ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure—Hypertension and the Proximal Tubule

Renal control of effective circulating volume (ECV) is key for circulatory performance. When renal sodium excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish ECV. Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule sodium hydrogen exchanger 3 (NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the proximal tubule, where it reabsorbs a significant fraction of the filtered sodium. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and the MV base, where NHE3 is less active. A high-salt diet and acute hypertension both retract NHE3 to the base and reduce proximal tubule sodium reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: The angiotensin-converting enzyme (ACE) inhibitor captopril redistributes NHE3 to the base and subsequent angiotensin II (AngII) infusion returns NHE3 to the body of the MV and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension; that is, NHE3 remains in the body of the MV, due to the high local AngII level and inflammation, and exhibits a compensatory decrease in abundance driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of the proximal tubule AngII receptors interleukin-17A or interferon-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance, which improves the pressure natriuresis response and reduces the need for elevated blood pressure to facilitate circulating volume balance

Mechanisms of Regulation of Proximal Tubule Sodium Transporters in Obesity-Induced Hypertension

Hypertension is one of the common complications of obesity. Using a rat model of diet induced obesity and hypertension we investigated some of the mechanisms that are involved in regulation of blood pressure in obesity. The first aim of this study was to determine the role of proximal tubule transporters on the renal sodium handling in obese hypertensive (OP) and lean normotensive (OR) rats. An acute increase in renal perfusion pressure resulted in a blunted natriuretic response in OP vs. OR rats and indicated that increased sodium reabsorption in the proximal tubule is casual, at least in part, for hypertension in OP rats. Subsequently, protein expression and activity of Na,K-ATPase and NHE3 were increased in obese rats compared to lean rats. Moreover, in OP rats more NHE3 was associated with its active pool located in the microvillus region. Together, these results suggest that hypertension in obese rats is characterized by the impaired pressure-natriuresis and diuresis that can be explained by the increased activity of proximal tubule sodium transporters. Previous results from our lab determined that peroxisome proliferator activated receptor γ (PPARγ) has reduced expression and activity in the kidney of OP vs. OR rats. Therefore, in the second aim we investigated the effect of PPARγ ligand activation on expression and activity of proximal tubule Na+ transporters in OP and OR rats. In addition, by employing in vitro studies using proximal tubule epithelial cells, we determined whether pioglitazone exerts its effect via direct PPARγ activation. Pioglitazone reduced systolic blood pressure in obese rats while having no effect in lean rats. However, it increased sodium retention in the lean group. Pioglitazone increased Na,K-ATPase activity in OP rats, while its protein expression was increased in both groups. In contrast, NHE3 activity was reduced in obese rats treated with pioglitazone and protein expression was decreased in both groups. Pioglitazone did not have an effect on NHE3 localization in obese rats, but in lean rats, it had tendency to redistribute NHE3 towards the more active membrane pool. In cells transiently transfected to overexpress or silence PPARγ, we demonstrated that pioglitazone reduced Na,K-ATPase and NHE3 abundance via PPARγ activation. Collectively, the results indicated that pioglitazone reduced blood pressure in the obese group most likely by decreasing activity of NHE3. However, other factors besides trafficking are involved in the transporter regulation. Pioglitazone did not reduce blood pressure in lean rats, suggesting that the metabolic milieu is an important determinant of the pioglitazone differential effect on the blood pressure and on the proximal tubule transporters. Nitric oxide (NO) plays an important role in regulating pressure natriuresis and diuresis and its availability seems to be altered in obese animals and humans. The third aim was designed to examine the role of NO on blood pressure, pressure natriuresis and expression of sodium transporters NHE3 and Na,K-ATPase in OP and OR rats. To determine the role of NO, we performed in vivo study using L-NAME for chronic NO inhibition. The NO inhibition did not change glomerular filtration rate in either of the groups. Natriuresis and diuresis was significantly decreased only in treated OR rats. Also, NHE3 protein expression and activity were significantly elevated in treated vs. non-treated OR rats, with no significant changes in OP rats. Moreover, L-NAME caused a shift of NHE3 to the active pool located in microvillus region in OR group only. In conclusion, normotensive OR rats are more susceptible to NO deficiency and the mechanism involves an increase in activity of NHE3 with the transporter redistribution playing a significant role. In addition, we investigated in cell culture whether hormones elevated in obesity can modulate Na,K-ATPase and NHE3 via cGMP production. In vitro experiments provided some evidence that angiotensin II and insulin interact with the NO signaling pathway at the level of cGMP. cGMP could affect transporter activity by phosphorylation which could account for the effects determined in vivo

The Increase in Renal Sodium Excretion in Response to Angiotensin II Infusion in Exercised Female Rats is Dependent on a Rise in Renal Perfusion Pressure

