Interactive Actions of Aldosterone and Insulin on Epithelial Na+ Channel Trafficking

Epithelial Na+ channel (ENaC) participates in renal epithelial Na+ reabsorption, controlling blood pressure. Aldosterone and insulin elevate blood pressure by increasing the ENaC-mediated Na+ reabsorption. However, little information is available on the interactive action of aldosterone and insulin on the ENaC-mediated Na+ reabsorption. In the present study, we tried to clarify if insulin would modify the aldosterone action on the ENaC-mediated Na+ reabsorption from a viewpoint of intracellular ENaC trafficking. We measured the ENaC-mediated Na+ transport as short-circuit currents using a four-state mathematical ENaC trafficking model in renal A6 epithelial cells with or without aldosterone treatment under the insulin-stimulated and -unstimulated conditions. We found that: (A) under the insulin-stimulated condition, aldosterone treatment (1 µM for 20 h) significantly elevated the ENaC insertion rate to the apical membrane ( k I ) 3.3-fold and the ENaC recycling rate ( k R ) 2.0-fold, but diminished the ENaC degradation rate ( k D ) 0.7-fold without any significant effect on the ENaC endocytotic rate ( k E ); (B) under the insulin-unstimulated condition, aldosterone treatment decreased k E 0.5-fold and increased k R 1.4-fold, without any significant effect on k I or k D . Thus, the present study indicates that: (1) insulin masks the well-known inhibitory action of aldosterone on the ENaC endocytotic rate; (2) insulin induces a stimulatory action of aldosterone on ENaC apical insertion and an inhibitory action of aldosterone on ENaC degradation; (3) insulin enhances the aldosterone action on ENaC recycling; (4) insulin has a more effective action on diminution of ENaC endocytosis than aldosterone.