INHIBITION OF PINACIDIL INDUCED I-K(ATP) IN HEART BY CHANGES IN EXTRACELLULAR PH

Objective: The aim was to determine whether extracellular protons influence the activation of ATP sensitive potassium channels (K-ATP) in heart by the drug pinacidil. Methods: Single channel and whole cell currents were measured in enzymatically dispersed guinea pig ventricular myocytes with patch clamp procedures. ATP sensitive potassium channel activity was induced by pinacidil in whole cell experiments and either by lowering cytoplasmic ATP or pinacidil in membrane patch experiments. Results: Extracellular acidification inhibited and extracellular alkalisation potentiated ATP sensitive potassium channel currents (I-K(ATP)) induced by pinacidil under whole cell conditions. In membrane patches, I-K(ATP) induced by low cytoplasmic ATP (1 mM) was not sensitive to changes in extracellular pH, but I-K(ATP) induced by cytoplasmic application of pinacidil was inhibited by extracellular acidification. Conclusions: Interactions between protons and pinacidil drug molecules underlie the pH dependent changes in I-K(ATP) induced by pinacidil. Based on the kinetics of pH dependent changes in pinacidil induced channel activity in inside-out membrane patches, our data suggest that the important inhibitory site of action of this drug is located on the extracellular side of the cell membrane.