Effects of age on mechanical and structural properties of myocardium of Fischer 344 rats.

To determine whether the mechanical and structural characteristics of the myocardium from the left and right ventricles are comparable in the adult rat and whether aging affects the two ventricles in a similar manner, the left and right posterior papillary muscles of rats at 10-11 and 19-20 mo of age were studied in terms of function and quantitative morphology. At 10-11 mo of age, the duration of isometric contraction was shorter and resting tension higher in the right muscle than in the left muscle. Isotonic velocity of shortening and relengthening were quicker in the left, but time-to-peak shortening was faster in the right. With aging (19-20 mo), velocity of shortening fell in the left ventricle while duration of contraction became prolonged. No such age-related effects were found in the right papillary muscle. On a structural basis, myocardial damage in the form of replacement scarring was present in the left and right muscles at 10-11 mo of age. However, the left muscle was more severely injured, and this difference persisted with age because a comparable increase in replacement fibrosis occurred in both muscles. It was concluded that cardiac performance is distinctly different in the left and right papillary muscles and aging exerts its detrimental impact on the left more than in the right myocardium, an observation that is further supported by the magnitude of tissue injury.