Prior studies in this lab have shown that chronically exercised female spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats excrete a greater amount of sodium in response to angiotensin II (Ang II) infusion than do sedentary rats. The current study determined if the difference in renal sodium excretion persisted when the renal perfusion pressure (RPP) was held constant. Female SHR and female WKY rats were separated into sedentary and exercised groups at 4 weeks of age. The exercise group voluntarily exercised for at least eight weeks using an exercise wheel and time/distance monitor. At 13 weeks of age or older, rats were anesthetized and catheters were placed into the jugular vein, and carotid and femoral arteries. A noose, placed around the abdominal aorta, was continually adjusted to maintain a constant RPP during Ang II infusion. Ang II was infused at 0, 0.125, 0.5, and 2.0 micrograms/ml saline, each for 15 minutes. MAP and RPP were measured continually. Urine was collected during each 15 minute period and analyzed for sodium excretion. Sodium excretion did not significantly increase with the Ang II infusions when RPP was held constant, and no difference was found between the exercised and sedentary rats. The increase in renal sodium excretion observed in exercised female rats compared to sedentary appears to be due to a difference in the pressure natriuresis response

The potential roles of osmotic and non-osmotic sodium handling in mediating effects of SGLT2 inhibitors on heart failure

Concomitant type 2 diabetes and chronic kidney disease (CKD) increases the risk of heart failure (HF). Recent STUDIES: demonstrate beneficial effects of sodium-glucose cotransporter 2 inhibitors (SGLT2i) on CKD progression and HF hospitalization in patients with and without diabetes. In addition to inhibiting glucose reabsorption, SGLT2i reduce proximal tubular sodium reabsorption, possibly leading to transient natriuresis. We review the hypothesis that SGLT2i's natriuretic and osmotic diuretic effects mediate their cardio-protective effects. The degree to which these benefits are related to changes in sodium, independent of the kidney, is currently unknown. Aside from effects on osmotically active sodium, we explore the intriguing possibility that SGLT2i could also modulate non-osmotic sodium storage. This alternative hypothesis is based on emerging literature that challenges the traditional two-compartment model of sodium balance to provide support for a three-compartment model that includes the binding of sodium to glycosaminoglycans, such as those in muscles and skin. This recent research on non-osmotic sodium storage, as well as direct cardiac effects of SGLT2i, provides possibilities for other ways in which SGLT2i might mitigate HF risk. Overall, we review the effects of SGLT2i on sodium balance and sensitivity, cardiac tissue, interstitial fluid and plasma volume, and non-osmotic sodium storage

Prostaglandin-independent protection by furosemide from oliguric ischemic renal failure in conscious rats

Prostaglandin-independent protection by furosemide from oliguric ischemic renal failure in conscious rats. In 38 conscious rats divided into seven groups, acute unilateral ischemic renal failure was induced by 1 hour of complete occlusion of the left renal artery while the contralateral kidney remained intact. Renal excretory function of the left kidney was monitored up to 144 hours after ischemia and revealed a typical course of oliguric renal failure with oligoanuria persisting for more than 48 hours. Urinary osmolality and sodium concentration became plasma isotonic after release of renal artery occlusion and approximated control values on day 6 after ischemia. In nine rats, the i.v. infusion of furosemide before (6 µg/min/100g body wt) and after (12 µg/min/ 100g body wt) renal artery occlusion protected the ischemic kidney from oligoanuria with endogenous creatinine clearance of 0.42 ± 0.11 ml/min/g kidney wt 5 hours after ischemia. Tubular absorption of sodium and water was at least partially preserved 36 hours after ischemia when infusion of furosemide was stopped. The loop diuretic significantly (P < 0.01) increased total urinary prostaglandin (PG) E2 excretion before and after renal artery occlusion; and 5 hours after ischemia, PGE2 excretion from the ischemic kidney significantly exceeded that from the intact kidney (P < 0.05). Indomethacin (1 mg/100g body wt) administered in six animals markedly suppressed control PGE2 excretion (P < 0.05) as well as the furosemide-induced rise in urinary PG excretion before and after ischemia but did not modify the protective effect of the diuretic in this experimental model. Inhibition of PG synthesis, however, reduced urinary flow rate and sodium and potassium excretion of the contralateral intact kidney and almost completely prevented its compensatory rise in creatinine clearance. The results indicate that mechanisms other than the intrarenal prostaglandin system must be considered to mediate the protective effects of furosemide in acute ischemic renal failure.Protection par le furosemide, indépendante des prostaglandins, de l'insuffisance rénale oligurique et ischémique chez le rat éveillé. Trente huit rats éveillés, répartis en sept groupes, ont été mis en insuffisance rénale aiguë ischémique unilatérale par occlusion complète de l'artère rénale gauche pendant 1 heure alors que le rein controlatéral était intact. La fonction excrétoire du rein a été surveillée pendant 144 heures après l'ischémie et elle a subi l'évolution typique de l'insuffisance rénale oligurique avec persistance de l'oligo-anurie pendant plus de 48 heures. L'osmolalité urinaire et la concentration de sodium sont devenues égales à celles du plasma après la levée de l'occlusion artérielle rénale et se sont rapprochées des valeurs contrôles au sixième jour après l'ischémie. Chez neuf rats la perfusion de furosemide avant (6 µg/min/100g poids corporel) et après (12 µg/ min/100g poids corporel) l'occlusion de l'artère rénale a protégé le rein ischémique de l'oligo-anurie, avec des clearances de la créatinine de 0,42 ± 0,11 ml/min/g de rein 5 heures après l'ischémie. La réabsorption tubulaire de sodium et d'eau était au moins partiellement préservée 36 heures après l'ischémie quand la perfusion de furosemide était arrêtée. Ce diurétique augmente significativement (P < 0,01) l'excrétion urinaire de prostaglan-dines (PG) E2 avant et après l'occlusion de l'artère rénale; 5 heures après l'ischémie l'excrétion de PGE2 par le rein lésé est significativement supérieure à celle du rein intact (P < 0,05). L'indométhacine (1 mg/100g poids corporel) administrée à six animaux diminue considérablement l'excrétion basale de PGE2 (P < 0,05) de même que l'augmentation, dépendante du furosémide, de l'excrétion urinaire de PG avant et après l'ischémie, mais ne change pas l'effet protecteur du diurétique dans ce modèle expérimental. L'inhibition de la synthèse de PG, cependant, diminue le débit urinaire et l'excrétion de sodium et de potassium du rein intact et empêche presque complètement l'augmentation compensatrice de la clearance de la créatinine. Ces résultats indiquent que d'autres mécanismes que le système des prostaglandines intrarénales doivent être invoqués comme médiateurs des effets protecteurs du furosémide dans l'insuffisance rénale aiguë ischémique

