Evidence That Binding of Cyclic GMP to the Extracellular Domain of NKA (Sodium-Potassium ATPase) Mediates Natriuresis.

BACKGROUND: Extracellular renal interstitial guanosine cyclic 3',5'-monophosphate (cGMP) inhibits renal proximal tubule (RPT) sodium (Na+) reabsorption via Src (Src family kinase) activation. Through which target extracellular cGMP acts to induce natriuresis is unknown. We hypothesized that cGMP binds to the extracellular α1-subunit of NKA (sodium-potassium ATPase) on RPT basolateral membranes to inhibit Na+ transport similar to ouabain-a cardiotonic steroid. METHODS: Urine Na+ excretion was measured in uninephrectomized 12-week-old female Sprague-Dawley rats that received renal interstitial infusions of vehicle (5% dextrose in water), cGMP (18, 36, and 72 µg/kg per minute; 30 minutes each), or cGMP+rostafuroxin (12 ng/kg per minute) or were subjected to pressure-natriuresis±rostafuroxin infusion. Rostafuroxin is a digitoxigenin derivative that displaces ouabain from NKA. RESULTS: Renal interstitial cGMP and raised renal perfusion pressure induced natriuresis and increased phosphorylated SrcTyr416 and Erk 1/2 (extracellular signal-regulated protein kinase 1/2)Thr202/Tyr204; these responses were abolished with rostafuroxin coinfusion. To assess cGMP binding to NKA, we performed competitive binding studies with isolated rat RPTs using bodipy-ouabain (2 µM)+cGMP (10 µM) or rostafuroxin (10 µM) and 8-biotin-11-cGMP (2 µM)+ouabain (10 µM) or rostafuroxin (10 µM). cGMP or rostafuroxin reduced bodipy-ouabain fluorescence intensity, and ouabain or rostafuroxin reduced 8-biotin-11-cGMP staining. We cross-linked isolated rat RPTs with 4-N3-PET-8-biotin-11-cGMP (2 µM); 8-N3-6-biotin-10-cAMP served as negative control. Precipitation with streptavidin beads followed by immunoblot analysis showed that RPTs after cross-linking with 4-N3-PET-8-biotin-11-cGMP exhibited a significantly stronger signal for NKA than non-cross-linked samples and cross-linked or non-cross-linked 8-N3-6-biotin-10-cAMP RPTs. Ouabain (10 µM) reduced NKA in cross-linked 4-N3-PET-8-biotin-11-cGMP RPTs confirming fluorescence staining. 4-N3-PET-8-biotin-11-cGMP cross-linked samples were separated by SDS gel electrophoresis and slices corresponding to NKA molecular weight excised and processed for mass spectrometry. NKA was the second most abundant protein with 50 unique NKA peptides covering 47% of amino acids in NKA. Molecular modeling demonstrated a potential cGMP docking site in the ouabain-binding pocket of NKA. CONCLUSIONS: cGMP can bind to NKA and thereby mediate natriuresis.

Renal AT2 Receptors Mediate Natriuresis via Protein Phosphatase PP2A.

BACKGROUND: How signals from activated angiotensin type-2 receptors (AT2R) mediate inhibition of sodium ion (Na+) reabsorption in renal proximal tubule cells is currently unknown. Protein phosphatases including PP2A (protein phosphatase 2A) have been implicated in AT2R signaling in tissues other than kidney. We investigated whether inhibition of protein phosphatase PP2A reduced AT2R-mediated natriuresis and evaluated changes in PP2A activity and localization after renal AT2R activation in normal 4- and 10-week-old control Wistar-Kyoto rats and 4-week-old prehypertensive and 10-week-old hypertensive spontaneously hypertensive rats. METHODS AND RESULTS: In Wistar-Kyoto rats, direct renal interstitial administration of selective AT2R nonpeptide agonist Compound-21 (C-21) increased renal interstitial cyclic GMP (cGMP) levels, urine Na+ excretion, and simultaneously increased PP2A activity ≈2-fold in homogenates of renal cortical tubules. The cyclic GMP and natriuretic responses were abolished by concurrent renal interstitial administration of protein phosphatase inhibitor calyculin A. In renal proximal tubule cells in response to C-21, PP2A subunits A, B55α and C, but not B56γ, were recruited to apical plasma membranes together with AT2Rs. Calyculin A treatment abolished C-21-induced translocation of both AT2R and PP2A regulatory subunit B55α to apical plasma membranes. Immunoprecipitation of AT2R solubilized from renal cortical homogenates demonstrated physical association of AT2R with PP2A A, B55α, and C but not B56γ subunits. In contrast, in spontaneously hypertensive rats, administration of C-21 did not alter urine Na+ excretion or PP2A activity and failed to translocate AT2Rs and PP2A subunits to apical plasma membranes. CONCLUSIONS: In renal proximal tubule cells of Wistar-Kyoto rats, PP2A is activated and PP2A subunits AB55αC are recruited to C-21-activated AT2Rs during induction of natriuresis. This response is defective in prehypertensive and hypertensive spontaneously hypertensive rats, presenting a potential novel therapeutic target for treating renal Na+ retention and hypertension.

