Sinoatrial node dysfunction induces cardiac arrhythmias in diabetic mice

Soltysinska et al. Cardiovascular Diabetology 2014, 13:122 http://www.cardiab.com/content/13/1/122 CARDIO VASCULAR DIABETOLOGY ORIGINAL INVESTIGATION Open Access Sinoatrial node dysfunction induces cardiac arrhythmias in diabetic mice Ewa Soltysinska, Tobias Speerschneider, Sine V Winther and Morten B Thomsen* Abstract Background: The aim of this study was to probe cardiac complications, including heart-rate control, in a mouse model of type-2 diabetes. Heart-rate development in diabetic patients is not straight forward: In general, patients with diabetes have faster heart rates compared to non-diabetic individuals, yet diabetic patients are frequently found among patients treated for slow heart rates. Hence, we hypothesized that sinoatrial node (SAN) dysfunction could contribute to our understanding of the mechanism behind this conundrum and the consequences thereof. Methods: Cardiac hemodynamic and electrophysiological characteristics were investigated in diabetic db/db and control db/+ mice. Results: We found improved contractile function and impaired filling dynamics of the heart in db/db mice, relative to db/+ controls. Electrophysiologically, we observed comparable heart rates in the two mouse groups, but SAN recovery time was prolonged in diabetic mice. Adrenoreceptor stimulation increased heart rate in all mice and elicited cardiac arrhythmias in db/db mice only. The arrhythmias emanated from the SAN and were characterized by large RR fluctuations. Moreover, nerve density was reduced in the SAN region. Conclusions: Enhanced systolic function and reduced diastolic function indicates early ventricular remodeling in obese and diabetic mice. They have SAN dysfunction, and adrenoreceptor stimulation triggers cardiac arrhythmia originating in the SAN. Thus, dysfunction of the intrinsic cardiac pacemaker and remodeling of the autonomic nervous system may conspire to increase cardiac mortality in diabetic patients. Keywords: Sinus node, Heart rate variability, ECG, Sympathetic nervous system Background Diabetes mellitus is a serious public health concern and the global prevalence was estimated to be 2.8% in 2000 rising to 4.4% in 2030 [1]. The incidence of cardiovascular disease is higher among diabetic patients [2], and they often die from cardiovascular complications [3]. Coronary artery disease secondary to atherosclerosis account for a large fraction of the comorbidity, likely underlying the reports of myocardial infarction as the main cause of mortality in diabetic patients [3]. The vast majority of diabetic patients have type-2 diabetes mellitus (T2DM), characterized by obesity, insulin resistance and very high blood glucose levels. * Correspondence: From the Danish National Research Foundation Centre for Cardiac Arrhythmia, Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Blegdamsvej 3b; bldg.: 12.5.36, Copenhagen DK-2200, Denmark Elevated resting heart rate is associated with an increased risk of cardiovascular complications and sudden cardiac death in the general population and in T2DM patients [4-6]. Conversely, in patient groups with electronic cardiac pacemakers due to slow heart rates, there is a statistically significant overrepresentation of diabetic patients, suggesting diabetes-induced impairment of the endogenous, natural pacemaker of the heart [7-9]. Sick sinus node syndrome is associated with T2DM in case reports only [10]. Reports of impaired atrio-ventricular node in diabetic patients [11,12] supports the clinical observation that the cardiac conduction system, especially the function of the nodes, are compromised in T2DM. Hence, there are strong clinical indications of altered sino-atrial node (SAN) function in T2DM and that this could contribute to the increased cardiovascular mortality in this large patient population. © 2014 Soltysinska et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Soltysinska et al. Cardiovascular Diabetology 2014, 13:122 http://www.cardiab.com/content/13/1/122 Leptin-receptor deficient homozygous db/db mouse lacks the hypothalamic leptin regulation resulting in development of obesity and severe T2DM [13]. The model shows age-dependent progression of metabolic abnormalities that mimics the pathogenesis of T2DM in humans, including early insulin resistance and hyperinsulinemia at mouse age 6–12 weeks, followed by an insulin-secretory defect and hypoinsulinemia after 12 weeks [14]. Previous experimental studies related to effects of diabetes on SAN function have been conducted primarily in T1DM animal models [15-17], which lack the metabolic complexity of T2DM. Reports from heart rate recordings in conscious or anesthetized db/db mice with T2DM have revealed profound inconsistencies in results with examples of faster [18], slower [19,20] and comparable [21-23] resting heart rates. The clinical finding of SAN dysfunction in a small subset of diabetic patients and the discrepancies in the experimental findings of heart rate in db/db mice imply that this mouse model may constitute a unique tool for studying the effects of diabetes on SAN function. In the present study, we hypothesized that db/db mice have a concealed SAN dysfunction in vivo that can be unmasked by the appropriate stress. We probed and challenged SAN function to elicit changes in heart rate and heart rate instability compatible with the clinical observations of augmented cardiovascular death. At basal conditions, heart rate is not different in anesthetized db/db mice; however, upon a challenge with β-adrenoreceptor stimulation we exposed pronounced SAN dysfunction inducing prominent heart-rate fluctuations. Invasive electrophysiological studies showed a prolonged sinus node recovery time and immunoblots of SAN tissue identified reduced density of autonomic nerve endings. Methods Animals Experiments were performed using male leptin-receptor deficient db/db mice (C57BL/KS-leprdb/leprdb) and lean control heterozygote db/+ mice (C57BL/KS-leprdb/lepr+), aged 14–16 weeks. Animals were purchased from Taconic (Denmark) and housed in a specific pathogen free facility with ad libitum access to water and standard chow food in a room with a 12-h light/dark schedule and an ambient temperature of 22°C. Mice were anesthetized in 1.5-2% isoflurane in 100% O2. Body temperature was constantly monitored and kept at 37 ± 0.5°C. Blood glucose levels were determined from the tail vein as the mean of 2 consecutive measurements by a Microdot glucometer (Kacey Diagnostics, USA). Bod (...truncated)