The Acute And Chronic Effects Of Resveratrol On Renal Function And Blood Pressure

We investigated the acute and chronic effects of resveratrol on renal function and blood pressure. We hypothesized that resveratrol would act as a renal vasodilator through a nitric oxide-dependent mechanism. In our acute studies, we found an intravenous bolus of resveratrol influenced changes in renal hemodynamics by increasing renal blood flow and decreasing renal vascular resistance. The mechanism of renal vasodilation was nitric oxide dependent and through a reduction of endogenous reactive oxygen species. Resveratrol-induced renal vasodilation was not influenced by COX metabolism and vasodilatory prostanoids. We found with continuous intravenous resveratrol infusion induced significant renal vasodilation while not altering either glomerular filtration rate or blood pressure in normal rats. Resveratrol infusion produced significant natriuresis at all doses, independent of hemodynamic responses, suggesting it may have a direct effect on renal tubular sodium handling independent of perfusion pressure, renal blood flow, or changes in renal vascular resistance. We hypothesized chronic ingestion of resveratrol would reduce Ang II-induced and fructose-induced salt-sensitive increases in blood pressure by decreasing sodium reabsorption through a NO-dependent mechanism. In both models of elevated blood pressure, resveratrol treatment did not blunt initial increases in blood pressure in either the Ang II or fructose-induced salt sensitive model nor did resveratrol increase sodium excretion or decrease sodium retention. However, following 3 weeks of treatment the increases in blood pressure began to reverse on the Ang II model, but not the fructose salt sensitive model (during 2 weeks of treatment). Resveratrol did not decrease oxidative stress, as measured by urinary excretion of 8-isoprostane, in either model. Notable, short term exposure to fructose (2 weeks), independent of blood pressure or salt, increased oxidative stress. It is well-defined from our data that resveratrol has distinct physiological actions on nitric oxide production, free radical formation, natriuresis, and sustained hypertension. However, the data does not suggest efficacy in prevention of hypertension

Effects of urodilatin on natriuresis in cirrhosis patients with sodium retention

BACKGROUND: Sodium retention and ascites are serious clinical problems in cirrhosis. Urodilatin (URO) is a peptide with paracrine effects in decreasing sodium reabsorption in the distal nephron. Our aim was to investigate the renal potency of synthetic URO on urine sodium excretion in cirrhosis patients with sodium retention and ascites. METHODS: Seven cirrhosis patients with diuretics-resistant sodium retention received a short-term (90 min) infusion of URO in a single-blind, placebo-controlled cross-over study. In the basal state after rehydration the patients had urine sodium excretion < 50 mmol/24 h. RESULTS: URO transiently increased urine sodium excretion from 22 ± 16 μmol/min (mean ± SD) to 78 ± 41 μmol/min (P < 0.05) and there was no effect of placebo (29 ± 14 to 44 ± 32). The increase of URO's second messenger after the receptor, cGMP, was normal. URO had no effect on urine flow or on blood pressure. Most of the patients had highly elevated plasma levels of renin, angiotensin II and aldosterone and URO did not change these. CONCLUSION: The short-term low-dose URO infusion increased the sodium excretion of the patients. The increase was small but systematic and potentially clinically important for such patients. The small response contrasts the preserved responsiveness of the URO receptors. The markedly activated systemic pressor hormones in cirrhosis evidently antagonized the local tubular effects of URO