EFFECT OF T-3 ADMINISTRATION ON ELECTROPHYSIOLOGICAL PROPERTIES OF LIZARD VENTRICULAR MUSCLE-FIBERS

We investigated the effects on the electrophysiological properties of ventricular muscle fibres from lizards kept at 20 degrees C of mild an d severe hyperthyroidism. The hyperthyroidism was induced by a 4-day t reatment with either 0.025 or 1.0 mu g triiodothyronine g(-1) body wei ght, documented by increased serum levels of thyroid hormone. Triiodot hyronine treatment did not modify the duration of the action potential recorded in vitro at 25 degrees C from ventricular muscles stimulated at 1 Hz. Recordings at higher temperatures were associated with a fas ter repolarization phase and a decrease of action potential duration i n both euthyroid and hyperthyroid animals. However, in lizards treated with 1.0 mu g triiodothyronine . g(-1) body weight, the 90% repolariz ation recovery times at 30 and 35 degrees C (95.6 +/- 14.9 ms and 53.0 +/- 6.0 ms, respectively), were significantly shorter than normal (17 7.6 +/- 29.2 and 107.2 +/- 18.1 ms, respectively). Action potential du ration was also dependent on stimulation frequency of the preparations . Increased frequency led to significant decrease of the duration of a ction potentials recorded at 25 degrees C. III euthyroid preparations the reductions in 90% repolarization recovery time, owing to increases in stimulation frequency to 2.5 and 5 Hz, were 19.3 +/- 1.7 and 35.6 +/- 2.0 ms, respectively. In hyperthyroid preparations, the reductions in the 90% recovery time due to stimulus frequency increases varied f rom 35.4 +/- 1.9 and 58.1 +/- 2.1 ms at low hormone doses to 38.9 +/- 2.0 and 58.2 +/- 2.1 ms at high hormone doses. As a result of these di fferences, the action potential durations recorded from the two hypert hyroid preparations at high stimulation rates were shorter than from e uthyroid preparations. The results obtained suggest that lizard cardia c tissue is responsive to hormone action at low environmental temperat ure, but the effects of such action become evident when the temperatur e and heart rate increase.