Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na+/H+ exchanger 3 (NHE3), a principal Na+ transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na+/H+exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2–12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

AngQb reduces blood pressure in SHR to levels obtained with an ACE inhibitor, and is immunogenic and well tolerated in humans. Therefore, vaccination against angiotensin II has the potential to become a useful antihypertensive treatment providing long-lasting effects and improving patient compliance.

Abstract. Excessive proteinuria due to loss of glomerular permselectivity in nephrotic syndrome can cause disturbances in renal salt and water handling with edema formation. Apart from oncotic and hydrostatic mechanisms associated with hypoalbuminemia, primary derangements in renal tubular sodium transport may contribute to the pathogenesis of nephrotic edema. Whereas there is evidence for an increase of cortical collecting duct sodium reabsorption in nephrotic rats, it remains controversial whether proximal tubule sodium transport may also be activated in this condition. The regulation of the cortical Na/H exchanger NHE3, the main pathway for Na reabsorption in the proximal tubule (PT), was investigated in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome. PAN rats developed reduced GFR, severe proteinuria, and sodium retention within 3 d. After 10 d, immunoblots of brush border vesicles revealed a decreased abundance of NHE3 in nephrotic animals. However, the Na/H antiporter activity in the same vesicle preparations was not significantly altered. Antiporter activity normalized for NHE3 protein was increased by 88% in nephrotic animals (P ϭ 0.025). Immunohistochemistry with the same polyclonal antibody as for immunoblots revealed a decrease of NHE3 abundance in PT. In contrast, immunoreactivity for the monoclonal antibody 2B9, which specifically recognizes the non-megalin-associated, transport-competent pool of NHE3, was higher in PANtreated rats than in controls. In conclusion, increased sodium reabsorption might be associated with a shift of NHE3 from an inactive pool to an active pool, thus contributing to sodium retention in a state of proteinuria.

Abstract. Albumin filtered by the glomerulus is reabsorbed in the proximal tubule. We have shown previously that proteinuria stimulates the proximal tubular Na ϩ /H ϩ exchanger 3 (NHE3) in rats. Activation of NHE3 may be a pathophysiologically important factor in the development of renal salt and water retention observed in the nephrotic syndrome. For examining whether albumin is a specific inducer of proximal tubular Na ϩ /H ϩ exchange and to determine the molecular mechanisms by which it regulates Na ϩ /H ϩ exchange, the effect of albumin on NHE3 in opossum kidney cells was studied. Albumin activated Na ϩ /H ϩ exchange in a time-and dosedependent manner up to 100% in 48 h. In the early phase of stimulation (2 to 12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, increased NHE3 activity was accompanied by increase in cell surface NHE3 protein. The increase in surface NHE3 was associated with increased bidirectional trafficking of NHE3 protein between intracellular compartments and the cell surface. At 48 h, total cell NHE3 protein abundance and mRNA were increased as well. Whereas NHE3 translation was increased, NHE3 protein half-life remained unchanged. The effects of albumin on NHE3 protein abundance were modified by hydrocortisone in a complicated pattern. These results indicate that albumin directly regulates proximal tubular NHE3 at multiple levels.

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