Long-Term Regulation of ENaC Expression in Kidney by Angiotensin II

Abstract

Abstract-We carried out semiquantitative immunoblotting of kidney to identify apical sodium transporter proteins whose abundances are regulated by angiotensin II. In NaCl-restricted rats (0.5 mEq Na/200 g BW/d), the type 1 angiotensin II receptor (AT 1 receptor) antagonist, candesartan, (1 mg/kg of body weight per day SC for 2 days) markedly decreased the abundance of the ␣ subunit of the epithelial sodium channel (ENaC). This subunit has been shown to be rate-limiting for assembly of mature ENaC complexes. In addition, systemic infusion of angiotensin II increased ␣ENaC protein abundance in rat kidney cortex. The decrease in ␣ENaC protein abundance in response to AT 1 receptor blockade was associated with a fall in ␣ENaC mRNA abundance (real-time RT-PCR), consistent with transcriptionally mediated regulation. The effect of AT 1 receptor blockade on ␣ENaC expression was not blocked by spironolactone, suggesting a direct role of the AT 1 receptor in regulation of ␣ENaC gene expression. Candesartan administration was also found to increase the abundances of the ␤ and ␥ subunits. The increase in ␤ and ␥ENaC protein abundance was not associated with a significant increase in the renal abundances of the corresponding mRNAs, suggesting a posttranscriptional mechanism. Immunocytochemistry confirmed the increase in ␤ and ␥ENaC protein abundance and demonstrated candesartan-induced ENaC internalization in collecting duct cells. The results support the view that the angiotensin II receptor regulates ENaC abundance, consistent with a role for angiotensin II in regulation of collecting duct function. Key Words: receptors, angiotensin II Ⅲ angiotensin antagonist Ⅲ sodium channels Ⅲ aldosterone L ong-term control of blood pressure is closely tied to sodium balance and extracellular fluid volume regulation, both of which are controlled in part by the renin-angiotensin-aldosterone system (RAAS). 1 Angiotensin II has important nonrenal effects that are instrumental in the control of blood pressure as both a vasoconstrictor and a regulator of aldosterone secretion. In addition, angiotensin II has direct effects on the renal tubule in regulating NaCl reabsorption. 2 The direct antinatriuretic effects of angiotensin II appear to be particularly important in conditions of dietary sodium restriction or contraction of extracellular fluid volume. 1 Regulation of renal tubule sodium transport by angiotensin II has been investigated chiefly in relatively short-term experiments with observations within a few minutes of angiotensin II addition. 3-7 However, there is growing evidence that a variety of mediators of transport regulation in the kidney, such as vasopressin 8 and aldosterone, 9 work by both short-term and long-term actions. The long-term actions are associated with adaptive increases in abundance of transporter proteins, whereas short-term actions are generally associated with regulated trafficking or posttranslational modifications of the transporter proteins.The antinatriuretic effects of angiotensin II on sodium transport are...

AT 1 Receptor Mediated Augmentation of Intrarenal Angiotensinogen in Angiotensin II-Dependent Hypertension

“…This group also demonstrated that angiotensinconverting enzyme is present, allowing Ang II to be formed intraluminally in the collecting duct. 34 Moreover, Beutler et al 10 showed that chronic Ang II infusions stimulate the mRNA expression of the ␣ subunit of the epithelial sodium channel in collecting duct cells of rats and that treatment with a chronic ARB prevents this augmentation. The present study demonstrates further that stimulation of intrarenal AGT by the AT 1 receptor creates an amplification mechanism for the generation of intratubular AGT ultimately leading to increased intratubular formation of Ang I and Ang II.…”

“…Furthermore, epithelial sodium channel gene expression in the cortical collecting duct is also upregulated by Ang II. 10 Thus, Ang II regulates tubular transport in distal as well as in proximal tubular segments via its action on basolateral and luminal receptors. 1 Ang II-infused hypertensive rats exhibit increases in renal angiotensinogen (AGT) mRNA 11 and protein, 12 and an enhancement of urinary excretion rate of AGT.…”

Renin-Angiotensin-Aldosterone System and Progression of Renal Disease

“…Proximal tubular cells actively produce AngII and also secrete angiotensinogen into the urine (21). Intraluminal angiotensinogen may be converted in the distal tubules to AngII, and recent observations suggest that it leads to induction of sodium channels independent of aldosterone (22). Hyperglycemia and proteinuria could stimulate local AngII synthesis, mainly by oxygen species as signal transducers (12).…”

Renal Renin-Angiotensin System

“…Peti-Peterdi et al [70] demonstrated that Ang II directly stimulates epithelial sodium channel activity in the cortical collecting duct via AT1R. Furthermore, epithelial sodium channel gene expression in the cortical collecting duct is upregulated by chronic infusion of Ang II [71]. Thus, Ang II exerts important effects on tubular transport rate in distal as well as in proximal tubular segments via its action on both basolateral and luminal receptors [65].…”