EFFECT OF THYROID STATE ON CARDIAC ELECTRICAL-ACTIVITY OF THE FROG RANA-ESCULENTA

The effects of altered thyroid states on the heart rate and ventricula r electrophysiological properties of the frog were examined. Hypothpro idism was induced by a 10-day treatment with propylthiouracil and prod uced decreased serum-free and total triiodothyronine levels below dete ctable concentrations. Hyperthyroidism, elicited by a 5-day treatment with triiodothyronine, was associated with increased serum thyroid hor mone levels. The hypothproid state was associated with a significantly decreased heart rate measured in vivo and an increased duration of th e action potential recorded in vitro from ventricular fibers. Hyperthy roidism was associated wit an increased heart rate and a decreased ven tricular action potential duration (APD). The dependence of APD on tem perature was affected by thyroid status. An increase from 25 to 30 deg rees barely shortened the repolarization phase in hyperthyroids, minim ally (13.3%) shortened that in euthyroids, and greatly (43.7%) shorten ed that in hypothyroids; the APD was similar in euthyroid and hypothyr oid frogs. The shortening of the repolarization phase, by increased st imulation frequency, was also greater for hypothyroid frogs. In this c ase, however, the differences in APD among groups remained significant at all the frequencies tested. (C) 1995 Academic Press, Inc.