Effect of Age on Blood and Tissue Lipid Levels in the Male Rat

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Abstract
Blood glucose concentration and lipid content of plasma, liver, heart and white and red skeletal muscle was determined per unit wet weight in male rats 1, 4, 9 and 18 months old. Blood glucose levels were slightly higher at 1 and 9 months than at 4 and 18 months. At these ages the concentration of free fatty acids and glycerol in plasma were lowest. The amount of cholesterol and phospholipids in plasma remained unchanged from 1 to 4 months, but increased then continuously. The plasma triglyceride level rose slightly from 1 to 4 months and sharply so from 4 to 9 months whereafter it was constant. In the liver the cholesterol concentration increased and the phospholipid content decreased continuously with age. The triglyceride content showed no definite age trend. There was no consistent age trend for the amount of lipids in the heart. The concentration of cholesterol followed the same pattern in red and white type of skeletal muscle with an initial drop, a constant level from 4-9 months and then an increase. The phospholipids decreased in both muscle types from 1 -4 months and then remained constant. The triglyceride content of the white type remained constant up to the age of 9 months. In the red type there was a decrease from 1-4 months and a constant triglyceride level up to 9 months. From 9-18 months there was, in both muscle types, a manifold increase of the triglyceride content. The mechanisms for the demonstrated changes in lipid content of various tissues with age are largely unknown, but some possible mechanisms were discussed. The similarities between the changes with age in plasma lipids in man and rat were stressed and the use of the rat as a model for further elucidation of age-dependent changes in lipid metabolism was suggested.