Background
Cardiovascular disease (CVD) accounts for the majority of deaths for people with type 2 diabetes mellitus. CVD is a broad term which includes any condition causing pathological changes in blood vessels, cardiac muscle or valves and cardiac rhythm. The electrocardiogram (ECG) offers a quick, non-invasive clinical and research screen for the early detection of CVD.
Electrocardiographic changes in raw and corrected QT intervals and R wave amplitudes are early indicators of evolving CVD and increased cardiovascular risk. Prolonged QT and QT
c intervals are considered reliable predictors of heart disease and fatal ventricular arrhythmias [
1‐
3]. A positive linear relationship exists between QT
c interval prolongation and diabetic cardiac autonomic neuropathy (DAN) severity in diabetic population [
4]. Heart rate variability (HRV), one indicator of DAN, decreases with diabetes which indicates increased mortality risk [
5]. QT and QT
c interval abnormalities reflect changes in cardiac architecture. A positive correlation between QT or QT
c interval prolongation and left ventricular (LV) mass has been reported [
6‐
8]. LV hypertrophy presents as exaggerated R wave amplitudes on ECG recordings. Elevated R wave amplitudes are an independent risk factor for cardiovascular events [
9]. LV hypertrophy and QT interval alterations coupled with decreased cardiac function are commonly observed with diabetes related CVD [
10].
Non pharmacological interventions for CVD focus primarily on lifestyle changes with physical activity as the primary focus and a risk reduction strategy. Physical activity reduces QT
c interval prolongation and cardiac dysfunction in healthy subjects [
11,
12]. Exercise lowers heart rate and increases HRV in healthy and diseased populations [
13,
14]. Physical activity can serve as potent prescription in the delay and attenuation of the CVD complications for persons with type 2 diabetes but additional comparative studies are needed regarding the cardiac response to exercise under diabetic conditions at various time points of the disease.
The Zucker Diabetic Fatty (ZDF) rat is a model of type 2 diabetes. The ZDF rat develops hyperglycemia and hyperlipidemia by week 8 and overt diabetes by week 12. The progression mimics the obesity-related insulin resistance and inflammation seen in humans [
15,
16]. The ZDF rat is commonly used to investigate prevention of diabetes; however, research related to the diabetic heart disease including ECG studies with this animal model is limited. We hypothesized that ECG changes occur in ZDF rats early in the disease process and aerobic exercise training is able to alleviate the changes. We detected changes in ECG parameters that were partially corrected by exercise training. Our findings add to the characterization of the ZDF model for studying type 2 diabetes effects on the heart and explore the benefits of an early exercise intervention in the presentation and progression of diabetes related CVD.
Discussion
CVD risk is increased up to four-fold in people with diabetes compared with their nondiabetic counterparts [
19]. Researchers are aggressively trying to identify early detection methods and explore the factors contributing to diabetes related heart disease. The ZDF rat model is routinely used to investigate physiological and molecular hypotheses regarding diabetes and its related complications. Reports are available indicating cardiac dysfunction in the ZDF rat established primarily with hemodynamic or echocardiographic measurements or through experiments on the isolated heart [
20‐
23]. However, limited information exists about the ECG changes that occur in the ZDF rat and its response to physical activity, specifically to aerobic exercise that is commonly recommended for those at risk or diagnosed with diabetes related heart disease [
24,
25]. Our project aimed to address this gap in the current literature.
Although we did not measure autonomic function directly, ECG indicators of autonomic dysfunction were observed in the ZDF rats at baseline with HRV alterations, tachycardia and QT interval prolongation. Autonomic innervations control HRV and cardiac function through a delicate balance of sympathetic and parasympathetic responses. Diabetes stimulates the sympathetic nervous system initially but prolonged exposure to hyperglycemia and elevated catecholamine levels cause a decrease in adrenergic receptors [
26]. As diabetes creates a sympathetic predominance, it also produces a corresponding parasympathetic imbalance by denervation of the vagus nerve. Subclinical symptoms of DAN, primarily decreased HRV, are believed to appear in humans within one year of type 2 diabetes diagnosis and clinical presentations may not emerge until years into the diagnosis [
27]. Bergstrom et al identified symptoms of DAN in type 1 diabetic patients with durations of diabetes as low as two months [
28]. In our animal study, diminished HRV was noted in ECG recordings as early as within one week of diabetes onset for the ZDF.
Clinical DAN symptoms include resting tachycardia, exercise intolerance and orthostatic hypotension and heart rate syndromes [
29]. Tachycardia was observed in diabetic animals at baseline, within a week of diabetes onset. Tachycardia is also a common finding in humans with uncontrolled diabetes [
30]. Similarly, our animals were not treated for their hyperglycemia as evident by blood glucose and HbA1C levels. The role of tachycardia is controversial because research indicates that it may be a diabetic complication or a causative factor of diabetes. Nagaya et al argue that elevated resting heart rates and systolic blood pressure increase the risk for type 2 diabetes [
31]. Another work by the same group indicated that prolonged QT
c intervals were also an independent risk factor for the development of diabetes [
32]. Our analysis indicates that widening of QT
c intervals was present with the onset of diabetes at twelve weeks of age in ZDF rats. Thus abnormalities of ventricular repolarization are present at an early stage of diabetes in this model. An earlier time study would be useful to verify the presence of tachycardia and QT
c interval prolongation during a pre-diabetic state. Of interest, several ECG parameters in our study showed an impact of aging, in agreement with findings by others [
33] indicating cardiac and renal changes in the ZDF rat model with aging, up to 36 months.
