Abstract 202: Aging Alters the Electrical and Mechanical properties of LV Myocytes

Abstract

Aging results in delays in the electrical recovery of the heart, enhancing the risk of malignant ventricular arrhythmias and sudden death. But whether altered electrical properties affect myocardial mechanical performance remains unclear. The aim of this study was to establish the ionic basis for the protracted repolarization of the senescent heart, and to determine whether these electrical changes impact on myocardial contractility. For this purpose, electrophysiological analyses were conducted in vivo and in isolated LV myocyte preparations from mice ranging from 3 to 30 months of age. Electrocardiographic parameters were preserved from 3 to 12 months of age, whereas animals 2 years or older presented prolonged PR, QRS and QT intervals. These in vivo results were confirmed in the isolated organ, using Langendorff perfused hearts. Myocytes from 30 months old mice showed longer early and late repolarization phases of the action potential (AP), in comparison with cells from animals at 3 months. By voltage-clamp, the density of the transient outward K+ current (Ito) was significantly reduced in old, whereas the late Na+ current (INaL) was increased, changes consistent with the prolonged AP of old cells. Additionally, by Western blotting, the SCN1B subunit, involved in Na+ channel gating, was reduced with age. Using field stimulation, old myocytes presented slower Ca2+ transient decay and relaxation, in comparison to young. Blockade of INaL with mexiletine in old myocytes shortened the AP duration, and fastened the decay of Ca2+ transients and relaxation. To assess the functional impact of INaL on the myocardium, papillary muscles were studied in an isometric system. Using tension-length protocols, old muscles presented elevated diastolic tension with respect to young. Importantly, inhibition of INaL in old muscles reduced diastolic tension, and attenuated the active developed force. Conversely, increasing INaL in the myocardium of young mice with anemonetoxin had opposite effects. Overall, these data indicate that aging results in a prolongation of the AP mediated, at least in part, by enhanced INaL. The prolonged repolarization of the AP contributes to the slower clearance of diastolic Ca2+ from the cytoplasm, resulting in protracted relaxation phase.