Triiodothyronine effects on some electrogenic properties of frog sartorius

At 21 degrees C in vitro, 0.2 and 2.0 muM of triiodothyronine (T3) produced an increase in resting membrane potential (RMP) of Rana pipens sartorius when the pH of the external solution was 7.4. The RMP was increased by 2.0 muM T3 in the presence of 10(-4) and 10(-3) M ouabain but not in 10(-3) M of 2,4 dinitrophenol. Small increases in RMP were observed with 2.0 muM T3 in solutions with low external Na. At pH 7.1 0.2 muM T3 produced a small transient increase in RMP. Membrane resistance (Rm) was found to decline gradually during exposure to 0.2 muM at a pH of 7.4. Treatment with 2.0 muM T3 at pH 7.4 was accompanied by a transient reduction in Rm. Similar transient changes in Rm were produced by 0.2 and 2.0 muM T3 at pH of 7.1 T3 reduced membrane resistance in isotonic K2SO4 and tris-buffered Mn (20 mM) solutions indicating that T3 increases potassium permeability. Direct action potentials were studied at pH 7.1. Overshoot, amplitude and rate of rise of the action potential underwent a gradual decrease in the presence of 0.2 muM T3 while thresholds remained unchanged. Thresholds were increased during exposure to 2.0 muM T3 whereas overshoot, amplitude and rate of rise underwent transient decreases followed by a return toward control levels.