Eight canine right hemidiaphragms were electrically stimulated at 2Hz (N = 3) and 10 Hz (N = 5) for up to 135 days. ATPase histochemical studies showed nearly total transformation of type II fast twitch fatigable muscle fibers to type I slow twitch fatigue resistant muscle fibers. The thickness and vascular perfusion of the stimulated muscle increased significantly. Electrical thresholds remained unchanged. The muscle contracted vigorously and without fatigue for up to 135 days at 10 Hz stimulation. Electrical conditioning of canine diaphragm skeletal muscle at 10 Hz frequency for 5 weeks results in a 95% and greater population of slow twitch fatigue resistant muscle fibers. This change in diaphragmatic skeletal muscle should make it a more suitable autograft for replacement or augmentation of diseased cardiac muscle.
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