Identification of a Primary Renal AT2 Receptor Defect in Spontaneously Hypertensive Rats.

RATIONALE: Previous studies identified a defect in Ang III (angiotensin III [des-aspartyl1-angiotensin II])-elicited AT2R (Ang type-2 receptor)-mediated natriuresis in renal proximal tubule cells of spontaneously hypertensive rats (SHR). OBJECTIVE: This study aimed to delineate in prehypertensive SHR kidneys the receptor or postreceptor defect causing impaired AT2R signaling and renal sodium (Na+) retention by utilizing the selective AT2R agonist compound-21 (C-21). METHODS AND RESULTS: Female 4-week-old Wistar Kyoto and SHR rats were studied after 24-hour systemic AT1R (Ang II type-1 receptor) blockade. Left kidneys received 30-minute renal interstitial infusions of vehicle followed by C-21 (20, 40, and 60 ng/[kg·min], each dose 30 minutes). Right kidneys received vehicle infusions. In Wistar Kyoto, C-21 dose-dependently increased urine Na+ excretion from 0.023±0.01 to 0.064±0.02, 0.087±0.01, and 0.089±0.01 µmol/min (P=0.008, P<0.0001, and P<0.0001, respectively) and renal interstitial fluid levels of AT2R downstream signaling molecule cGMP (cyclic guanosine 3',5' monophosphate) from 0.91±0.3 to 3.1±1.0, 5.9±1.2 and 5.3±0.5 fmol/mL (P=nonsignificant, P<0.0001, and P<0.0001, respectively). In contrast, C-21 did not increase urine Na+ excretion or renal interstitial cGMP in SHR. Mean arterial pressure was slightly higher in SHR but within the normotensive range and unaffected by C-21. In Wistar Kyoto, but not SHR, C-21 induced AT2R translocation to apical plasma membranes of renal proximal tubule cells, internalization/inactivation of NHE-3 (sodium-hydrogen exchanger-3) and Na+/K+ATPase (sodium-potassium-atpase) and phosphorylation of AT2R-cGMP downstream signaling molecules Src (Src family kinase), ERK (extracellular signal-related kinase), and VASP (vasodilator-stimulated phosphoprotein). To test whether cGMP could bypass the natriuretic defect in SHR, we infused 8-bromo-cGMP. This restored natriuresis, Na+ transporter internalization/inactivation, and Src and VASP phosphorylation, but not apical plasma membrane AT2R recruitment. In contrast, 8-bromo-cAMP administration had no effect on natriuresis or AT2R recruitment in SHR. CONCLUSIONS: The results demonstrate a primary renal proximal tubule cell AT2R natriuretic defect in SHR that may contribute to the development of hypertension. Since the defect is abrogated by exogenous intrarenal cGMP, the renal cGMP pathway may represent a viable target for the treatment of hypertension. Visual Overview: An online visual overview is available for this article.

Intrarenal ghrelin receptor inhibition ameliorates angiotensin II-dependent hypertension in rats.

The intrarenal ghrelin receptor (GR) is localized to collecting duct (CD) cells, where it increases epithelial Na+ channel (αENaC)-dependent sodium reabsorption in rodents. We hypothesized that chronic GR inhibition with intrarenal GR siRNA lowers blood pressure (BP) in angiotensin II-dependent hypertension via reductions in αENaC-dependent sodium reabsorption. Uninephrectomized Sprague-Dawley rats ( n = 121) received subcutaneous osmotic pumps for chronic systemic delivery of angiotensin II or vehicle (5% dextrose in water). Rats also received intrarenal infusion of vehicle, GR siRNA, or scrambled (SCR) siRNA. In rats receiving intrarenal vehicle or intrarenal SCR siRNA, systemic angiotensin II infusion increased sodium retention and BP on day 1, and BP remained elevated throughout the 5-day study. These rats also demonstrated increased CD GR expression after 5 days of infusion. However, intrarenal GR siRNA infusion prevented angiotensin II-mediated sodium retention on day 1, induced a continuously negative cumulative sodium balance compared with angiotensin II alone, and reduced BP chronically. Glomerular filtration rate and renal blood flow remained unchanged in GR siRNA-infused rats. Systemic angiotensin II infusion also increased serum aldosterone levels, CD αENaC, and phosphorylated serum and glucocorticoid-inducible kinase 1 expression in rats with intrarenal SCR siRNA; however, these effects were not observed in the presence of intrarenal GR siRNA, despite exposure to the same systemic angiotensin II. These data demonstrate that chronic inhibition of intrarenal GR activity significantly reduces αENaC-dependent sodium retention, resulting in a negative cumulative sodium balance, thereby ameliorating angiotensin II-induced hypertension in rats. Renal GRs represent a novel therapeutic target for the treatment of hypertension and other sodium-retaining states.

