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An intravenous infusion of a large dose (0.51 mg/kg) of norepinephrine sensitizes the heart to the action of catechol amines for a period of 2–4 days. During this period, small doses of either epinephrine or norepinephrine induce ventricular tachycardia. In dogs killed the day after infusion, frozen sections of the myocardium stained with Oil red O reveal fatty changes, the severity of which roughly parallels that of the induced tachycardias. Fatty changes were still present in the hearts of dogs killed the 3rd day after infusion, but were negligible 5 days after infusion. Similar, but less marked, cardiac effects were elicited by infusions of large doses of epinephrine.

Young adult male rats received 1–20 successively daily 6-hr exercise tests in a rotating cage. Serum glutamic oxalacetic transaminase and aldolase and blood urea nitrogen values rose sharply and serum alkaline phosphatase fell immediately after each of the first four tests with only partial recovery after an overnight rest. Subsequently, the serum alkaline phosphatase and blood urea nitrogen values returned to normal, but the transaminase and aldolase values were slightly elevated even after 17–20 tests. Weight loss was 15% in 3–6 days and 7% after 17–20 tests. Transient fatty changes were noted in the thigh muscles after the first test. Necrotic muscle lesions, most pronounced after three tests, regressed after the 1st week. Rats given 17–20 successive daily 6-hr exercise tests, contrary to untrained rats, showed no apparent fatigue, no muscle lesions, and no significant changes in serum enzymes immediately after a 16-hr exercise test. These findings emphasize the importance of properly graded training exercises.