Do Rats Have Epicardial Adipose Tissue?

The most frequently used laboratory animals for studies on adipose tissue properties and obesity are rodents. However, there are significant differences in the types of visceral fat depots between rodents and humans, including fat depots in the heart area. The large human fat depot of greatest interest in cardiac research is the epicardial adipose tissue (EAT). Its properties are widely investigated, because the EAT lies directly on the heart's surface and can easily affect myocardial physiology. The major fat depot in rodents' chest-pericardial fat-is located on the ventral surface of the parietal lamina of the pericardium and is often incorrectly referred to as the EAT. Further confusion arises from reports claiming that rodents are entirely devoid of the EAT. We decided to verify adipose tissues in the heart area of 16 male Sprague Dawley rats under physiological conditions and in obesity. The animals in the NFD group (n = 8) were fed with a standard diet while these in the HFD group (n = 8) were fed with a high-fat diet (31% fat) starting from 4 weeks after birth. When the animals reached 12 weeks, the presence of fat deposits was verified. Additionally, their blood was collected to characterize carbohydrate and lipid metabolism changes, adipokine profile alterations, and their systemic inflammation status. The obesogenic diet caused significant disturbances in their carbohydrate and lipid metabolism, as well as hyperleptinemia. A high-fat diet primarily promoted the accumulation of pericardial fat, which was absent in the NFD rats and observed in 6 out of the 8 HFD animals. In both groups, adipocytes were also found directly on the hearts' surfaces (EAT), albeit in very small numbers and limited to the atrioventricular groove on the dorsal side of the hearts. These adipocytes were dispersed among the vessels, making quantitative assessment and separation difficult, however, macroscopic evaluation revealed no noticeable differences in its extent. In conclusion, although rats are not entirely devoid of the EAT, their suitability for studying the properties of the EAT appears to be considerably limited.