Impaired renal hemodynamics and glomerular hyperfiltration contribute to hypertension-induced renal injury

Recently, we reported a mutation in γ-adducin (ADD3) was associated with an impaired myogenic response of the afferent arteriole and hypertension-induced chronic kidney disease (CKD) in fawn hooded hypertensive (FHH) rats. However, the mechanisms by which altered renal blood flow (RBF) autoregulation promotes hypertension-induced renal injury remain to be determined. The present study compared the time course of changes in renal hemodynamics and the progression of CKD during the development of DOCA-salt hypertension in FHH 1BN congenic rats [wild-type (WT)] with an intact myogenic response versus FHH 1BN Add3KO ( Add3KO) rats, which have impaired myogenic response. RBF was well autoregulated in WT rats but not in Add3KO rats. Glomerular capillary pressure rose by 6 versus 14 mmHg in WT versus Add3KO rats when blood pressure increased from 100 to 150 mmHg. After 1 wk of hypertension, glomerular filtration rate increased by 38% and glomerular nephrin expression decreased by 20% in Add3KO rats. Neither were altered in WT rats. Proteinuria doubled in WT rats versus a sixfold increase in Add3KO rats. The degree of renal injury was greater in Add3KO than WT rats after 3 wk of hypertension. RBF, glomerular filtration rate, and glomerular capillary pressure were lower by 20%, 28%, and 19% in Add3KO rats than in WT rats, which was associated with glomerular matrix expansion and loss of capillary filtration area. The results indicated that impaired RBF autoregulation and eutrophic remodeling of preglomerular arterioles increase the transmission of pressure to glomeruli, which induces podocyte loss and accelerates the progression of CKD in hypertensive Add3KO rats.

Myogenic Vasoconstriction Requires Canonical Gq/11 Signaling of the Angiotensin II Type 1a Receptor in the Murine Vasculature

BackgroundThe myogenic response is an inherent vasoconstrictive property of resistance arteries to keep blood flow constant in response to increases in intravascular pressure. Angiotensin II (Ang II) type 1 receptors (AT1R) are broadly distributed, mechanoactivated receptors, which have been proposed to transduce myogenic vasoconstriction. However, the AT1R subtype(s) involved and their downstream G protein- and β-arrestin-mediated signaling pathways are still elusive.ObjectiveTo characterize the function of AT1aR and AT1bR in the regulation of the myogenic response of resistance size arteries and possible downstream signaling cascades mediated by Gq/11 and/or β-arrestins.MethodsWe used Agtr1a-/-, Agtr1b-/- and tamoxifen-inducible smooth muscle-specific AT1aR knockout mice (SM-Agtr1a mice). FR900359, [Sar1, Ile4, Ile8] Ang II (SII) and TRV120055 were used as selective Gq/11 protein inhibitor and biased agonists to activate non-canonical β-arrestin and canonical Gq/11 signaling of the AT1R, respectively.ResultsMyogenic and Ang II-induced vasoconstrictions were diminished in the perfused renal vasculature of Agtr1a-/- and SM-Agtr1a mice. Similar results were observed in isolated pressurized mesenteric and cerebral arteries. Myogenic tone and Ang II-induced vasoconstrictions were normal in arteries from Agtr1b-/- mice. The Gq/11 blocker FR900359 decreased myogenic tone and Ang II vasoconstrictions while selective biased targeting of AT1R β-arrestin signaling pathways had no effects.ConclusionThe present study demonstrates that myogenic arterial constriction requires Gq/11-dependent signaling pathways of mechanoactivated AT1aR but not G protein-independent, noncanonical alternative signaling pathways in the murine mesenteric, cerebral and renal circulation.

Abstract MP46: Knockout Of Add3 Impairs Autoregulation Of Renal Blood Flow And Promotes Hypertension-induced Renal Injury By Increasing Glomerular Capillary Pressure And Podocyte Loss

