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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.

The effect of prophylactic digitalization on the development of left ventricular hypertrophy was studied in adult rats. Digitoxin, 0.1 mg/100 g body wt or solvent was given daily for 1 wk prior to either aortic constriction or sham operation and was continued until the animals were killed, either 1 or 4 wk after surgery. A hemodynamic study was done in those animals killed 1 wk after surgery; hearts of all animals were examined for evidence of myocardial hypertrophy. Constriction of the ascending aorta had no significant effect on cardiac output but did reduce peak flow velocity and flow acceleration. An increase in left ventricular mass, RNA, and hydroxyproline was found in the animals with aortic constriction. Digitoxin treatment did not alter peak flow velocity or flow acceleration, but did significantly increase isovolumic (dP/dt)P-1. Digitoxin had no effect on body weight, heart weight, RNA, or hydroxyproline in either the sham-operated animals or in the animals with aortic constriction. Therefore, despite plasma digitoxin levels sufficient to affect myocardial contractility, left ventricular hypertrophy still developed after aortic constriction.