P wave amplitudes were unaffected with the onset of diabetes. Future studies need to investigate the relationship between the dispersion of the P wave and the onset of diabetes. Obesity is commonly linked to diabetes and is reported to increase the dispersion of the P wave [
34]. PR intervals were widened in diabetic animals at baseline, but normalized at termination. PR intervals are commonly associated with atrial fibrillation [
35,
36]; however our ECG assessments of diabetic animals did not indicate atrial dysfunction. Diabetes and exercise appeared to have no effect on T wave amplitudes. T wave amplitudes were skewed at baseline and therefore change scores were used for analysis. However, the morphology of the T wave has shown to have prognostic value for CVD, not amplitude changes [
37,
38].
After seven weeks of aerobic exercise, ED animals showed R wave amplitudes comparable to control animals. Only SD animals had an increase in R wave amplitude. Hyperglycemia is associated with left ventricular hypertrophy in type 2 diabetes [
39]. The increase in R wave amplitude may indicate a change in left ventricular mass. With diabetes, pathological hypertrophy results from myocardial damage and fibrosis [
40‐
42]. Fibrosis affects the filling and contractility of the ventricles. Subsequently, cardiac dysfunction presents as decreased activity tolerance, ejection fraction, cardiac output and heart failure [
43]. However, other factors can alter R wave amplitudes, including electrical axis deviations, altered electrodes position and differences in chest wall thickness. Ideally, the ECG should be correlated with an echocardiography. Unfortunately, we were unable to use echocardiography approach, thus we recognize this as a limitation of this study. In analyzing other indications of heart hypertrophy, we found no difference in the heart weight/body weight ratio in our study. Alternatively, the LV weight/tibial length ratio has been validated as an index of cardiac hypertrophy in mature rats [
44]. Due to the fact that the onset of diabetes in the ZDF rat occurs early in their age, we were restricted to using relatively young animals that were in the latter stages of their growth phase. Thus, normalizing heart size to the changing tibial bone length during growth may not have provided an accurate estimation in our study. Darmellah et al reported that normalization of the heart weight per body weight or tibial length resulted in similar measurements of cardiac hypertrophy in Goto-Kakizaki animals, another rat model of type 2 diabetes [
45].
Aerobic exercise did not impact the hyperglycemia. Exercise is postulated to improve glucose uptake and decrease lipid accumulation in persons with controlled diabetes but the protective mechanism of exercise is lost if hyperglycemia persist [
46]. A comparative study of fenobirate and metformin validated the role of lipid oxidation in the development and progression of diabetes related heart disease, with fenobirate decreasing triglycerides content and fibrosis in diabetic myocardium [
47]. The switch in myocardial substrate from glucose to fatty acids has been shown to result in systolic and diastolic dysfunction in the ZDF model [
48]. Exercise training has also been suggested to improve microcirculation through enhanced endothelial function [
49] through normalizing glycemic levels. However, the severity of diabetes will determine if the body can adapt to the demands of exercise or regional flow has already been compromised beyond recovery [
50]. Microcirculatory disturbances or small vessel disease may lead to declines in myocardial blood flow which could influence ECG parameters [
51].
A 31% difference in QT intervals was reported between SC and SD animals at baseline. We hypothesize that cardiac remodeling was already in the process in the SD rats when we were taking the baseline measures, prior to any training, due to animals already displaying hyperglycemia and obesity associated with the genotype, in contrast to the SC rat. QTc intervals did not respond to exercise, but the chronic tachycardia shortened the intervals as a compensatory effect. Commonly, exercise may cause a decrease in heart rate and increased ventricular relaxation which presents as longer QT intervals. A study investigating the effects of a seven month endurance training program in dogs revealed an increase in QT intervals [
52]. An acute resistance exercise resulted in a similar effect on QT
c intervals [
53]. Since our obese, diabetic animals were showing early signs of autonomic disturbance, exercise tolerance was lowered and animals required frequent rest breaks during our training program. The mode and duration of exercise might not have been sufficient for QT
c interval adaptation. In a future study we will investigate longer durations of exercise training to see whether it may return diabetic animals to normal heart rates or restore HRV. Pagkalos et al reported improvements in cardiac autonomic function with six months of aerobic exercise training [
54]. Another alternative is the evaluation of HRV during post-exercise recovery. Training may not affect resting HRV, but the benefits may be evident with the post-exercise recovery. This conclusion is supported by a study investigating cardiac autonomic function in women with and without diabetes [
55].
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
LVH performed data analysis and interpretation and drafted the manuscript. YS performed data analysis and interpretation. RL handled the animals, completed the electrocardiograms and participated in data analysis. JV participated in data analysis and interpretation and revision of the manuscript draft. LSB participated in the design of the study and animal handling, contributed to the interpretation of results and revision of the manuscript draft. LN participated in exercise training, data collection and coordination of the study. MAJ participated with the exercise training of the animals and data collection. IVS conceived the study, participated in the design, oversaw all the work, handled animals, performed data collection and revised the manuscript draft. All authors approved of the final manuscript.