Recently, we reported that a mutation in γ-Adducin (ADD3) alters the actin cytoskeleton and is associated with an impaired myogenic response of the afferent arteriole and enhanced hypertension-induced renal disease. However, it remains to be determined whether the loss of ADD3 function promotes renal injury by increasing glomerular capillary pressure (Pgc) and podocyte loss or other mechanisms. The present study compared the time course of changes in renal hemodynamics and the progression of renal injury during the development of DOCA-salt hypertension in FHH 1 BN rats (WT) with an intact myogenic response vs. FHH 1 BN Add3KO rats (Add3KO) in which the myogenic response is impaired. When transmural pressure rose from 40 to 100 mmHg, the inner diameter of the preglomerular artery constricted by 19% (47.7 ± 4.3 to 38.4 ± 3.4 μm, n = 5) in WT, but it dilated by 28% (53.0 ± 2.2 to 67.9 ± 4.3 μm, n = 7) in Add3KO. Pgc was similar (50.1 ± 0.4 vs. 51.2 ± 0.8 mmHg, n = 6) at 100 mmHg, but rose by 6 and 14 mmHg in WT vs. Add3KO when perfusion pressure rose to 150 mmHg. Mean arterial pressure increased similarly and reached 177.7 ± 3.5 vs. 182.6 ± 2.3 mmHg (n = 9) after 3 weeks of DOCA-salt hypertension in WT vs. Add3KO. After 1 week of DOCA-salt hypertension, glomerular filtration rate (GFR) increased by 38% (1.2 ± 0.1 to 1.6 ± 0.1 ml/min/kidney, n = 6) and glomerular nephrin expression decreased by 20% (165.4 ± 4.5 to 131.614 ± 5.2 RFU, n = 7) in Add3KO. Both were unaltered in WT. Proteinuria increased 2 folds in WT (56.9 ± 4.7 to 168.6 ± 26.7 mg/day, n = 12) in the first week of hypertension vs. a 6-fold increase in Add3KO (64.6 ± 4.1 to 446.0 ± 41.9 mg/day, n = 9). After 3 weeks of hypertension, the degree of glomerulosclerosis (3.4 ± 0.1 vs. 2.4 ± 0.1 glomerular injury score, n = 9~12), protein cast formation (9.0% ± 0.8% vs. 4.8% ± 0.4% of area, n = 6), epithelial-mesenchymal transition, interstitial fibrosis (17.9% ± 0.8% vs. 9.5% ± 0.3% of area, n = 6), and inflammation was significantly greater in Add3KO vs. WT. GFR and Pgc were 28% and 19% lower in Add3KO than WT. These results indicate that the impaired myogenic response increases the transmission of pressure to the glomerulus to induce the loss of podocytes, which accelerates the progression of renal injury during the development of hypertension in Add3KO.

Myogenic Response to Increasing Concentrations of Ammonia Differs between Mammalian, Avian, and Fish Species: Cell Differentiation and Genetic Study

Ammonia is very toxic to the body and has detrimental effects on many different organ systems. Using cultured myoblast cells, we examined ammonia’s effect on myostatin expression, a negative regulator of skeletal muscle growth, and myotube diameters. The objective of this study was to examine how murine, avian, and fish cells respond to increasing levels of ammonia up to 50 mM. The murine myoblast cell line (C2C12), primary chick, and primary tilapia myoblast cells were cultured and then exposed to 10, 25, and 50 mM ammonium acetate, sodium acetate, and an untreated control for 24 h. High levels of ammonia were detrimental to the C2C12 cells, causing increased Myostatin (MSTN) expression and decreased myotube diameters between 10 and 25 mM (p < 0.002). Ammonia at 10 mM continued the positive myogenic response in the chick, with lower MSTN expression than the C2C12 cells and larger myotube diameters, but the myotube diameter at 50 mM ammonium acetate was significantly smaller than those at 10 and 25 mM (p < 0.001). However, chick myotubes at 50 mM were still significantly larger than the sodium acetate-treated and untreated control (p < 0.001). The tilapia cells showed no significant difference in MSTN expression or myotube diameter in response to increasing the concentrations of ammonia. Overall, these results confirm that increasing concentrations of ammonia are detrimental to mammalian skeletal muscle, while chick cells responded positively at lower levels but began to exhibit a negative response at higher levels, as the tilapia experienced no detrimental effects. The differences in ammonia metabolism strategies between fish, avian, and mammalian species could potentially contribute to the differences between species in response to high levels of ammonia. Understanding how ammonia affects skeletal muscle is important for the treatment of muscle wasting observed in liver failure patients.

