Nephrotic syndrome is associated with increased plasma K+ concentration, intestinal K+ losses, and attenuated urinary K+ excretion: a study in rats and humans.

The present study tested the hypotheses that nephrotic syndrome (NS) leads to renal K+ loss because of augmented epithelial Na+ channel (ENaC) activity followed by downregulation of renal K+ secretory pathways by suppressed aldosterone. The hypotheses were addressed by determining K+ balance and kidney abundance of K+ and Na+ transporter proteins in puromycin aminonucleoside (PAN)-induced rat nephrosis. The effects of amiloride and angiotensin II type 1 receptor and mineralocorticoid receptor (MR) antagonists were tested. Glucocorticoid-dependent MR activation was tested by suppression of endogenous glucocorticoid with dexamethasone. Urine and plasma samples were obtained from pediatric patients with NS in acute and remission phases. PAN-induced nephrotic rats had ENaC-dependent Na+ retention and displayed lower renal K+ excretion but elevated intestinal K+ secretion that resulted in less cumulated K+ in NS. Aldosterone was suppressed at day 8. The NS-associated changes in intestinal, but not renal, K+ handling responded to suppression of corticosterone, whereas angiotensin II type 1 receptor and MR blockers and amiloride had no effect on urine K+ excretion during NS. In PAN-induced nephrosis, kidney protein abundance of the renal outer medullary K+ channel and γ-ENaC were unchanged, whereas the Na+-Cl- cotransporter was suppressed and Na+-K+-ATPase increased. Pediatric patients with acute NS displayed suppressed urine Na+-to-K+ ratios compared with remission and elevated plasma K+ concentration, whereas fractional K+ excretion did not differ. Acute NS is associated with less cumulated K+ in a rat model, whereas patients with acute NS have elevated plasma K+ and normal renal fractional K+ excretion. In NS rats, K+ balance is not coupled to ENaC activity but results from opposite changes in renal and fecal K+ excretion with a contribution from corticosteroid MR-driven colonic secretion.

Physiology and pathophysiology of the plasminogen system in the kidney.

The plasminogen system is important for fibrinolysis in addition to tissue remodeling and inflammation with significance for kidney disease. The system consists of the circulating zymogen plasminogen (Plg) and the tissue- and urokinase-type plasminogen activators, tPA and uPA, expressed in the glomeruli, endothelium and tubular epithelium, respectively, and the inhibitors α2-antiplasmin and plasminogen activator inhibitor-type1, PAI-1. Plasminogen is activated by surface receptors, some with renal expression: urokinase-type plasminogen activator receptor (uPAR), plasminogen receptor KT (Plg-RKT), and tPA, most evident in the endothelium. Plasmin may exert effects through protease-activated receptors, PARs, expressed in the kidney. Deletion of plasminogen system component genes confers no major developmental or renal phenotypes in normal mice. In glomerular injury and renal interstitial fibrosis, deletion of various components, notably Plg, uPA, PAI, and uPAR is associated with protection suggesting a disease promoting effect of plasmin, in some cases exerted through PAR1 receptor activation. Plasminogen and uPA are aberrantly filtrated across the glomerular barrier in proteinuria, and plasminogen is activated in the tubular fluid. In the tubular fluid, plasmin may activate proteolytically the epithelial sodium channel (ENaC) and inhibit the apical calcium transporter transient receptor potential cation channel subfamily V member 5 (TRPV5), which could explain impaired sodium excretion and enhanced calcium excretion in proteinuria. Amiloride, a potassium-sparing diuretic, inhibits urokinase and plasmin activation in the tubular fluid and uPAR expression in vitro, which highlights new indications for an old drug. Protease inhibitors lowered blood pressure and antagonized fibrosis in salt-sensitive Dahl rats. Current knowledge indicates that the plasminogen system aggravates renal disease by direct and indirect hypertensive effects and is a promising target to antagonize disease progression.

Urine exosomes from healthy and hypertensive pregnancies display elevated level of α-subunit and cleaved α- and γ-subunits of the epithelial sodium channel-ENaC.

Preeclampsia is characterized by hypertension, proteinuria, suppression of plasma renin-angiotensin-aldosterone, and impaired urine sodium excretion. Aberrantly filtered plasmin in urine may activate proteolytically the γ-subunit of the epithelial sodium channel (ENaC) and promote Na+ reabsorption and urine K+ loss. Plasma and urine was sampled from patients with preeclampsia, healthy pregnant controls and non-pregnant women, and from patients with nephrostomy catheters. Aldosterone concentration, urine plasminogen, and protein were determined. Exosomes were isolated by ultracentrifugation. Immunoblotting was used to detect exosome markers; γ-ENaC (two different epitopes within the inhibitory peptide tract), α-ENaC, and renal outer medullary K-channel (ROMK) and compared with human kidney cortex homogenate. Urine total plasmin(ogen) was significantly increased in preeclampsia, plasma and urine aldosterone was higher in pregnancy compared to non-pregnancy, and the urine Na/K ratio was lower in preeclampsia compared to healthy pregnancy. Exosome markers ALIX and AQP-2 were stably associated with exosomes across groups. Exosomal α-ENaC-subunit migrated at 75 kDa and dominantly at 50 kDa and was significantly elevated in pregnancy. In human kidney cortex tissue and two of four pelvis catheter urine, ~90-100 kDa full-length γ-ENaC was detected while no full-length γ-ENaC but 75, 60, and 37 kDa variants dominated in voided urine exosomes. There was no difference in γ-ENaC protein abundances between healthy pregnancy and preeclampsia. ROMK was detected inconsistently in urine exosomes. Pregnancy and preeclampsia were associated with increased abundance of furin-cleaved α-ENaC subunit while γ-subunit appeared predominantly in cleaved form independently of conditions and with a significant contribution from post-renal cleavage.

The acute blood pressure-lowering effect of amiloride is independent of endothelial ENaC and eNOS in humans and mice.

AIMS: The epithelial sodium channel (ENaC) is expressed in cultured endothelial cells and inhibitory coupling to eNOS activity has been proposed. The present study tested the hypothesis that ENaC blockers increase systemic NO-products and lower blood pressure in patients and mice, depending on eNOS. METHODS: NO-products and cGMP were measured in diabetes patient urine and plasma samples before and after amiloride treatment (20-40 mg for two days, plasma n = 22, urine n = 12 and 5-10 mg for eight weeks, plasma n = 52, urine n = 55). Indwelling catheters were implanted in the femoral artery and vein in mice for continuous arterial blood pressure and heart rate recordings and infusion. RESULTS: Treatment with amiloride for two days increased plasma and urine NO-products, while plasma cGMP decreased and urinary cGMP was unchanged in patient samples. Eight weeks of treatment with amiloride did not alter NO-products and cGMP. In mice, amiloride boli of 5, 50, and 500 µg/kg lowered heart rate and arterial blood pressure significantly and acutely. Benzamil had no effect on pressure and raised heart rate. In hypertensive eNOS-/- and L-NAME-treated mice, amiloride lowered blood pressure significantly. L-NAME infusion significantly decreased NO-products in plasma; amiloride and eNOS-deletion had no effect. An acetylcholine bolus resulted in acute blood pressure drop that was attenuated in eNOS-/- and L-NAME mice. ENaC subunit expressions were not detected consistently in human and mouse arteries and endothelial cells. CONCLUSION: Amiloride has an acute hypotensive action not dependent on ENaC and eNOS and likely related to the heart.