Differences in the temporal effects of aging on the structure and function of rat myocardium

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Abstract
To determine the effects of age on the heart, the mechanical characteristics of left and right ventricular myocardium of rats at 3, 11, and 19 months of age were studied, and these measurements were complemented with quantitative analysis of the capillary network and the subcellular components of myocytes. Several indices of mechanical performance of papillary muscles from both ventricles did not show marked changes with age. However, time-to-peak isometric tension was prolonged, and peak velocities of isotonic shortening and relengthening were decreased in the left muscle at 19 months. Comparisons between left and right papillary muscles indicated that resting tension was greater in the right muscle. Moreover, time-to-peak tension and time-to-one-half relaxation were shorter in the right, while the rate of force rise and velocity of shortening and relengthening were faster in the left muscle. Structurally, a relative decrease in the capillary luminal volume and surface, and an increase in the diffusion distance for oxygen, ie, the average distance from the capillary wall to the mitochondria of myocytes, were demonstrated in both ventricles with aging. The concentration of collagen and ground substance also increased with time. The changes in these quantitative properties occurred first in the left ventricle. The volume fraction of mitochondria and myofibrils in myocyte cytoplasm, however, remained essentially constant in both ventricles at the three age time-points. In conclusion, aging affects the structural properties of the capillary microvasculature, while sparing many mechanical characteristics and the component structures implicated in energy production and contractile performance.