Effects of altitude and exercise intensity on cardiac function in rats.

High-altitude exercise affects cardiac function. This study investigated how altitude and exercise intensity interacted to affect cardiac function of Sprague-Dawley rats. Four altitudes (410, 3600, 4600 and 5600 m) and three exercise intensities (non-exercise (N), low-intensity exercise (L) and high-intensity exercise (H)) were tested combinatorically. After 4 weeks of exercise, cardiac function and specific markers of myocardial injury were assessed. With regard to cardiac function, (a) at the same intensity, stroke volume and left ventricular end-diastolic volume were higher in the 3600 m group but lower in the 4600 and 5600 m groups; and (b) the heart rate increased with altitude and intensity. The biochemical results showed that the levels of creatine kinase, myoglobin and cardiac troponin I generally increased with increasing altitude and exercise intensity, significantly for creatine kinase and myoglobin at 4600 and 5600 m. For pathological results, (a) in the non-exercise group, pathological damage was observed only in the 5600 N group; and (b) in the exercised state, varying degrees of injury were noted, except for the 410 and 3600 L groups. There may be a turning point at 3600 m where the injury to the heart increases. Myocardial injury markers exhibited abnormalities before cardiac dysfunction. Detecting these markers is crucial to provide warnings for the individual from cardiac disease during high-altitude exercise.