Thyroxine effects on temperature dependence of ionic currents in single rabbit cardiac myocytes

Macroscopic whole cell currents were measured from single rabbit cardiac myocytes, using the suction electrode voltage-clamp technique, under euthyroid, hyperthyroid, and hypothyroid conditions. In ventricular myocytes, the temperature dependence of the transient outward current (I(t)) was greatly reduced in hyperthyroid conditions, with Q10 values (between 22 and 32 degrees C) reduced from normal values of 6.14 +/- 0.93 (SE, n = 8) to 2.14 +/- 0.14 (n = 6). In contrast, two of the other major currents in these cells were relatively unaffected. Under hyperthyroid conditions, there was very little change in the amplitudes or temperature dependence of L-type calcium currents and of steady-state currents, which reflect mainly the inwardly rectifying potassium current. In atrial cells no changes in the temperature dependence of I(t) were observed, with virtually identical Q10 values (close to 4) in eu- and hyperthyroid conditions. Under hypothyroid conditions, there was no change in the temperature dependence of I(t) in either ventricular or atrial cells. We conclude that the regulation of I(t) in ventricular cells is unique, rendering it extremely sensitive to temperature changes and to elevations in thyroxine levels. These results are discussed in the context of long-term regulation of ionic channels.