Mien T. X. Nguyen

The absence of intrarenal ACE protects against hypertension.

Activation of the intrarenal renin-angiotensin system (RAS) can elicit hypertension independently from the systemic RAS. However, the precise mechanisms by which intrarenal Ang II increases blood pressure have never been identified. To this end, we studied the responses of mice specifically lacking kidney angiotensin-converting enzyme (ACE) to experimental hypertension. Here, we show that the absence of kidney ACE substantially blunts the hypertension induced by Ang II infusion (a model of high serum Ang II) or by nitric oxide synthesis inhibition (a model of low serum Ang II). Moreover, the renal responses to high serum Ang II observed in wild-type mice, including intrarenal Ang II accumulation, sodium and water retention, and activation of ion transporters in the loop of Henle (NKCC2) and distal nephron (NCC, ENaC, and pendrin) as well as the transporter activating kinases SPAK and OSR1, were effectively prevented in mice that lack kidney ACE. These findings demonstrate that ACE metabolism plays a fundamental role in the responses of the kidney to hypertensive stimuli. In particular, renal ACE activity is required to increase local Ang II, to stimulate sodium transport in loop of Henle and the distal nephron, and to induce hypertension.

Intrarenal localization of the plasma membrane ATP channel pannexin1

In the renal tubules, ATP released from epithelial cells stimulates purinergic receptors, regulating salt and water reabsorption. However, the mechanisms by which ATP is released into the tubular lumen are multifaceted. Pannexin1 (Panx1) is a newly identified. ubiquitously expressed protein that forms connexin-like channels in the plasma membrane, which have been demonstrated to function as a mechanosensitive ATP conduit. Here, we report on the localization of Panx1 in the mouse kidney. Using immunofluorescence, strong Panx1 expression was observed in renal tubules, including proximal tubules, thin descending limbs, and collecting ducts, along their apical cell membranes. In the renal vasculature, Panx1 expression was localized to vascular smooth muscle cells in renal arteries, including the afferent and efferent arterioles. Additionally, we tested whether Panx1 channels expressed in renal epithelial cells facilitate luminal ATP release by measuring the ATP content of urine samples freshly collected from wild-type and Panx1(-/-) mice. Urinary ATP levels were reduced by 30% in Panx1(-/-) compared with wild-type mice. These results suggest that Panx1 channels in the kidney may regulate ATP release and via purinergic signaling may participate in the control of renal epithelial fluid and electrolyte transport and vascular functions.

Effects of ACE inhibition and ANG II stimulation on renal Na-Cl cotransporter distribution, phosphorylation, and membrane complex properties

The renal distal tubule Na-Cl cotransporter (NCC) reabsorbs <10% of the filtered Na(+) but is a key control point for blood pressure regulation by angiotensin II (ANG II), angiotensin-converting enzyme inhibitors (ACEI), and thiazide diuretics. This study aimed to determine whether NCC phosphorylation (NCCp) was regulated by acute (20-30 min) treatment with the ACEI captopril (12 μg/min × 20 min) or by a sub-pressor dose of ANG II (20 ng·kg(-1)·min(-1)) in Inactin-anesthetized rats. By immuno-EM, NCCp was detected exclusively in or adjacent to apical plama membranes (APM) in controls and after ACEI or ANG II treatment, while NCC total was detected in both APM and subapical cytoplasmic vesicles (SCV) in all conditions. In renal homogenates, neither ACEI nor ANG II treatment altered NCCp abundance, assayed by immunoblot. However, by density gradient fractionation we identified a pool of low-density APM in which NCCp decreased 50% in response to captopril and was restored during ANG II infusion, and another pool of higher-density APM that responded reciprocally, indicative of regulated redistribution between two APM pools. In both pools, NCCp was preferentially localized to Triton-soluble membranes. Blue Native gel electrophoresis established that APM NCCp localized to ~700 kDa complexes (containing γ-adducin) while unphosphorylated NCC in intracellular membranes primarily localized to ~400 kDa complexes: there was no evidence for native monomeric or dimeric NCC or NCCp. In summary, this study demonstrates that phosphorylated NCC, localized to multimeric complexes in the APM, redistributes in a regulated manner within the APM in response to ACEI and ANG II.

