Heart functional responses to pressure overload in exercised and sedentary rats.

Female rats that had been subjected to a moderate treadmill running program were compared with sedentary animals on the basis of heart weight, selected biochemical measurements, and heart function. Exercised animals maintained normal growth rate, and cardiac hypertrophy was not present. Left ventricular RNA, DNA, and cytochrome c levels were unchanged. Heart functional measurements obtained in situ were similar in sedentary and exercised animals under control conditions. When subjected to sustained (1-3 days) aortic constriction pressure overload, exercised animals maintained or increased myocardial contractility. Contractility was depressed in sedentary animals. Both sedentary and exercised animals increased left ventricular end diastolic pressure without changing contractility during acute (1-3 min) pressure overload. However, exercised animals were able to fully regain normal cardiac output when the acute overload was relieved. Cardiac output remained approximately 10% below control in sedentary animals. The improved ability of previously exercised animals to withstand pressure overload appears to be due to alterations in adaptation rather than preliminary augmentation of metabolism or function.