MATERIALS AND METHODS: We tested a hypothesis implicating Na+ current alterations as a mechanism underlying these electrophysiological phenotypes. We applied loose patch-clamp techniques to intact young and aged, WT and Pgc-1β-/-, atrial cardiomyocyte preparations preserving their in vivo extracellular and intracellular conditions.
RESULTS AND DISCUSSION: Depolarising steps activated typical voltage-dependent activating and inactivating inward (Na+) currents whose amplitude increased or decreased with the amplitudes of the activating, or preceding inactivating, steps. Maximum values of peak Na+ current were independently influenced by genotype but not age or interacting effects of genotype and age on two-way ANOVA. Neither genotype, nor age, whether independently or interactively, influenced voltages at half-maximal current, or steepness factors, for current activation and inactivation, or time constants for recovery from inactivation following repolarisation. In contrast, delayed outward (K+) currents showed similar activation and rectification properties through all experimental groups. These findings directly demonstrate and implicate reduced Na+ in contrast to unchanged K+ current, as a mechanism for slowed conduction causing atrial arrhythmogenicity in Pgc-1β-/- hearts.
EXPERIMENTAL APPROACH: We compared the ability of fully charged (QX-FL) and neutral (NU-FL) derivatives of flecainide to block individual recombinant human RyR2 channels incorporated into planar phospholipid bilayers, and their effects on the properties of Ca(2) (+) sparks in intact adult rat cardiac myocytes.
KEY RESULTS: Both QX-FL and NU-FL were partial blockers of the non-physiological cytosolic to luminal flux of cations through RyR2 channels but were significantly less effective than flecainide. None of the compounds influenced the physiologically relevant luminal to cytosol cation flux through RyR2 channels. Intracellular flecainide or QX-FL, but not NU-FL, reduced Ca(2) (+) spark frequency.
CONCLUSIONS AND IMPLICATIONS: Given its inability to block physiologically relevant cation flux through RyR2 channels, and its lack of efficacy in blocking the cytosolic-to-luminal current, the effect of QX-FL on Ca(2) (+) sparks is likely, by analogy with flecainide, to result from Na(+) channel block. Our data reveal important differences in the interaction of flecainide with sites in the cytosolic vestibules of Na(+) and RyR2 channels.