDUCED INTRACELLULAR PH

We have used three-microelectrode voltage clamp in conjunction with the ammonium prepulse method to investigate the effects of lowered intracellular pH (pH(i)) on resting potassium currents of frog skeletal muscle fibres. Potassium currents were recorded in 40 mM K+, Cl--free solution in response either to voltage steps or ramps. An ammonium prepulse (2 h) reduced pH(i) to 6.45 from a control value of 7.19. The intracellular ATP concentration, measured with high-pressure liquid chromatography (HPLC), was unchanged by this procedure. Mean outward potassium currents were larger in low pH(i) than in control fibres, being about twice as large at + 40 mV, whereas mean inward currents were very similar in control and low-pH(i) fibres. The sulphonylurea glibenclamide blocked single K(ATP) channels in excised patches with a K(d) of 3-mu-M. In intact fibres 50-mu-M glibenclamide had no effect on K+ currents in controls but reduced currents in low-pH(i) fibres. In the presence of glibenclamide, K+ currents in low-pH(i) fibres were not significantly different from those in control fibres. We suggest that reduced pH(i) in intact skeletal muscle fibres opens ATP-dependent potassium channels (K(ATP) channels), as has been shown to occur in excised patches of membrane.