Hypotonic Stress Increases Efficacy of Rilmakalim, but Not Pinacidil, to Activate ATP-Sensitive K+ Current in Guinea Pig Ventricular Myocytes

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 (KATP) openings by hypo-osmotic stress. The whole-cell KATP currents (IK,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 Emax value for pinacidil-activated IK,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 IK,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, (ii) the absence of Ca2+ from the internal and external solutions, and (iii) the presence of creatine phosphate (3 mmol/L), which affects creatine kinase regulation of the KATP 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 IK,ATP through intracellular mechanisms involving the phosphatidylinositol kinase pathway.

Differentiating electrophysiological effects and cardiac safety of drugs based on the electrocardiogram: a blinded validation

BACKGROUND

The ventricular components (QRS and QT) on the electrocardiogram (ECG) depend on the properties of ventricular action potentials that can be modulated by drugs via specific ion channels. However, the correlation of ECG ventricular waveforms with underlying ion actions is not well established and has been extensively debated.

OBJECTIVE

To conduct a blinded in vitro assessment of the ionic mechanisms for drug-induced ECG changes.

METHODS AND RESULTS

Fourteen cardiac and noncardiac drugs with known effects on cardiac ion channels were selected by the study sponsor, and were tested in the rabbit left ventricular wedge preparation with recording of the ECG and contractility. The investigators who performed the experiments and analyzed the data were blinded to names, concentrations, and molecular weights of the drugs. The compounds were prepared by the sponsor and sent to the investigators as 56 stock solutions. The effects of I(Kr), I(Ks), I(Ca,L), I(Na) blocker, and I(KATP) opener on QRS, QT, and T(p-e), were evaluated. Disclosure of the names and concentrations after completion of the study revealed that there were highly correlated ECG changes with underlying ionic mechanisms and proarrhythmic potential of drugs that, respectively, target I(Kr), I(Ks), I(Ca,L), I(Na), and I(KATP). Among ECG parameters, T(p-e) was more useful in differentiating drugs' actions.

CONCLUSIONS

Specific electrophysiological action and the consequent proarrhythmic potential of a drug can be accurately determined by analysis of drug-induced changes in ECG in the rabbit left ventricular wedge preparation. Change in T(p-e) provides the most relevant information.

Hypotonic stress enhances slope conductivity of ATP-sensitive K+ channels activated pharmacologically

The aim of this paper was to find out the effects of hypotonic stress on the slope conductivity of ATP-sensitive K(+) channels. Using patch clamp technique, rilmakalim and pinacidil as the activators and glibenclamide as an inhibitor of mentioned channels, we have demonstrated that short hypotonic challenge doubled slope conductivity of exploring channels by augmentation of their open probability.