Role of Angiotensin II Type 1a Receptor (AT1aR) of Renal Tubules in Regulating Inwardly Rectifying Potassium Channels 4.2 (Kir4.2), Kir4.1, and Epithelial Na⁺ Channel (ENaC)

Kir4.2 and Kir4.1 play a role in regulating membrane transport in the proximal tubule (PT) and in the distal-convoluted-tubule (DCT), respectively.

We generated kidney-tubule-specific-AT1aR-knockout (Ks-AT1aR-KO) mice to examine whether renal AT1aR regulates Kir4.2 and Kir4.1.

Ks-AT1aR-KO mice had a lower systolic blood pressure than Agtr1a^flox/flox (control) mice. Ks-AT1aR-KO mice had a lower expression of NHE₃ (Na⁺/H⁺-exchanger 3) and Kir4.2, a major Kir-channel in PT, than Agtr1a^flox/flox mice. Whole-cell recording also demonstrated that the membrane potential in PT of Ks-AT1aR-KO mice was lesser negative than Agtr1a^flox/flox mice. The expression of Kir4.1 and Kir5.1, Kir4.1/Kir5.1-mediated K⁺ currents of DCT and DCT membrane potential in Ks-AT1aR-KO mice, were similar to Agtr1a^flox/flox mice. However, angiotensin II perfusion for 7 days hyperpolarized the membrane potential in PT and DCT of the control mice but not in Ks-AT1aR-KO mice, while angiotensin II perfusion did not change the expression of Kir4.1, Kir4.2, and Kir5.1. Deletion of AT1aR did not significantly affect the expression of αENaC (epithelial Na⁺ channel) and βENaC but increased cleaved γENaC expression. Patch-clamp experiments demonstrated that deletion of AT1aR increased amiloride-sensitive Na⁺-currents in the cortical-collecting duct but not in late-DCT. However, tertiapin-Q sensitive renal outer medullary potassium channel currents were similar in both genotypes.

AT1aR determines the baseline membrane potential of PT by controlling Kir4.2 expression/activity but AT1aR is not required for determining the baseline membrane potential of the DCT and Kir4.1/Kir5.1 activity/expression. However, AT1aR is required for angiotensin II-induced hyperpolarization of basolateral membrane of PT and DCT. Deletion of AT1aR had no effect on baseline renal outer medullary potassium channel activity but increased ENaC activity in the CCD.