After the 5-week treatment period, animals were anesthetized (pentobarbitone sodium, 60 mg/kg intraperitoneal), tracheostomized, and placed on mechanical ventilation. A femoral artery and vein were cannulated with the aid of a microscope in order to record arterial BP and to administer supplementary anesthetic, respectively. Core temperature was maintained at 37°C using a thermostatically controlled heating blanket and radiant heat. The digastric muscles were separated to reveal the omohyoid muscle, which was cut to expose the underlying geniohyoid muscles that run as two parallel strips from the midpoint of the mandible to the hyoid bone. The sternohyoid muscles running from the sternum to the hyoid bone were also exposed. The muscles were removed rapidly and prepared for adenosine triphosphatase (ATPase) staining or for contractile studies using longitudinal strips of 1 to 2 mm in diameter that were suspended vertically in a water-jacketed bath in warmed (30°C), oxygenated (95% oxygen, 5% carbon dioxide) physiologic saline solution (pH 7.4) containing the following: NaCl, 120 mmol; KCl, 5 mmol; Ca gluconate, 2.5 mmol; MgSO4, 1.2 mmol; NaH2PO4, 1.2 mmol; NaHCO3, 25; and glucose, 11.5 mmol. One end of the muscle strip was fixed, and the other end was attached to an isometric force transducer mounted on a micropositioner. Isometric twitch tension, tetanic tension, twitch/tetanic tension ratio, contraction time, half-relaxation time, the tension-frequency relationship, fatigue, and recovery from fatigue were measured using field stimulation (supramaximal voltage, 1 ms in duration) with platinum electrodes and recorded using an analog-to-digital converter and microcomputer.