The Effects of Exercise Training (ET) on Cardiac Arrhythmias and Left Ventricular (LV) Connexin 43 (Cx43) Expression in Aged and Young Rats

Reduced LV expression of the myocardial gap junction protein Cx43 is associated with increased arrhythmogenesis. The occurrence of cardiac arrhythmias increases with age although mechanisms are unknown. We hypothesized that aging would result in increased cardiac arrhythmias and decreased LV Cx43 expression while ET would reverse these effects in young (4–6 mo) and aged (24–25 mo) F344 rats. Rats underwent 10–11 weeks of treadmill ET or sedentary handling. Subcutaneous electrocardiographic leads were implanted after the young sedentary (YS n=9), young exercise (YEx n=7), aged sedentary (AS n=6), and aged exercise (AEx n=6) protocols to allow for data acquisition via the Actiwave telemetry system. The arrhythmic index (AR) was calculated using a modified scoring system during baseline (BL), sympathoexcitation (isoproterenol (ISO), 0.15mg/kg, s.c.) and brief restraint (BR). The total AR (BL+ISO+BR) was significantly higher in aged vs young rats (p<0.05) while ET showed a trend toward reversing this effect (p=0.065). Western blot analysis (YS=3, YEx=3, AS=3, AEx=2) revealed reduced Cx43 expression in aged vs young rats while exercise partially reversed this effect in AEx rats. These data support the hypothesis that ET provides a protective benefit against cardiac arrhythmias in young and aged rats. Future studies will further investigate the role of Cx43 in mediating this response.