Effect of thyroid hormone on canine cardiac Purkinje fiber transmembrane potential

The electrophysiological effects of chronic 3,5,3'-triiodo-L-thyronine (T3) administration on cardiac Purkinje fibers were studied using intracellular recording techniques. Adult mongrel dogs receiving T3 injections (1 mg/kg sc) three times daily for 7-12 days demonstrated elevated resting heart rates and serum total T3 content, atrophied thyroid glands, and a small body weight loss when compared with controls. Isolated Purkinje fibers from control and T3-treated animals were superfused with an oxygenated Tyrode solution containing 4 mM K+ at 35 degrees C. Preparations were stimulated externally at 1.0, 2.0, and 3.1 Hz for 2-3 min while measurements of action potentials were made. At 1.0 Hz, action potential duration (APD) of T3-treated Purkinje fibers was significantly (P less than 0.01) greater than controls at both 20% (APD20) and 80% (APD80) of repolarization to the maximum diastolic potential (Emax). The overshoot, plateau height, and Emax were not different. Raising the stimulus frequency to 2.0 and 3.1 Hz produced a slight increase in the overshoot and a rate-dependent shortening of the action potential duration until it resembled controls. APD20 shortened proportionately more than APD80, giving the action potential a triangular appearance. The K+ selectivity of the resting membrane was not affected as evidenced by Nernst plots, which were identical in normal and T3-treated fibers. These results demonstrate that thyroid hormone influences the action potential of isolated cardiac Purkinje fibers.