A K_ATP channel opener protects cardiomyocytes from Ca²⁺ waves: A laser confocal microscopy study

Laser confocal microscopy was used to visualize intracellular spatiotemporal Ca²⁺ patterns in single guinea pig ventricular myocytes loaded with the Ca²⁺ indicator, fluo 3-acetoxymethyl ester (fluo 3-AM), and exposed to moderately elevated extracellular K⁺ to induce partial membrane depolarization. Analysis of K⁺-induced intracellular Ca²⁺ elevation revealed three distinct paradigms: 1) diffuse, nonoscillatory Ca²⁺ elevation across the myocyte; 2) localized Ca²⁺ elevation in anatomically restricted areas (Ca²⁺ sparks); and 3) regenerative frontal propagations of Ca²⁺ that traversed the length of the cell (Ca²⁺ waves). The first two patterns were more frequently observed when the extracellular K⁺ concentration was raised to 8 mM. Ca²⁺ waves became more common when extracellular K⁺ concentration was increased to 16 mM, suggesting that a minimum threshold of increase in intracellular Ca²⁺ is necessary for the organization of Ca²⁺ waves. The velocity of propagation was typically ∼60 μm/s with an average frequency of one wave per second crossing at a given point in the cell. Wave propagation resulted in spatial and temporal oscillations in cytosolic and nuclear Ca²⁺ concentration. Treating cardiac cells with aprikalim, a potassium channel-opening drug, prevented 16 mM K⁺ (but not 32 mM K⁺) from inducing an increase in Ca²⁺ concentration and from generating Ca²⁺ waves. In cardiomyocytes treated with glyburide, a selective antagonist of ATP-sensitive K channels, aprikalim failed to prevent 16 mM K⁺ from inducing Ca²⁺ waves. In summary, moderate hyperkalemia induces distinct nonuniform patterns of intracellular Ca²⁺ elevation in ventricular cells, which can be prevented by a potassium channel-opening drug through a glyburide-sensitive mechanism.