Aging Impairs Renal Autoregulation in Mice

Impaired renal autoregulation permits more transmission of disturbance in systemic blood pressure, which initiates barotrauma in intrarenal microvasculatures such as glomerular and tubulointerstitial capillaries, contributing to the development of kidney damage and deterioration in renal function, especially under the conditions with high blood pressure. Although it has been postulated that autoregulatory efficiency is attenuated in the aging kidney, direct evidence remains lacking. In the present study, we measured the autoregulation of renal blood flow, myogenic response of afferent arteriole (Af-Art), tubuloglomerular feedback in vivo with micropuncture, as well as tubuloglomerular feedback in vitro in isolated perfused juxtaglomerular apparatus in young and aged C57BL/6 mice. We found that renal blood flow was not significantly changed in response to a defined elevation of renal arterial pressure in young mice but significantly increased in aged mice. Additionally, myogenic response of Af-Art measured by microperfusion with a stepwise increase in perfusion pressure was significantly blunted in the aging kidney, which is associated with the attenuation of intraluminal pressure-induced intracellular calcium increases, as well as the reduced expression of integrin α5 (Itga5) in Af-Art. Moreover, both tubuloglomerular feedback in vivo and in vitro were nearly inactive in the aging kidney, which is associated with the significantly reduced expression of adenosine A1 receptor (A1AR) and suppressed vasoconstrictor response to adenosine in Af-Art. In conclusion, this study demonstrates that aging impairs renal autoregulation with blunted myogenic response and inhibited tubuloglomerular feedback response. The underlying mechanisms involve the downregulations of integrin α5 and A1AR in the Af-Art.

Glutamine synthetase in avian muscle contributes to a positive myogenic response to ammonia compared with mammalian muscle

In mammalian models of cirrhosis, plasma ammonia concentration increases, having numerous adverse effects, including sarcopenia. The objective of this study was to identify differences between avian and mammalian myogenic response to applied ammonia and glutamine. Primary chicken breast and thigh, primary rat, and C2C12 myotubes were treated with ammonium acetate (AA, 10 mM) or glutamine (10 mM) for 24 h and compared with sodium acetate (10 mM) and untreated controls. Myostatin mRNA was significantly higher in C2C12 and rat myotubes treated with AA compared with glutamine and controls ( P < 0.01), whereas myostatin was unchanged in chicken myotubes. AA-treated C2C12 myotubes had significantly higher glutamine synthetase (GS) mRNA expression compared with controls, but GS protein expression was unchanged. In contrast, GS mRNA expression was unchanged in thigh myotubes, but GS protein expression was significantly higher in AA-treated thigh myotubes ( P < 0.05). In both breast and thigh myotubes, intracellular glutamine concentration was significantly increased in AA- and glutamine-treated myotubes compared with controls but was only increased in glutamine-treated C2C12 and rat myotubes ( P < 0.05). Glutamine concentration was significantly higher in all treatment media collected from avian myotube cultures compared with both C2C12 and rat media ( P < 0.01). Myotube diameter was significantly larger in avian myotubes after treatment with both AA and glutamine ( P < 0.05). C2C12 and rat myotubes had a significantly smaller myotube diameter after AA treatment ( P < 0.001). Altogether, these data support species differences in skeletal muscle ammonia metabolism and suggest that glutamine synthesis is a mechanism of ammonia utilization in avian muscle.

Exercise training restores the myogenic response in skeletal muscle resistance arteries and corrects peripheral edema in rats with heart failure

Impairment of the myogenic response can affect capillary hydrostatic pressure and contribute to peripheral edema and exercise intolerance, which are markers of heart failure (HF). The aim of this study was to assess the effects of exercise training (ET) on myogenic response in skeletal muscle resistance arteries and peripheral edema in HF rats, focusing on the potential signaling pathways involved in these adjustments. Male Wistar rats were submitted to either coronary artery occlusion or a sham-operated surgery. After 4 wk, an exercise test was performed, and the rats were divided into the following groups: untrained normal control (UNC) and untrained HF (UHF) and exercise- trained (on treadmill, 50–60% of maximal capacity) NC (TNC) and exercise-trained HF (THF). Caudal tibial artery (CTA) myogenic response was impaired in UHF compared with UNC, and ET restored this response in THF to NC levels and increased it in TNC. Rho kinase (ROCK) inhibitor abolished CTA myogenic response in the untrained and blunted it in exercise-trained groups. CTA-stored calcium (Ca2+) mobilization was higher in exercise-trained rats compared with untrained rats. The paw volume was higher in UHF rats, and ET decreased this response compared with UNC. Myogenic constriction was positively correlated with maximal running distance and negatively correlated with paw volume. The results demonstrate, for the first time, that HF impairs the myogenic response in skeletal muscle arteries, which contributes to peripheral edema in this syndrome. ET restores the myogenic response in skeletal muscle arteries improving Ca2+ sensitization and handling. Additionally, this paradigm also improves peripheral edema and exercise intolerance. NEW & NOTEWORTHY The novel and main finding of the present study is that moderate intensity exercise training restores the impaired myogenic response of skeletal muscle resistance arteries, exercise intolerance and peripheral edema in rats with heart failure. These results also show for the first time to our knowledge that exercise training improving calcium sensitization through the ROCK pathway and enhancing intracellular calcium handling could contribute to restoration of flow autoregulation to skeletal muscle in heart failure.