Paracellular epithelial sodium transport maximizes energy efficiency in the kidney

Efficient oxygen utilization in the kidney may be supported by paracellular epithelial transport, a form of passive diffusion that is driven by preexisting transepithelial electrochemical gradients. Claudins are tight-junction transmembrane proteins that act as paracellular ion channels in epithelial cells. In the proximal tubule (PT) of the kidney, claudin-2 mediates paracellular sodium reabsorption. Here, we used murine models to investigate the role of claudin-2 in maintaining energy efficiency in the kidney. We found that claudin-2-null mice conserve sodium to the same extent as WT mice, even during profound dietary sodium depletion, as a result of the upregulation of transcellular Na-K-2Cl transport activity in the thick ascending limb of Henle. We hypothesized that shifting sodium transport to transcellular pathways would lead to increased whole-kidney oxygen consumption. Indeed, compared with control animals, oxygen consumption in the kidneys of claudin-2-null mice was markedly increased, resulting in medullary hypoxia. Furthermore, tubular injury in kidneys subjected to bilateral renal ischemia-reperfusion injury was more severe in the absence of claudin-2. Our results indicate that paracellular transport in the PT is required for efficient utilization of oxygen in the service of sodium transport. We speculate that paracellular permeability may have evolved as a general strategy in epithelial tissues to maximize energy efficiency.

Maintaining Balance Under Pressure: Integrated Regulation of Renal Transporters During Hypertension

Hypertension is the leading cause of stroke and cardiovascular diseases and a leading risk factor for global disease burden, affecting 30% of the adult population in Western cultures.1 Blood pressure (BP) can be elevated by vasoconstriction and by increasing the circulating volume. Evidence from studies of mutations in renal Na+ transporters, renal transplantation, and diuretic action supports Guyton’s hypothesis that long-term regulation of effective circulating volume and BP depends on fractional renal Na+ reabsorption.2–4 Ultimately, excess Na+ reabsorption (Figure, red arrows) raises effective circulating volume and BP which provoke counteracting natriuretic responses to match Na+ output to Na+ intake at the expense of elevated BP (Figure, blue arrow).2 We and others have determined that pressure natriuresis responses involve Na+ transporter inhibition at multiple levels of regulation.5–13 According to Guyton, kidneys possess the capacity to excrete enough Na+ and volume to normalize BP in the face of expanded effective circulating volume.2 Thus, hypertension can be characterized as a failure of compensatory renal pressure natriuresis. Indeed, there is strong evidence that the pressure natriuresis response is impaired during experimental hypertension by inflammation, immune cell infiltration, and intrarenal angiotensin II (AngII) production, secondary to initiating stimuli such as AngII infusion, reduced NO production, high-salt diet, or elevated renal sympathetic nerve activity.14–17 This brief review focuses on the natriuretic effectors and addresses (1) the renal tubular locations and transporters that participate in pressure natriuresis and (2) the mechanisms that blunt the response in experimental models of hypertension. Figure. Renal transporter profile reveals region specific transporter regulation during experimental hypertension. A , Anatomic arrangement of renal cortex and renal medulla in a kidney cross-section …

How does potassium supplementation lower blood pressure

to the editor: Hypertension is one of the most common diseases in the United States, not easily controlled with medication, and its sequelae are among the most common causes of mortality. Interestingly, hypertension is found mainly in industrialized societies, with very low prevalence in isolated

Short‐term nonpressor angiotensin II infusion stimulates sodium transporters in proximal tubule and distal nephron

In Sprague Dawley rats, 2‐week angiotensin II (AngII) infusion increases Na+ transporter abundance and activation from cortical thick ascending loop of Henle (TALH) to medullary collecting duct (CD) and raises blood pressure associated with a pressure natriuresis, accompanied by depressed Na+ transporter abundance and activation from proximal tubule (PT) through medullary TALH. This study tests the hypothesis that early during AngII infusion, before blood pressure raises, Na+ transporters’ abundance and activation increase all along the nephron. Male Sprague Dawley rats were infused via osmotic minipumps with a subpressor dose of AngII (200 ng/kg/min) or vehicle for 3 days. Overnight urine was collected in metabolic cages and sodium transporters’ abundance and phosphorylation were determined by immunoblotting homogenates of renal cortex and medulla. There were no significant differences in body weight gain, overnight urine volume, urinary Na+ and K+ excretion, or rate of excretion of a saline challenge between AngII and vehicle infused rats. The 3‐day nonpressor AngII infusion significantly increased the abundance of PT Na+/H+ exchanger 3 (NHE3), cortical TALH Na‐K‐2Cl cotransporter 2 (NKCC2), distal convoluted tubule (DCT) Na‐Cl cotransporter (NCC), and cortical CD ENaC subunits. Additionally, phosphorylation of cortical NKCC2, NCC, and STE20/SPS1‐related proline–alanine‐rich kinase (SPAK) were increased; medullary NKCC2 and SPAK were not altered. In conclusion, 3‐day AngII infusion provokes PT NHE3 accumulation as well as NKCC2, NCC, and SPAK accumulation and activation in a prehypertensive phase before evidence for intrarenal angiotensinogen accumulation.