The most prevalent form (90%) of diabetes mellitus is Type II (non-insulin-dependent) diabetes [1]. Epidemiological and clinical evidence shows that type II diabetes mellitus is a prevalent disease that results in a marked increase in cardiovascular complications that are in part due to a specific cardiomyopathy, characterized by ventricular dysfunction in the absence of atherosclerotic coronary heart disease or hypertension [2, 3]. Abnormalities in cardiac function and structure in diabetic subjects have been demonstrated in animal [4‐7] and human studies [8‐11]. However, there is still controversy regarding the severity or time of appearance of cardiac impairment following the appearance of type II diabetes.

Most reports of diabetes-induced cardiac dysfunction have used insulin-deficient (TypeI) models [12‐14]. Relatively few studies on cardiac function have been conducted with type II diabetic animal models [15]. The leptin receptor-deficient db/db mouse is a well established model of type II diabetes, with obesity and insulin resistance [16, 17]. In vivo evidence for left ventricle structure and dysfunction in diabetic mice is commonly obtained from conventional echocardiography (M-mode, 2D, Doppler) [18, 19], which is typically considered late manifestations of disease and insensitive to detect subtle left ventricle dysfunction. The presence of resting abnormalities of sensitive indices of myocardial function measured by speckle tracking echocardiography (STE) has been attributed to a subclinical diabetic cardiomyopathy. Tracking the movement of myocardial tissue, STE is used to determine longitudinal strain, radial and circumferential strain for detecting global and regional myocardial dysfunction [20]. Therefore, the objective of this study was to use STE to assess systolic myocardial strain in vivo with diabetic db/db and control mice in order to study early changes of left ventricle contractile function in type II diabetes model.

All studies have been approved by Beijing Anzhen Hospital Ethics Committee and performed in accordance with the ethical standards.

Diabetic db/db mice provide one of more commonly used animal models used to investigate diabetic cardiomyopathy, with obesity and insulin resistance [16, 17]. In the literature, db/db mice develop severe dabetes by 8 weeks of age [22, 23]. Our study showed at 8, 12 and 16 weeks of age, diabetic (db/db) mice weighed significantly more than control mice. As expected, at 8 weeks of age diabetic mice had significantly elevated fasting blood glucose with hyperglycemia.

Diabetic cardiomyopathy is accompanied by functional, structural and pathologic cardiac characteristics. Echocardiography is a noninvasive method that can assess cardiac function in mice [24‐26]. We used M-mode measurements of LV dimensions in end systole and end diastole and myocardium thickness to provide information on morphology and systolic function. However, we observed that left ventricular dimensions and myocardium thickness in db/db mice were not significantly increased until 16 weeks of age compared to control mice. The parameters reflecting the left ventricular systolic function such as LVEF and LVFS could not be detected unchanged in db/db mice. These findings were supported by Barouch and colleages as they reported echocardiographically in 24-week-old db/db mice on C57BL/6J background an unchanged LVEF and FS [27]. However, conventional echocardiographic measures lack sensitivity for capturing subtle variations in left ventricular (LV) performance. Changes in LV structure and global function, when detected by conventional echocardiographic parameters, are typically considered late manifestations of disease.

Speckle tracking echocardiography (STE) is a novel, non-Doppler-based technique used to detect myocardial wall motion and myocardial deformation. In clinical studies STE based on tissue deformation has provided an improved accuracy of myocardial contractility and quantification of global and regional myocardial function. Cardiovascular risk factors result in impaired strain measures before the development of decreased LVEF or ventricular dilatation [28, 29]. In experimental studies, the adoption of strain analysis in small animal models has been limited, primarily because of technical differences in imaging mice versus humans, including limited echocardiographic views, translational motion during image acquisition, and the effect of very high heart rates. However, recently developed speckle-tracking based techniques now allow for angle-independent, reproducible, and accurate strain measurements that may be applied to mice. STE can accurately predict pathophysiology during the evolution of MI [30, 31] and TAC-induced heart failure in mice [32]. LV contraction is a complex process involving deformation resulting in shortening in 3 normal directions; longitudinal, radial strain (RS) and circumferential strain (CS). RS and CS are more influenced by transmural fiber dysfunction (especially the midmyocardium), and are generally more suited for identifying dysfunction in ventricles with reduced left ventricle systolic function. Therefore our study used STE to assess systolic myocardial radial strain and circumferential strain with diabetic db/db and control mice. With the use of STE, we were able to detect lower myocardial radial and circumferential strain in db/db mice at 16 weeks of age, indicating LV contractile function was indeed abnormal in this type II diabetic mice. Our study showed that subtle abnormalities in strain were associated with the presence of diabetic cardiomyopathy even in the setting of normal cardiac structure and function by conventional measures. In other words, the present study therefore indicate that radial and circumferential strain are more sensitive and can be used for detection of early LV contractile dysfunction in this type II diabetic mice model.

It should be mentioned that the HRs play an important role in the determination of cardiac function in mice by echocardiography. Inhalation anesthesia with isoflurane has currently been considered ideal and useful for experimental studies in the mouse because of its rapid conduction, easy control of the depth of anesthesia, and stable HRs during observations [33, 34]. Usually, the range of HRs in anesthetic mice for echocardiography is 300 to 550 beats per minute (bpm), as reported [35]. In our experiment HRs were not significantly different between db/db mice and age-matched controls. Heart rate values of our mice were low but in this range. It may be related to anesthesia time.

The present study shows that speckle tracking echocardiography can be used to evaluate cardiac functional alterations in mouse models of cardiovascular disease. Radial and circumferential strain are more sensitive and can be used for detection of early left ventricular contractile dysfunction in db/db type II diabetic mice.