BACKGROUND & AIMS: Both hyperthyroidism and hypothyroidism as well as hyperaldosteronism have been associated with changes of epithelial transport. The aim of this study was to evaluate the role of thyroid hormones in the regulation of electrogenic amiloride-sensitive sodium transport by aldosterone in the distal colon of immature and adult rats. METHODS: The changes in amiloride-sensitive short-circuit current (Isc) and Na+, K(+)-adenosine triphosphatase (ATPase) activity were measured in suckling, weanling, and adult euthyroid and methimazole-induced hypothyroid rats. RESULTS: The developmental increase of thyroid hormones in control pups was associated with an increase in plasma aldosterone concentration and amiloride-sensitive Isc. The inhibition of the developmental increase of thyroxine and triiodothyronine in hypothyroid pups was followed by suppression of amiloride-sensitive Isc while aldosterone concentrations remained elevated. Moreover, the induction of amiloride-sensitive Isc by secondary hyperaldosteronism was inhibited in hypothyroid adult rats, and the effect of thyroid hormones on amiloride-sensitive Isc could not be explained by changes in the plasma aldosterone concentration. Hypothyroidism also led to a decrease of colonic Na+, K(+)-ATPase activity. Replacement therapy of hypothyroid pups with triiodothyronine restored amiloride-sensitive Isc and increased Na+, K(+)-ATPase activity. CONCLUSIONS: Thyroid hormones have a permissive role in the regulation of electrogenic amiloride-sensitive Na+ transport by aldosterone.

Institute of Physiology Czech Academy of Sciences Prague Czech Republic

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