Ionic Mechanisms that Underlie Ventricular Action Potential Prolongation following Loss of Caveolin-3 in Adult Transgenic Mice

Abstract

Caveolin proteins are involved in establishing membrane microstructure, lipid raft organization, and cell signaling. In the heart, caveolin-3 (Cav3) predominates. Inherited or disease-induced Cav3 loss increases risk of sudden cardiac death (SCD). We aimed to explore connections between Cav3 loss and arrhythmogenic changes in the ventricular action potential (AP) by investigating the Cav3 dependence of ionic currents. Drugs commonly used to disrupt or remove Cav3 in cultured cells exclude any compensatory process likely to occur in vivo. This motivated us to engineer a novel conditional Cav3 knockout (Cav3-/-) mouse that survives to adulthood. We isolated ventricular cells for electrophysiological experimentation. AP duration (APD90) was prolonged from 2454 ms in WT to 9659 ms in Cav3-/-, and several currents were affected. Reduced peak: L-type Ca 2þ current (ICaL), 21%; slow K þ current, 81%; transient outward K þ current, 57%; steady state outward K þ current (Iss), 43%. Late Na þ current was enhanced~10-fold. These changes were partially offsetting -preventing a simple account for the APD90 increase. To relate changes in currents to changes in the AP, we developed a computational representation of Cav3-/-based on the Morotti et al. mouse ventricular cell model and defined by fractional change in currents. Unexpectedly, the relatively small change in relatively small Iss caused 33% of total simulated AP prolongation. Though Iss conductance was reduced, peak Iss actually increased in the dynamic setting of the simulated AP. Early in the AP, lower Iss indirectly enhanced inward currents (importantly late ICaL) by extending the plateau phase, which in turn allowed Iss to more fully activate. This Iss/ ICaL process largely accounted for the pro-arrhythmic APD90 increase following Cav3 loss and is therefore a candidate target for normalizing SCD risk. Diabetes mellitus is associated with sinoatrial node dysfunction, as evidenced by an increased risk of atrial fibrillation, pacemaker implantation due to bradycardia and cardiac death in diabetic patients. While sinoatrial node myocytes (SAMs) generate the spontaneous action potentials (APs) that initiate each heartbeat, little is known about how diabetes affects SAMs directly. In this study, we used streptozoticin (STZ) -treated mice as a model of diabetic hyperglycemia. Four weeks after STZ injections, we found that both intrinsic heart rate (measured during autonomic blockade) and maximum heart rate (measured during restraint stress) were reduced in diabetic animals compared to pretreatment values. Current-clamp recordings from acutely isolated SAMs from diabetic animals revealed corresponding reductions in spontaneous AP firing rates. AP waveform analysis showed that the reduced firing rates in diabetic cells resulted from a prolongation of the AP duration and a slowing of the rate of repolarization. Accordingly, we observed significant decreases in steady state and transient outward K þ current densities in whole-cell voltage-clamp recordings from SAMs from diabet...