Mechanisms responsible for the altered cardiac repolarization dispersion in experimental hypothyroidism

Aims: To identify the causes for the inhomogeneity of ventricular repolarization and increased QT dispersion in hypothyroid mice. Methods: We studied the effects of 5‐propyl‐2‐thiouracil‐induced hypothyroidism on the ECG, action potential (AP) and current density of the repolarizing potassium currents I_to,fast, I_to,slow, I_K,slow and I_ss in enzymatically isolated myocytes from three different regions of mouse heart: right ventricle (RV), epicardium of the left ventricle (Epi‐LV) and interventricular septum. K⁺ currents were recorded with the patch‐clamp technique. Membranes from isolated ventricular myocytes were extracted by centrifugation. Kv4.2, Kv4.3, KChIP and Na/Ca exchanger proteins were visualized by Western blot. Results: The frequency or conduction velocity was not changed by hypothyroidism, but QTc was prolonged. Neither resting membrane potential nor AP amplitude was modified. The action potential duration (APD)₉₀ increased in the RV and Epi‐LV, but not in the septum. Hypothyroid status has no effect either on I_to,slow, I_k,slow or I_ss in any of the regions analysed. However, I_to,fast was significantly reduced in the Epi‐LV and in the RV, whereas it was not altered in cells from the septum. Western blot analysis reveals a reduction in Kv4.2 and Kv4.3 protein levels in both the Epi‐LV and the RV and an increase in Na/Ca exchanger. Conclusion: From these results we suggest that the regional differences in APD lengthening, and thus in repolarization inhomogeneity, induced by experimental hypothyroidism are at least partially explained by the uneven decrease in I_to,fast and the differences in the relative contribution of the depolarization‐activated outward currents to the repolarization process.