Age-dependent alterations in rat ventricular myocardium: a quantitative analysis.

A quantitative electron microscopic examination of the ventricular myocardium of the male Fischer 344 rat was undertaken to determine the extent of age-dependent changes in ultrastructure. Rats of 6, 16 and 30 months of age were examined using a non-biased stereological test system. Volume fractions of mitochondria, myofibrillar mass and lipid remained unchanged during aging. Dense body volume fraction increased four-fold over the period studied, while the sarcoplasmic reticulum volume increased only from 6 months to 16 months and did not change thereafter. Biochemical analysis of the specific activities of the two lysosomal enzymes, acid phosphatase and beta-glucuronidase demonstrated a differential response to aging, with acid phosphatase remaining constant and beta-glucuronidase increasing slightly. Neither enzyme activity correlated with the pronounced change in dense body volume fraction. This study provides clear evidence that the previously observed changes in cardiac performance associated with aging are not readily explicable on the basis of a degradation of cardiac ultrastructure.