Thyroid status and diabetes modulate regional differences in potassium currents in rat ventricle.

1. The rate dependence and recovery kinetics of the Ca(2+)-independent transient (I(t)) and steady-state or 'pedestal' (Iss) outward potassium (K+) currents were studied in single myocytes isolated from epicardial and endocardial regions of rat left ventricles. The whole-cell, suction microelectrode method was used to measure baseline (fully reactivated) I(t), as well as its rate-dependent attenuation. Results from a group of control animals were compared with data from three other groups having an experimentally altered hormonal status. 2. I(t) was significantly smaller in endocardial cells than in epicardial cells, in part due to a very large difference in the recovery kinetics of this current in endocardial cells. This was reflected in a pronounced rate-dependent prolongation of endocardial action potentials. In contrast, the non-inactivating 'pedestal' current, Iss, was very similar in magnitude and showed comparable rate dependence in cells from both epicardium and endocardium. 3. Changing the thyroid status had selective, differential actions on the amplitude and rate dependence of It in epicardial and endocardial cells. Under hypothyroid conditions there was a more pronounced reduction of baseline I(t) in epicardial than in endocardial cells. Moreover, a slowing of the recovery kinetics in epicardial cells resulted in an enhanced attenuation of this current at high rates. Changing thyroid status had no effect on the magnitude or rate dependence of Iss in cells from either region of the left ventricle. 4. Following establishment of hyperthyroid conditions, there was no significant change in I(t) magnitude at baseline. However, when compared with control data, the recovery of I(t) was considerably faster in endocardial cells, and marginally faster in epicardial cells. 5. Streptozotocin-induced diabetic conditions resulted in a much greater attenuation of I(t) in epicardial cells than in endocardial cells. Epicardial action potentials in these conditions showed prominent rate-dependent prolongation. Iss was reduced to a similar extent in cells from these two regions. 6. Our findings demonstrate that altered hormonal status can selectively change the amplitude and kinetics of It in the epi- and endocardium of rat left ventricle. These changes can reduce the epicardial-endocardial gradients in the magnitude and recovery kinetics of It and hence diminish the intrinsic differences in both action potential duration and refractoriness.