AT2 Receptor Activation Prevents Sodium Retention and Reduces Blood Pressure in Angiotensin II-Dependent Hypertension.

RATIONALE: Compound 21 (C-21) is a highly selective nonpeptide angiotensin AT2 receptor (AT2R) agonist. OBJECTIVE: To test the hypothesis that chronic AT2R activation with C-21 induces natriuresis via an action at the renal proximal tubule (RPT) and lowers blood pressure (BP) in experimental angiotensin II (Ang II)-dependent hypertension. METHODS AND RESULTS: In rats, Ang II infusion increased both sodium (Na(+)) retention and BP on day 1, and BP remained elevated throughout the 7-day infusion period. Either intrarenal or systemic administration of C-21 prevented Ang II-mediated Na(+) retention on day 1, induced continuously negative cumulative Na(+) balance compared with Ang II alone, and reduced BP chronically. The effects of C-21 are likely to be mediated by action on the RPT as acute systemic C-21-induced natriuresis was additive to that induced by chlorothiazide and amiloride. At 24 hours of Ang II infusion, AT2R activation with C-21, both intrarenally and systemically, translocated AT2Rs from intracellular sites to the apical plasma membranes of RPT cells without altering the total cellular pool of AT2Rs and internalized/inactivated major RPT Na(+) transporters Na(+)-H(+)-exchanger-3 and Na(+)/K(+)ATPase. C-21 lowered BP to a similar degree whether administered before or subsequent to the establishment of Ang II-dependent hypertension. CONCLUSIONS: Chronic AT2R activation initiates and sustains receptor translocation to RPT apical plasma membranes, internalizes/inactivates Na(+)-H(+)-exchanger-3 and Na(+)/K(+)ATPase, prevents Na(+) retention resulting in negative cumulative Na(+) balance, and lowers BP in experimental Ang II-induced hypertension. Acting uniquely at the RPT, C-21 is a promising candidate for the treatment of hypertension and Na(+)-retaining states in humans.

When I'm 64: Effects of an interdisciplinary gerontology course on first-year undergraduates' perceptions of aging.

One challenge for gerontology is getting more students interested in aging at an earlier point in their academic career. This study evaluated the impact of an interdisciplinary course on aging designed for first-year undergraduate students. The course aimed to expand students' appreciation of the personal and professional relevance of aging issues, with the goal of expanding their aging-related curricular and career interests. Main outcome variables of the study included knowledge of older adults and aging, attitudes toward older adults, and anxiety about personal aging. Participants included an intervention group enrolled in the course and a control group not enrolled in the course. Compared to baseline, at the end of the semester students in the class had more knowledge about aging and more positive explicit attitudes toward older adults, but their implicit attitudes toward older adults and anxiety about aging did not change. Control students showed no changes. These findings suggest that objective knowledge of aging and explicit attitudes improve with curricular intervention, but implicit attitudes and anxiety might be more difficult to change. Gerontology education is a complex undertaking whose diverse goals must be clearly articulated in order to guide curricular interventions and incite curiosity among young undergraduate students.

AT2 receptor activation induces natriuresis and lowers blood pressure.

RATIONALE: Compound 21 (C-21) is a highly selective nonpeptide AT2 receptor (AT2R) agonist. OBJECTIVE: To test the hypothesis that renal proximal tubule AT2Rs induce natriuresis and lower blood pressure in Sprague-Dawley rats and mice. METHODS AND RESULTS: In rats, AT2R activation with intravenous C-21 increased urinary sodium excretion by 10-fold (P<0.0001); this natriuresis was abolished by direct renal interstitial infusion of specific AT2R antagonist PD-123319. C-21 increased fractional excretion of Na(+) (P<0.05) and lithium (P<0.01) without altering renal hemodynamic function. AT2R activation increased renal proximal tubule cell apical membrane AT2R protein (P<0.001) without changing total AT2R expression and internalized/inactivated Na(+)-H(+) exchanger-3 and Na(+)/K(+)ATPase. C-21-induced natriuresis was accompanied by an increase in renal interstitial cGMP (P<0.01); C-21-induced increases in urinary sodium excretion and renal interstitial cGMP were abolished by renal interstitial nitric oxide synthase inhibitor l-N(6)-nitroarginine methyl ester or bradykinin B2 receptor antagonist icatibant. Renal AT2R activation with C-21 prevented Na(+) retention and lowered blood pressure in the angiotensin II infusion model of experimental hypertension. CONCLUSIONS: AT2R activation initiates its translocation to the renal proximal tubule cell apical membrane and the internalization of Na(+)-H(+) exchanger-3 and Na(+)/K(+)ATPase, inducing natriuresis in a bradykinin-nitric oxide-cGMP-dependent manner. Intrarenal AT2R activation prevents Na(+) retention and lowers blood pressure in angiotensin II-dependent hypertension. AT2R activation holds promise as a renal proximal tubule natriuretic/diuretic target for the treatment of fluid-retaining states and hypertension.

