Effects of thyroid status on expression of voltage-gated potassium channels in rat left ventricle.

OBJECTIVE: Thyroid hormone modifies cardiac action potentials and outward potassium currents directly and indirectly e.g. through beta-adrenergic signaling pathway. We thus examined the expression of six voltage-gated potassium channel alpha-subunits in the rat left ventricle under hypo- and hyperthyroid status, and tested roles of beta-adrenergic signaling pathway in their expressions under both status.

METHODS: Hypothyroidism and hyperthyroidism were induced by administration of methimazole (MMI) for 4 weeks and by injection of L-thyroxine (T4) to the MMI-treated rats for the last 7 days, respectively. To distinguish the effects of T4 and the beta-adrenergic system, propranolol (Pro) was administered to the MMI-treated rats together with T4, and isoproterenol (Iso) was injected to MMI-treated rats for the last 7 days. The mRNA levels of Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv4.2 and Kv4.3 in the left ventricles were determined by ribonuclease protection assay.

RESULTS: MMI treatment induced hypothyroidism and resulted in a significant decrease in the mRNA levels of Kv1.5, Kv2.1 and Kv4.2 (19%, 77% and 61% of control value, respectively; n = 6, p < 0.05). T4 administration induced hyperthyroidism and cardiac hypertrophy, and it increased the Kv1.5 and Kv2.1 mRNA levels over the control value (212% and 140%, respectively; n = 6, p < 0.05). Kv4.2 mRNA level was restored to the control level by T4. In contrast, the Kv1.2 and Kv1.4 mRNA levels increased in hypothyroid rats (161% and 186% of control value, respectively; n = 6, p < 0.01) and decreased in hyperthyroid rats (14% and 33% of control value, respectively; n = 6, p < 0.01). The Kv4.3 mRNA level was not altered by thyroid status. Pro did not inhibit the T4-induced hypertrophy. Iso induced cardiac hypertrophy. Pro or Iso by itself did not alter Kv mRNA levels except for Kv1.2, the message of which was decreased by Iso.

CONCLUSION: Thyroid hormone differentially regulates the expression of Kv1.4, Kv1.5, Kv2.1 and Kv4.2 mRNA levels in the rat left ventricle. This effect is not mediated through beta-adrenergic signaling pathway. On the other hand, the reduction in Kv1.2 mRNA level was associated with cardiac hypertrophy induced by T4 or Iso.