Involvement of the phosphatidylinositol kinase pathway in augmentation of ATP-sensitive K⁺ channel currents by hypo-osmotic stress in rat ventricular myocytes

The objective of this study was to investigate the mechanisms of increase in the efficacy of ATP-sensitive K⁺ channel (K_ATP) openings by hypo-osmotic stress. The whole-cell K_ATP currents (I_k,atp) stimulated by 100 μmol/L pinacidil, a K⁺ channel opening drug, were significantly augmented during hypo-osmotic stress (189 mOsmol/L) compared with normal conditions (303 mOsmol/L). The EC₅₀ and E_max value for pinacidil-activated I_k,atp (measured at 0 mV) was 154 μmol/L and 844 pA, respectively, in normal solution and 16.6 μmol/L and 1266 pA, respectively, in hypo-osmotic solution. Augmentation of I_{K ATP} during hypo-osmotic stress was attenuated by wortmannin (50 μmol/L), an inhibitor of phosphatidylinositol 3- and 4-kinases, but not by (i) phalloidin (30 μmol/L), an actin filament stabilizer, (it) the absence of Ca²⁺ from the internal and external solutions, and (tit) the presence of creatine phosphate (3 mmol/L), which affects creatine kinase regulation of the K_ATP channels. In the single-channel recordings, an inside-out patch was made after approximately 5 min exposure of the myocyte to hypo-osmotic solution. However, the IC₅₀ value for ATP under such conditions was not different from that obtained in normal osmotic solution. In conclusion, hypo-osmotic stress could augment cardiac I_k,atp through intracellular mechanisms involving the phosphatidylinositol kinase pathway.