Loss of collectrin, an angiotensin-converting enzyme 2 homolog, uncouples endothelial nitric oxide synthase and causes hypertension and vascular dysfunction.

BACKGROUND: Collectrin is an orphan member of the renin-angiotensin system and is a homolog of angiotensin-converting enzyme 2, sharing ≈50% sequence identity. Unlike angiotensin-converting enzyme 2, collectrin lacks any catalytic domain. Collectrin has been shown to function as a chaperone of amino acid transporters. In rodents, the renal expression of collectrin is increased after subtotal nephrectomy and during high-salt feeding, raising the question of whether collectrin has any direct role in blood pressure regulation. METHODS AND RESULTS: Using a susceptible genetic background, we demonstrate that deletion of collectrin results in hypertension, exaggerated salt sensitivity, and impaired pressure natriuresis. Collectrin knockout mice display impaired endothelium-dependent vasorelaxation that is associated with vascular remodeling, endothelial nitric oxide synthase uncoupling, decreased nitric oxide production, and increased superoxide generation. Treatment with Tempol, a superoxide scavenger, attenuates the augmented sodium sensitivity in collectrin knockout mice. We report for the first time that collectrin is expressed in endothelial cells. Furthermore, collectrin directly regulates l-arginine uptake and plasma membrane levels of CAT1 and y(+)LAT1 amino acid transporters in endothelial cells. Treatment with l-arginine modestly lowers blood pressure of collectrin knockout mice. CONCLUSIONS: Collectrin is a consequential link between the transport of l-arginine and endothelial nitric oxide synthase uncoupling in hypertension.

Intrarenal ghrelin receptors regulate ENaC-dependent sodium reabsorption by a cAMP-dependent pathway.

The main distal nephron segment sodium transporters are the distal tubule chlorothiazide-sensitive sodium chloride cotransporter (NCC) and the collecting duct amiloride-sensitive epithelial sodium channel (ENaC). The infusion of ghrelin into the renal interstitium stimulates distal nephron-dependent sodium reabsorption in normal rats, but the mechanism is unknown. Here we localize renal ghrelin receptors (GR) to the cortical collecting duct (CCD). Ghrelin significantly increased phosphorylated serum/glucocorticoid-regulated kinase-1 (pSGK1), a major upstream signaling intermediate regulating ENaC. To test whether increased apical membrane αENaC induced the antinatriuresis, ghrelin was infused in the presence of acute and chronic amiloride, a selective inhibitor of ENaC. In the presence of amiloride, renal interstitial ghrelin failed to reduce urine sodium excretion, suggesting that ghrelin-induced sodium reabsorption is dependent on intact ENaC activity. While the main sodium transporter of the CCD is ENaC, NCC is also present. In response to renal interstitial ghrelin infusion, neither total nor phosphorylated NCC levels are altered. Ghrelin-induced sodium reabsorption persisted in the presence of chlorothiazide (selective inhibitor of NCC), indicating that intact NCC activity is not necessary for ghrelin-induced antinatriuresis. Finally, renal interstitial ghrelin infusion significantly increased interstitial cAMP levels and adenylyl cyclase blockade abolished ghrelin-induced antinatriuresis. Thus, GRs expressed in the CCD regulate sodium reabsorption by cAMP-induced trafficking of ENaC to the apical membrane.

Progressive Cardiac Metabolic Defects Accompany Diastolic and Severe Systolic Dysfunction in Spontaneously Hypertensive Rat Hearts.

