Effects of aging on single cardiac myocyte function in Fischer 344 x Brown Norway rats.

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The Fischer 344 x Brown Norway (F344xBN) rat has been demonstrated to have a lower incidence of age-related pathology than other rat strains. Therefore, to elucidate the effects of aging on cardiac function, uncomplicated by compensatory effects caused by age-related pathology, cardiac myocytes were isolated from female F344xBN rats at 6 (young) and 32-33 (old) mo of age. Myocytes showed an increase in the relative amount of beta-myosin heavy chain with advanced age and a significant rightward shift in the tension-pCa curve from 5.78 +/- 0.02 pCa units in young adult myocytes to 5.66 +/- 0.03 pCa units. Consistent with a shift to a slower myosin isoform, the time from stimulation to peak sarcomere shortening increased with age from 50.5 +/- 1.3 to 58.9 +/- 1.0 ms. In contrast, no age-related difference was found in either the relengthening parameters or the Ca(2+) transient, indicating that relaxation is not directly altered by aging. This latter finding is at variance with previous studies in rat strains with higher rates of pathology. We conclude, therefore, that the primary effect of aging in isolated cardiac myocytes from the F344xBN rat model is a shift in the myosin heavy chain isoform. Changes in relaxation seen in other rat strains may result from compensatory mechanisms induced by pathological conditions.