title: Effect of thyroid hormone on canine cardiac Purkinje fiber transmembrane potential.
abstract: 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.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D004285:Dogs; D004558:Electric Stimulation; D005260:Female; D006321:Heart / Q000502:physiology* / Q000503:physiopathology; D006329:Heart Conduction System / Q000502:physiology*; D006980:Hyperthyroidism / Q000503:physiopathology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000494:pharmacology; D011690:Purkinje Fibers / Q000187:drug effects / Q000502:physiology*; D014284:Triiodothyronine / Q000494:pharmacology*
chemical_list: D014284:Triiodothyronine; D011188:Potassium