Background Cardiac metabolic abnormalities are present in heart failure. Few studies have followed metabolic changes accompanying diastolic and systolic heart failure in the same model. We examined metabolic changes during the development of diastolic and severe systolic dysfunction in spontaneously hypertensive rats (SHR). Methods and Results We serially measured myocardial glucose uptake rates with dynamic 2-[18F] fluoro-2-deoxy-d-glucose positron emission tomography in vivo in 9-, 12-, and 18-month-old SHR and Wistar Kyoto rats. Cardiac magnetic resonance imaging determined systolic function (ejection fraction) and diastolic function (isovolumetric relaxation time) and left ventricular mass in the same rats. Cardiac metabolomics was performed at 12 and 18 months in separate rats. At 12 months, SHR hearts, compared with Wistar Kyoto hearts, demonstrated increased isovolumetric relaxation time and slightly reduced ejection fraction indicating diastolic and mild systolic dysfunction, respectively, and higher (versus 9-month-old SHR decreasing) 2-[18F] fluoro-2-deoxy-d-glucose uptake rates (Ki). At 18 months, only few SHR hearts maintained similar abnormalities as 12-month-old SHR, while most exhibited severe systolic dysfunction, worsening diastolic function, and markedly reduced 2-[18F] fluoro-2-deoxy-d-glucose uptake rates. Left ventricular mass normalized to body weight was elevated in SHR, more pronounced with severe systolic dysfunction. Cardiac metabolite changes differed between SHR hearts at 12 and 18 months, indicating progressive defects in fatty acid, glucose, branched chain amino acid, and ketone body metabolism. Conclusions Diastolic and severe systolic dysfunction in SHR are associated with decreasing cardiac glucose uptake, and progressive abnormalities in metabolite profiles. Whether and which metabolic changes trigger progressive heart failure needs to be established.

Inhibition of renal caveolin-1 reduces natriuresis and produces hypertension in sodium-loaded rats.

Renal dopamine receptor function and ion transport inhibition are impaired in essential hypertension. We recently reported that caveolin-1 (CAV1) and lipid rafts are necessary for normal D(1)-like receptor-dependent internalization of Na-K-ATPase in human proximal tubule cells. We now hypothesize that CAV1 is necessary for the regulation of urine sodium (Na(+)) excretion (U(Na)V) and mean arterial blood pressure (MAP) in vivo. Acute renal interstitial (RI) infusion into Sprague-Dawley rats of 1 µg·kg⁻¹·min⁻¹ fenoldopam (FEN; D(1)-like receptor agonist) caused a 0.46 ± 0.15-µmol/min increase in U(Na)V (over baseline of 0.29 ± 0.04 µmol/min; P < 0.01). This increase was seen in Na(+)-loaded rats, but not in those under a normal-sodium load. Coinfusion with ß-methyl cyclodextrin (ßMCD; lipid raft disrupter, 200 µg·kg⁻¹·min⁻¹) completely blocked this FEN-induced natriuresis (P < 0.001). Long-term (3 day) lipid raft disruption via continuous RI infusion of 80 µg·kg⁻¹·min⁻¹ ßMCD decreased renal cortical CAV1 expression (47.3 ± 6.4%; P < 0.01) and increased MAP (32.4 ± 6.6 mmHg; P < 0.001) compared with vehicle-infused animals. To determine whether the MAP rise was due to a CAV1-dependent lipid raft-mediated disruption, Na(+)-loaded rats were given a bolus RI infusion of CAV1 siRNA. Two days postinfusion, cortical CAV1 expression was decreased by 73.6 ± 8.2% (P < 0.001) and the animals showed an increase in MAP by 17.4 ± 2.9 mmHg (P < 0.01) compared with animals receiving scrambled control siRNA. In summary, acute kidney-specific lipid raft disruption decreases CAV1 expression and blocks D(1)-like receptor-induced natriuresis. Furthermore, chronic disruption of lipid rafts or CAV1 protein expression in the kidney induces hypertension.

Circulating Extracellular Vesicles in Normotension Restrain Vasodilation in Resistance Arteries.

Extracellular vesicles (EVs) have been described as novel biomarkers and bioactivators in vascular dysfunction in hypertension. However, the mechanism(s) by which EVs affect vascular function is unknown. To examine the effects of EVs on endothelial-dependent vasodilation (acetylcholine), we isolated circulating EVs from platelet-poor plasma using a low centrifugation speed (17 000g) and mesenteric resistance arteries from 12-week-old normotensive WKYs (Wistar-Kyoto rats) and SHRs (spontaneously hypertensive rats). Arteries were cannulated on a pressure myograph, and EVs were added to the vessel lumen and circulating bath. We found that circulating EVs from normotensive WKY reduced vasodilation of normotensive WKY arteries but had no effect on hypertensive SHR arteries. In contrast, EVs from hypertensive SHR failed to reduce vasodilation of arteries from both WKY and SHR. The restraining effect on vasodilation by EVs from normotensive WKY may be mediated by inhibition of eNOS (endothelial NO synthase), as addition of L-nitro-arginine methyl ester did not provide any additive effect. Moreover, circulating EVs from normotensive 6-week-old SHR-an age where SHRs have not yet developed hypertension-had similar restraining effect on vasodilation. In addition, delipidation of EVs did not alter the restraining effect of EVs from WKY but did restore the restraining effect of EVs from SHR. Finally, EVs from normotensive humans also restrained vasodilation of normotensive mouse arteries-an effect not observed in EVs from hypertensive humans. Taken together, our data support a vasoactive role of EVs that is altered in hypertension.

