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Integrity of the myocardium following a swim to exhaustion (STE) was assessed by quantitative light and electron microscopy in young adult female rats. Myocardial fiber diameter and mitochondrial (MITO) number, size, and volume fraction (VF) were unaffected by the STE, Also, the STE did not alter ventricular water content. In contrast, myofibrillar (MYO) VF was reduced and sarcoplasmic (SARCO) VF elevated (both P less than 0.05) at exhaustion. As both cell and MITO absolute volume were apparently unchanged at exhaustion, an intracellular fluid shift from MYO to SARCO space must have occurred, without affecting MITO volume. Neither swim training nor low-protein diet modified these exhaustion-induced changes. These data provide a possible explanation at the ultrastructural level for the documented reduction in performance of the heart in the exhausted state.

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