Glucose fluctuations increase inducibility of atrial fibrillation in diabetic rats

Abstract

Purpose: Glucose instability in diabetes mellitus has been attracting much attention due to increasing cardiovascular complications more than stable hyperglycemia. Diabetes is one of the major risk factors of atrial fibrillation (AF). Recent studies revealed that atrial fibrosis is common feature of structural remodeling in clinical AF. In the present study, we investigated whether glucose fluctuations aggravate cardiac fibrosis and increase the occurrence of AF in diabetic rats. Methods: Adult male Sprague-Dawley rats were divided into 3 groups: the control group (CNT), diabetes group (DM), and DM with glucose fluctuations group (DF). Diabetes was induced by single intravenous injection of streptozotocin (60 mg/kg). Glucose fluctuations were induced by 24h-fasting and additional insulin injection (0.5 IU/kg) three times a week for consecutive 3 weeks. After glucose fluctuations period, to test the inducibility of AF, we applied programmed electrical stimulation under Langendorff perfusion. Left atrial and ventricular tissues were collected differently to evaluate interstitial fibrosis by Masson trichrome staining. The protein expression relating to profibrotic signals were also estimated by Western blot analysis. Results: Cardiac fibrosis evaluated in atrium and ventricle by histological examination and the expressions of collagen type I and alpha-smooth muscle actin were accelerated in DM more than CNT, which was more pronounced in DF. Consistently, left ventricular diastolic function was more deteriorated in DF compared with DM and CNT. The inducibility of AF by double extrastimuli was significantly increased in DM compared to CNT, which was more exaggerated in DF. To explore the mechanism of cardiac fibrosis, we investigated the level of reactive oxygen species (ROS) and apoptosis. The expression of malondialdehyde, an indicator of ROS level, was significantly up-regulated in DF more than CNT and DF concomitant with increased expression of thioredoxin interacting protein (Txnip). Further, expression of caspase-3 and the number of TUNEL-positive cells were significantly increased in DF group. These results indicate that up-regulation of Txnip and resulting oxidative stress may facilitate apoptosis and cardiac fibrosis. Conclusions: Glucose fluctuations increased the inducibility of AF. Atrial fibrosis and left ventricular diastolic dysfunction are possible mechanisms of increased susceptibility to AF.