Adipocyte lipolysis drives acute stress-induced insulin resistance.

Stress hyperglycemia and insulin resistance are evolutionarily conserved metabolic adaptations to severe injury including major trauma, burns, or hemorrhagic shock (HS). In response to injury, the neuroendocrine system increases secretion of counterregulatory hormones that promote rapid mobilization of nutrient stores, impair insulin action, and ultimately cause hyperglycemia, a condition known to impair recovery from injury in the clinical setting. We investigated the contributions of adipocyte lipolysis to the metabolic response to acute stress. Both surgical injury with HS and counterregulatory hormone (epinephrine) infusion profoundly stimulated adipocyte lipolysis and simultaneously triggered insulin resistance and hyperglycemia. When lipolysis was inhibited, the stress-induced insulin resistance and hyperglycemia were largely abolished demonstrating an essential requirement for adipocyte lipolysis in promoting stress-induced insulin resistance. Interestingly, circulating non-esterified fatty acid levels did not increase with lipolysis or correlate with insulin resistance during acute stress. Instead, we show that impaired insulin sensitivity correlated with circulating levels of the adipokine resistin in a lipolysis-dependent manner. Our findings demonstrate the central importance of adipocyte lipolysis in the metabolic response to injury. This insight suggests new approaches to prevent insulin resistance and stress hyperglycemia in trauma and surgery patients and thereby improve outcomes.

Metformin Improves Cardiac Metabolism and Function, and Prevents Left Ventricular Hypertrophy in Spontaneously Hypertensive Rats.

Background In spontaneously hypertensive rats (SHR) we observed profound myocardial metabolic changes during early hypertension before development of cardiac dysfunction and left ventricular hypertrophy. In this study, we evaluated whether metformin improved myocardial metabolic abnormalities and simultaneously prevented contractile dysfunction and left ventricular hypertrophy in SHR. Methods and Results SHR and control Wistar-Kyoto rats were treated with metformin from 2 to 5 months of age, when SHR hearts exhibit metabolic abnormalities and develop cardiac dysfunction and left ventricular hypertrophy. We evaluated the effect of metformin on myocardial glucose uptake rates with dynamic 2-[18F] fluoro-2-deoxy-D-glucose positron emission tomography. We used cardiac MRI in vivo to assess the effect of metformin on ejection fraction, left ventricular mass, and end-diastolic wall thickness, and also analyzed metabolites, AMP-activated protein kinase and mammalian target-of-rapamycin activities, and mean arterial blood pressure. Metformin-treated SHR had lower mean arterial blood pressure but remained hypertensive. Cardiac glucose uptake rates, left ventricular mass/tibia length, wall thickness, and circulating free fatty acid levels decreased to normal, and ejection fraction improved in treated SHR. Hearts of treated SHR exhibited increased AMP-activated protein kinase phosphorylation and reduced mammalian target-of-rapamycin activity. Cardiac metabolite profiling demonstrated that metformin decreased fatty acyl carnitines and markers of oxidative stress in SHR. Conclusions Metformin reduced blood pressure, normalized myocardial glucose uptake, prevented left ventricular hypertrophy, and improved cardiac function in SHR. Metformin may exert its effects by normalizing myocardial AMPK and mammalian target-of-rapamycin activities, improving fatty acid oxidation, and reducing oxidative stress. Thus, metformin may be a new treatment to prevent or ameliorate chronic hypertension-induced left ventricular hypertrophy.

Ghrelin-Induced Sodium Reabsorption Is Mediated by PKA and Microtubule-Dependent αENaC Translocation in Female Rats.

