Activation of the Hypoxia-Inducible Factor Pathway Inhibits Epithelial Sodium Channel-Mediated Sodium Transport in Collecting Duct Principal Cells

Background Active sodium reabsorption is the major factor influencing renal oxygen consumption and production of reactive oxygen species (ROS). Increased sodium reabsorption uses more oxygen, which may worsen medullary hypoxia and produce more ROS via enhanced mitochondrial ATP synthesis. Both mechanisms may activate the hypoxiainducible factor (HIF) pathway. Because the collecting duct is exposed to low oxygen pressure and variations of active sodium transport, we assessed whether the HIF pathway controls epithelial sodium channel (ENaC)-dependent sodium transport. Methods We investigated HIF's effect on ENaC expression in mpkCCDcl4 cells (a model of collecting duct principal cells) using real-time PCR and Western blot and ENaC activity by measuring amiloride-sensitive current. We also assessed the effect of hypoxia and sodium intake on abundance of kidney sodium transporters in wild-type and inducible kidney tubule-specific Hif1α knockout mice. Results In cultured cells, activation of the HIF pathway by dimethyloxalylglycine or hypoxia inhibited sodium transport and decreased expression of βENaC and γENaC, as well as of Na,K-ATPase. HIF1α silencing increased βENaC and γENaC expression and stimulated sodium transport. A constitutively active mutant of HIF1α produced the opposite effect. Aldosterone and inhibition of the mitochondrial respiratory chain slowly activated the HIF pathway, suggesting that ROS may also activate HIF. Decreased γENaC abundance induced by hypoxia in normal mice was abolished in Hif1α knockout mice. Similarly, Hif1α knockout led to increased γENaC abundance under high sodium intake. Conclusions This study reveals that γENaC expression and activity are physiologically controlled by the HIF pathway, which may represent a negative feedback mechanism to preserve oxygenation and/or prevent excessive ROS generation under increased sodium transport.

Cryo-EM structure of the sodium-driven chloride/bicarbonate exchanger NDCBE

SLC4 transporters play significant roles in pH regulation and cellular sodium transport. The previously solved structures of the outward facing (OF) conformation for AE1 (SLC4A1) and NBCe1 (SLC4A4) transporters revealed an identical overall fold despite their different transport modes (chloride/bicarbonate exchange versus sodium-carbonate cotransport). However, the exact mechanism determining the different transport modes in the SLC4 family remains unknown. In this work, we report the cryo-EM 3.4 Å structure of the OF conformation of NDCBE (SLC4A8), which shares transport properties with both AE1 and NBCe1 by mediating the electroneutral exchange of sodium-carbonate with chloride. This structure features a fully resolved extracellular loop 3 and well-defined densities corresponding to sodium and carbonate ions in the tentative substrate binding pocket. Further, we combine computational modeling with functional studies to unravel the molecular determinants involved in NDCBE and SLC4 transport.

Transepithelial nasal potential difference in patients with, and at risk of acute respiratory distress syndrome

BackgroundImpaired alveolar fluid clearance, determined in part by alveolar sodium transport, is associated with acute respiratory distress syndrome (ARDS). Nasal sodium transport may reflect alveolar transport. The primary objective of this prospective, observational study was to determine if reduced nasal sodium transport, as measured by nasal potential difference (NPD), was predictive of the development of and outcome from ARDS.MethodsNPD was measured in 15 healthy controls and in 88 patients: 40 mechanically ventilated patients defined as ‘at-risk’ for ARDS, 61 mechanically ventilated patients with ARDS (13 who were previously included in the ‘at-risk’ group) and 8 ARDS survivors on the ward.ResultsIn at-risk subjects, maximum NPD (mNPD) was greater in those who developed ARDS (difference –8.4 mV; 95% CI –13.8 to –3.7; p=0.005) and increased mNPD predicted the development of ARDS before its onset (area under the curve (AUC) 0.75; 95% CI 0.59 to 0.89). In the ARDS group, mNPD was not significantly different for survivors and non-survivors (p=0.076), and mNPD was a modest predictor of death (AUC 0.60; 95% CI 0.45 to 0.75). mNPD was greater in subjects with ARDS (−30.8 mV) than in at-risk subjects (−24.2 mV) and controls (−19.9 mV) (p<0.001). NPD values were not significantly different for survivors and controls (p=0.18).ConclusionsIncreased NPD predicts the development of ARDS in at-risk subjects but does not predict mortality. NPD increases before ARDS develops, is greater during ARDS, but is not significantly different for controls and survivors. These results may reflect the upregulated sodium transport necessary for alveolar fluid clearance in ARDS. NPD may be useful as a biomarker of endogenous mechanisms to stimulate sodium transport. Larger studies are now needed to confirm these associations and predictive performance.

Dynamics of proximal and distal transport of sodium in case of bilateral gonadectomy and the efficacy of hormonal replacement therapy

Introduction. The characteristic feature of current trauma in Ukraine and in the world is the prevalence of highly energetic skeletal trauma which is followed by the appearance of combined and multiple injuries with the formation of multiorgan dysfunction and failure. Because of the increase of life duration, the high interest of current traumatology is the investigation process of trauma course in older people. Among them the high interest is given to females in postmenopausal period. The decrease of estrogen level causes very unfavorable ground for the trauma course. However, the influence of severe trauma on changes in proximal and distal transport of sodium in case of hypoestrogenic state is not fully investigated. There is no available data concerning the effectiveness of hormonal replacement therapy in this condition.The goal of the study: is to investigate the influence of cranioskeletal injury on the dynamics of proximal and distal sodium transport in rats with bilateral ovariectomy in the period of late changes of traumatic disease and evaluate the effectiveness of hormonal replacement therapy. Materials and methods. Experiments were performed on 42 non-linear white female rats with the weight of 200-220 g. The model of hypoestrogenic state was performed via surgical removal of the ovaries. 1 month after the gonadectomy in rats skeletal trauma was performed. As a treatment option in one subgroup of rats with gonadectomy and cranioskeletal injury we performed hormonal replacement therapy. The control groups were formed by intact rats and osteoporotic rats 1 month after gonadectomy which were not injured. In control groups and after 1 and 2 months of posttraumatic period we were investigating the functional state of kidneys by performing the water upload method. Afterwards we performed the laboratory checkup of creatinine and sodium levels in serum and urine and additionally calculated the values of proximal and distal sodium transport. Results and its discussion. It was found that the level of proximal and distal sodium transport depends on the estrogen level, the decrease of its secretion leads to the decrease of sodium reabsorption in tubules and the increase in secretion. These changes were more prevalent in proximal tubules. In the course of trauma 1 month after posttraumatic period the value was decreasing in both research groups. However, in rats with gonadectomy the changes were much bigger and stayed at the same level until the 2nd month of the experiment. In rats without gonadectomy these values were achieving the same levels as control group until 2nd month. The use of combined hormonal replacement therapy caused the increase of proximal and distal sodium transport in comparison to the group without receiving treatment. The results that we got though was not achieving the level of rats without gonadectomy, but it showed the perspective for the use of combined hormonal replacement therapy in case of hypoestrogenic state for the decrease of renal dysfunction. Conclusions: Bilateral ovariectomy in female rats after 1 month shows the decrease of proximal and distal sodium levels comparing to rats with preserved gonads. The cranioskeletal injury that happened 1 month after bilateral ovariectomy causes the much bigger decrease of proximal and distal sodium transport comparing to rats with preserved gonads. The use of combined hormonal replacement therapy caused lower changes in proximal and distal sodium transport comparing to rats without this treatment.