Stereological characterization of left ventricular cardiomyocytes, capillaries, and innervation in the nondiabetic, obese mouse

Abstract

Background

Obesity is associated with left ventricular hypertrophy and dysfunction, but little is known about the structural remodeling of cardiomyocytes, capillaries, and nerve fibers in this state. We hypothesized that all three compartments should show quantitative structural alterations.

Methods

Ten C57Bl6 mice were randomly assigned to a control or obesity group. Lean mice received standard chow, whereas obese mice received a high-fat Western diet. After 28 weeks, the mice were sacrificed, and the hearts were prepared for design-based stereology using light and electron microscopy.

Results

Body mass and left ventricular mass were significantly elevated in obese vs. control mice. The left ventricular hypertrophy was accompanied by a significant increase in cardiomyocyte lipid droplets and total myocyte volume. The volume fractions of myofibrils, free sarcoplasm, and mitochondria did not differ between the groups. The total length of capillaries was significantly enhanced in obese vs. control mice, whereas the total length of axons ramifying between cardiomyocytes was not different.

Conclusions

Obesity is associated with significant structural alterations in cardiomyocytes and capillaries, whereas no structural changes in the myocardial innervation were observed. The structural characteristics in obese mice do not provide a clear basis for functional changes observed in obesity-related cardiac hypertrophy.

Introduction

Obesity and excess weight have reached pandemic proportions in the Western world due to the high intake of calorie-dense food and a lack of exercise. Associated with obesity are metabolic disorders, such as type II diabetes and the metabolic syndrome, which are in turn linked to the accelerated development of cardiovascular diseases. However, even in the absence of traditional risk factors (e.g., diabetes, arterial hypertension), obesity is considered as an independent pathogenic factor in the development of cardiomyopathy and heart failure in humans [1]. The pathological features of the heart are thought to be characterized by the loss of cardiomyocytes as a consequence of increased apoptosis, with fibrotic tissue replacing lost cells [2], [3], hypertrophy of remaining cardiomyocytes [4], and intracellular lipid accumulation [5]. Functionally, there is evidence for obesity-associated left ventricular dysfunction, particularly in the diastole [6], [7]. The adverse effects of obesity on the structure and function and on the metabolism of the heart were explained by the concept of cardiac lipotoxicity introduced a few years ago [8]. Lipotoxicity in the heart involves the increased uptake of palmitate into cardiomyocytes, accumulation of ceramide, release of Ca²⁺ from the endoplasmic reticulum, generation of reactive oxygen species, release of cytochrome c from mitochondria, and initiation of apoptotic pathways [8], [9].

In addition to the cardiomyocytes, the innervation of the heart also seems to be affected in obese individuals. The left ventricle is densely innervated by adrenergic postganglionic neurons whose perikarya are located in the sympathetic ganglia [10]. In addition to noradrenalin, these postganglionic neurons contain neuropeptides, predominantly neuropeptide Y, which are packed into large dense core vesicles and released upon burst or high-frequency firing [11]. Interestingly, the regional spillover of noradrenalin in myocardium is decreased by 50% in normotensive obese compared to normotensive nonobese humans [12], whereas muscle sympathetic neural activity (measured by microneurography) is significantly increased in obese patients [13], [14]. Numerous studies have addressed the pathological changes of the heart and its innervation in obesity-related disorders, such as diabetes (e.g., Refs. [15], [16], [17], [18], [19]) or hypertension (e.g., Refs. [20], [21], [22]). Although cardiomyocyte alterations in diabetes share some similarities with those in obesity, little is known about the innervation of the myocardium in obesity. Moreover, experimentally induced diabetes is often induced by destruction of insulin-producing pancreatic beta cells and therefore does not mimic obesity-related diabetes. The pathology of the autonomic innervation of the heart in experimental diabetes can therefore not be simply transferred to the state of nondiabetic or prediabetic obesity. Although functional changes of the innervation observed in obesity are likely to alter cardiac function and suggest the presence of a dysfunctional cardiac autonomic innervation, there are currently no qualitative or quantitative studies on the effects of obesity on the morphology of the cardiac innervation.

In the present study, we tested the hypothesis that diet-induced obesity at a pre- or nondiabetic stage is associated with structural alterations of the myocardium and its autonomic innervation in the mouse. To test this hypothesis, we investigated the left ventricular innervation in relation to cardiomyocytes and capillaries by design-based stereology in diet-induced nondiabetic obese mice.