Intrarenal ghrelin infusion activates ghrelin receptors in the kidney collecting duct (CD) to increase α epithelial sodium (Na+) channel (αENaC)-dependent Na+ reabsorption in vivo, but the underlying mechanisms are unknown. Seventy-two hours following uninephrectomy, 12-week-old female Sprague-Dawley rats received the following renal interstitial (RI) infusions for 1 hour after a 1-hour control: vehicle (n = 10), ghrelin (3 µg/minute; n = 8), ghrelin + phosphatidylinositol 3-kinase (PI3K) inhibitor LY-294002 (0.1 µg/kg/minute; n = 7), ghrelin + protein kinase A (PKA) inhibitor adenosine 3'5'-cyclic monophosphorothioate, Rp-isomer (10 µg/kg/minute; n = 8), ghrelin + microtubule polymerization inhibitor nocodazole (0.3 µg/kg/minute; n = 7), or ghrelin + actin polymerization inhibitor cytochalasin D (0.3 µg/kg/minute; n = 6). Compared with vehicle infusion, RI ghrelin induced a significant anti-natriuresis (urine Na+ excretion was reduced by 53.7% ± 6.8%; P < 0.001). This effect was abolished during concomitant PKA or microtubule inhibition (106.4% ± 9.4% and 109.7% ± 10.6% of vehicle infusion, respectively; P < 0.01 from ghrelin) but not during concomitant PI3K or actin inhibition (reduced by 48.6% ± 3.9% and 52.8% ± 12.7%, respectively; P < 0.001 and P < 0.01 from vehicle, respectively; P = not significant from ghrelin). Infusions had no effect on mean arterial pressure. Western blot analysis demonstrated that CD membrane but not total αENaC expression increased in response to ghrelin infusion compared with vehicle, (0.39 ± 0.05 vs 0.12 ± 0.02 arbitrary units; P < 0.01). This effect was abolished during PKA or microtubule inhibition but persisted during PI3K or actin inhibition. Neural precursor cell expressed, developmentally down-regulated 4 isoform 2 (Nedd4-2) dependent internalization of αENaC was not affected by ghrelin, indicating that microtubule-dependent forward trafficking of αENaC is necessary for anti-natriuretic responses to ghrelin. Taken together, these studies highlight the importance of PKA and microtubule polymerization in ghrelin-induced αENaC-mediated Na+ reabsorption.

Defective Renal Angiotensin III and AT2 Receptor Signaling in Prehypertensive Spontaneously Hypertensive Rats.

Background Previous studies demonstrated that angiotensin (Ang) III , not Ang II , is the predominant endogenous agonist for Ang type-2 receptor ( AT 2R)-induced natriuresis in normal rats, and that hypertensive 12-week-old spontaneously hypertensive rats ( SHR ) lack natriuretic responses to Ang III . This study tested whether prehypertensive SHR already have defective Ang III -induced natriuresis and determined possible mechanisms. Methods and Results Female and male normotensive 4-week-old SHR and Wistar Kyoto rats were studied after 24-hour systemic AT 1R blockade. Left kidneys received 30 minute renal interstitial infusions of vehicle followed by Ang III (3.5, 7.0, 14, and 28 nmol/kg per min; each dose for 30 minutes). Right kidneys received vehicle infusions. In 4-week-old Wistar Kyoto rats, renal interstitial Ang III increased urine sodium (Na+) excretion but failed to induce natriuresis in 4-week-old SHR . Renal Ang III levels were similar between Wistar Kyoto rats and SHR , making increased Ang III degradation as a possible cause for defective natriuresis in SHR unlikely. In Wistar Kyoto rats, renal interstitial Ang III induced translocation of AT 2Rs to apical plasma membranes of renal proximal tubule cells. Simultaneously, Ang III induced retraction of the major Na+ transporter Na+-H+ exchanger-3 ( NHE -3) from apical membranes and internalization of Na+/K+ ATP ase ( NKA ) from basolateral membranes of renal proximal tubule cells. Consistent with NHE -3 and NKA retraction, Ang III increased pS er552- NHE -3 and decreased pS er23- NKA . In contrast, in SHR , intrarenal Ang III failed to induce AT 2R translocation, NHE -3 or NKA retraction, pS er552- NHE -3 phosphorylation, or pS er23- NKA dephosphorylation. Conclusions These results indicate impaired Ang III / AT 2R signaling as a possible primary defect in prehypertensive SHR .

Metabolic Changes in Spontaneously Hypertensive Rat Hearts Precede Cardiac Dysfunction and Left Ventricular Hypertrophy.

Background Sustained pressure overload leads to changes in cardiac metabolism, function, and structure. Both time course and causal relationships between these changes are not fully understood. Therefore, we studied spontaneously hypertensive rats (SHR) during early hypertension development and compared them to control Wistar Kyoto rats. Methods and Results We serially evaluated myocardial glucose uptake rates (Ki) with dynamic 2-[18F] fluoro-2-deoxy-D-glucose positron emission tomography, and ejection fraction and left ventricular mass to body weight ratios with cardiac magnetic resonance imaging in vivo, determined glucose uptake and oxidation rates in isolated perfused hearts, and analyzed metabolites, mammalian target of rapamycin activity and endoplasmic reticulum stress in dissected hearts. When compared with Wistar Kyoto rats, SHR demonstrated increased glucose uptake rates (Ki) in vivo, and reduced ejection fraction as early as 2 months of age when hypertension was established. Isolated perfused SHR hearts showed increased glucose uptake and oxidation rates starting at 1 month. Cardiac metabolite analysis at 2 months of age revealed elevated pyruvate, fatty acyl- and branched chain amino acid-derived carnitines, oxidative stress, and inflammation. Mammalian target of rapamycin activity increased in SHR beginning at 2 months. Left ventricular mass to body weight ratios and endoplasmic reticulum stress were elevated in 5 month-old SHR. Conclusions Thus, in a genetic hypertension model, chronic cardiac pressure overload promptly leads to increased myocardial glucose uptake and oxidation, and to metabolite abnormalities. These coincide with, or precede, cardiac dysfunction while left ventricular hypertrophy develops only later. Myocardial metabolic changes may thus serve as early diagnostic markers for hypertension-induced left ventricular hypertrophy.

