Abstract
At 21 Β°C in vitro, 0.2 and 2.0 mΜM of triiodothyronine (T^3^) produced an increase in resting membrane potential (RMP) of Rana pipiens sartorius when the pH of the external solution was 7.4. The RMP was increased by 2.0 mΜM T^3^ in the presence of 10^β4^ and 10^β3^ M ouabain but not in 10^β3^ M of 2, 4 dinitrophenol. Small increases in RMP were observed with 2.0 mΜM T^3^ in solutions with low external Na. At pH 7.1 0.2 mΜM T^3^ produced a small transient increase in RMP. Membrane resistance (Rm) was found to decline gradually during exposure to 0.2 mΜM at a pH of 7.4. Treatment with 2.0 mΜM T^3^ 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 mΜM T^3^ at pH of 7.1. T^3^ also reduced membrane resistance in isotonic K^2^SO^4^ and trisβbuffered Mn (20 mM) solutions indicating that T^3^ 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 mΜM T^3^ while thresholds remained unchanged. Thresholds were increased during exposure to 2.0 mΜM T^3^ whereas overshoot, amplitude and rate of rise underwent transient decreases followed by a return toward control levels.