Histological evidence for reversible cardiomyocyte changes and serum cardiac troponin T elevation after exercise in rats

This study characterized cardiac troponin T (cTnT) appearance and associated histological evidence of reversible or irreversible changes in myocardial ultrastructure, determined via electron microscopy, in rats undertaking isoproterenol (ISO) infusion or an endurance exercise challenge. Male rats were randomized into ISO and exercise groups. In ISO trials rats were killed 5 h (ISO-5H) and 24 h (ISO-REC19H) after a single ISO or saline injection (SAL-5H; SAL-REC19H). In the exercise trials rats were killed before, as a control (EXE-CON), immediately after (EXE-END5H) and 19 h after (EXE-REC19H) a 5-h bout of swimming with 5% body weight attached to their tail. Serum cTnT was quantified by electrochemiluminescence, and myocardial samples in ISO-REC19H, EXE-REC19H and SAL-REC19H were harvested for assessment of specific mitochondrial injury scores using electron-microscopy. cTnT was undetectable in all control animals (SAL-5H/SAL-REC19H and EXE-CON). cTnT increased in all animals after ISO and exercise but the response was significantly higher (P < 0.05) at ISO-5H (median [range]: 2.60 [1.76–6.18] μg · L^-1) than at EXE-END5H (median [range]: 0.05 [0.02–0.14] μg · L^-1). cTnT returned to baseline at EXE-REC19H, but had not completely recovered at ISO-REC19H (median [range]: 0.17 [0.09–1.22] μg · L^-1). Mitochondrial “injury scores” were significantly higher (P < 0.05) in ISO-REC19H compared to EXE-REC19H and SAL-REC19H, with no difference between EXE-REC19H and SAL-REC19H. Mitochondria from EXE-REC19H appeared aggregated in nonlinear clusters in a small number of scans. These findings suggest that acute exercise-induced appearance of cTnT in this animal model is only associated with reversible changes in cardiomyocyte structure.