Intrarenal ghrelin receptor antagonism prevents high-fat diet-induced hypertension in male rats.

Excess weight gain contributes up to 65% of the risk of primary hypertension, and the increase in blood pressure in response to high-fat diet (HFD) is preceded by significant increases in renal tubular sodium (Na(+)) reabsorption. In normal rats, intrarenal ghrelin infusion increases distal nephron-dependent Na(+) reabsorption via activation of the intrarenal ghrelin receptor (GHSR). This study focusses on the role of intrarenal GHSR-mediated Na(+) reabsorption in HFD-induced hypertension. Dahl salt-sensitive rats received standard diet or HFD for 6 weeks. Rats underwent uninephrectomy and osmotic minipump implantation for chronic intrarenal delivery of vehicle (0.25 µL/h × 28 d), selective GHSR antagonist [D-Lys-3]-growth hormone releasing peptide-6 (0.2µM/d), or GHSR inverse agonist [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-substance P (SUB-P) (3.6µM/d). HFD rats with vehicle pumps had significantly increased renal GHSR expression compared with standard diet (0.092 ± 0.005 vs 0.065 ± 0.004 arbitrary units; P < .05), whereas acyl ghrelin levels were similar (16.3±6.2 vs 15.7±8.7 pg/g tissue). HFD rats with vehicle pumps became hypertensive after 2 weeks (P < .05) and showed a significant reduction in 24-hour urine Na(+) before hypertension. At this time, these rats showed an increase in collecting duct α-epithelial Na(+) channel, thereby providing a potential mechanism for the excess Na(+) reabsorption. In contrast, HFD rats with [D-Lys-3]-growth hormone releasing peptide-6 or SUB-P pumps never became hypertensive and did not show the reduction in urine Na(+). Because SUB-P blocks the constitutive, but not ghrelin-dependent, activity of the GHSR, and HFD-induced α-epithelial Na(+) channel up-regulation was abolished during GHSR antagonism, these data suggest that HFD increases the constitutive activity of renal GHSR to increase Na(+) reabsorption and induce hypertension in rats.

Mechanisms of dopamine D(1) and angiotensin type 2 receptor interaction in natriuresis.

Renal dopamine D(1)-like receptors (D(1)Rs) and angiotensin type 2 receptors (AT(2)Rs) are important natriuretic receptors counterbalancing angiotensin type 1 receptor-mediated tubular sodium reabsorption. Here we explore the mechanisms of D(1)R and AT(2)R interactions in natriuresis. In uninephrectomized, sodium-loaded Sprague-Dawley rats, direct renal interstitial infusion of the highly selective D(1)R agonist fenoldopam induced a natriuretic response that was abolished by the AT(2)R-specific antagonist PD-123319 or by microtubule polymerization inhibitor nocodazole but not by actin polymerization inhibitor cytochalasin D. By confocal microscopy and immunoelectron microscopy, fenoldopam translocated AT(2)Rs from intracellular sites to the apical plasma membranes of renal proximal tubule cells, and this translocation was abolished by nocodazole. Because D(1)R activation induces natriuresis via an adenylyl cyclase/cAMP signaling pathway, we explored whether this pathway is responsible for AT(2)R recruitment and AT(2)R-mediated natriuresis. Renal interstitial coinfusion of the adenylyl cyclase activator forskolin and 3-isobutly-1-methylxanthine induced natriuresis that was abolished either by PD-123319 or nocodazole but was unaffected by specific the D(1)R antagonist SCH-23390. Coadministration of forskolin and 3-isobutly-1-methylxanthine also translocated AT(2)Rs to the apical plasma membranes of renal proximal tubule cells; this translocation was abolished by nocodazole but was unaffected by SCH-23390. The results demonstrate that D(1)R-induced natriuresis requires AT(2)R recruitment to the apical plasma membranes of renal proximal tubule cells in a microtubule-dependent manner involving an adenylyl cyclase/cAMP signaling pathway. These studies provide novel insights regarding the mechanisms whereby renal D(1)Rs and AT(2)Rs act in concert to promote sodium excretion in vivo.