Unusual resistance of the ground squirrel to the development of dietary-induced hypercholesterolemia and atherosclerosis

This study was designed to examine the apparently unusual resistance to atherosclerosis in the adult male ground squirrel. In one study, serum lipid and lipoprotein patterns and concentrations of ground squirrels were observed after feeding for 26 weeks with either rodent chow or rodent chow supplemented with cholesterol. These two groups were compared to a third group fed chow diet for three weeks, then killed for baseline values to distinguish between aging effects and those induced by cholesterol feeding. No outstanding changes in the concentration and patterns of the serum cholesterol, triglycerides, and lipoproteins were observed in any groups. A higher serum phospholipid concentration in cholesterol-fed animals compared to animals fed chow diet alone for three weeks suggested that the rise was associated with aging rather than diet. The predominant lipoprotein fraction in all three groups was high-density lipoprotein (HDL). No lesions were found in the coronary arteries or thoracic and abdominal aortas.In a second study a primate diet with 50% egg yolk was given for 1 year. This diet caused elevation of serum cholesterol and phospholipid levels, and increased HDL, which had an unusual ability to carry as much cholesterol as the low-density lipoprotein (LDL). However, no arterial lipid accumulation was observed. Light microscopic and ultrastructural studies of the coronary artery and aorta revealed nothing unusual, and in structure, they resembled those of other small rodents. The only striking finding was that very few degenerative aging changes were found in the aorta when compared to rats of a similar age. The study demonstrates that the ground squirrel does not develop marked hypercholesterolemia with cholesterol feeding and that the majority of the elevated levels of serum lipids, particularly cholesterol, are transported as α-lipoprotein. Both of these factors, plus the lack of degenerative aging changes in the arterial wall may play an important role in the resistance of this particular species to atherosclerosis.