Revisiting voltage-coupled H+ secretion in the collecting duct.

Experimental studies have shown that V-type ATPase-driven H+ secretion is dependent on the transepithelial voltage. On this basis the "voltage hypothesis" of urinary acidification by the collecting duct was derived. Accordingly, it has been supposed that the lumen-negative potential created by the reabsorption of Na+ via the epithelial sodium channel (ENaC) enhances electrogenic H+ secretion via the V-type H+-ATPase. This concept continues to be widely used to explain acid/base disorders. Importantly, however, a solid proof-of-principle for the voltage hypothesis in physiologically relevant situations has not been reached. Rather, it has been challenged by recent in vivo functional studies. In this review we outline the arguments and experimental observations explaining why voltage-coupled H+ secretion in the collecting duct often appears poorly applicable to rationalize for changes of H+ secretion as a function of more or less ENaC function in the collecting duct.

Low potassium activation of proximal mTOR/AKT signaling is mediated by Kir4.2.

The renal epithelium is sensitive to changes in blood potassium (K+). We identify the basolateral K+ channel, Kir4.2, as a mediator of the proximal tubule response to K+ deficiency. Mice lacking Kir4.2 have a compensated baseline phenotype whereby they increase their distal transport burden to maintain homeostasis. Upon dietary K+ depletion, knockout animals decompensate as evidenced by increased urinary K+ excretion and development of a proximal renal tubular acidosis. Potassium wasting is not proximal in origin but is caused by higher ENaC activity and depends upon increased distal sodium delivery. Three-dimensional imaging reveals Kir4.2 knockouts fail to undergo proximal tubule expansion, while the distal convoluted tubule response is exaggerated. AKT signaling mediates the dietary K+ response, which is blunted in Kir4.2 knockouts. Lastly, we demonstrate in isolated tubules that AKT phosphorylation in response to low K+ depends upon mTORC2 activation by secondary changes in Cl- transport. Data support a proximal role for cell Cl- which, as it does along the distal nephron, responds to K+ changes to activate kinase signaling.

High dietary K+ intake inhibits proximal tubule transport.

The impact of chronic dietary K+ loading on proximal tubule (PT) function was measured using free-flow micropuncture along with measurements of overall kidney function, including urine volume, glomerular filtration rate, and absolute and fractional Na+ and K+ excretion in the rat. Feeding animals a diet with 5% KCl [high K+ (HK)] for 7 days reduced glomerular filtration rate by 29%, increased urine volume by 77%, and increased absolute K+ excretion by 202% compared with rats on a 1% KCl [control K+ (CK)] diet. HK did not change absolute Na+ excretion but significantly increased fraction excretion of Na+ (1.40% vs. 0.64%), indicating that fractional Na+ absorption is reduced by HK. PT reabsorption was assessed using free-flow micropuncture in anesthetized animals. At 80% of the accessible length of the PT, measurements of inulin concentration indicated volume reabsorption of 73% and 54% in CK and HK, respectively. At the same site, fractional PT Na+ reabsorption was 66% in CK animals and 37% in HK animals. Fractional PT K+ reabsorption was 66% in CK and 37% in HK. To assess the role of Na+/H+ exchanger isoform 3 (NHE3) in mediating these changes, we measured NHE3 protein expression in total kidney microsomes as well as surface membranes using Western blots. We found no significant changes in protein in either cell fraction. Expression of the Ser552 phosphorylated form of NHE3 was also similar in CK and HK animals. Reduction in PT transport may facilitate K+ excretion and help balance Na+ excretion by shifting Na+ reabsorption from K+-reabsorbing to K+-secreting nephron segments.NEW & NOTEWORTHY In rats fed a diet rich in K+, proximal tubules reabsorbed less fluid, Na+, and K+ compared with those in animals on a control diet. Glomerular filtration rates also decreased, probably due to glomerulotubular feedback. These reductions may help to maintain balance of the two ions simultaneously by shifting Na+ reabsorption to K+-secreting nephron segments.

Regulation of glomerulotubular balance. IV. Implication of aquaporin 1 in flow-dependent proximal tubule transport and cell volume.

The water channel aquaporin-1 (AQP1) is the principal water pathway for isotonic water reabsorption in the kidney proximal tubule (PT). We investigated flow-mediated fluid (Jv) and [Formula: see text] ([Formula: see text]) reabsorption in PTs of the mouse kidney by microperfusion in wild-type (WT) and AQP1 knockout (KO) mice. Experiments were simulated in an adaptation of a mathematical model of the rat PT. An increase in perfusion rate from 5 to 20 nL/min increased Jv and [Formula: see text] in PTs of WT mice. AQP1 KO mice significantly decreased Jv at low and high flow rates compared with control. In contrast, [Formula: see text] was not reduced at either low or high flow rates. Cell volume showed no significant difference between WT and AQP1 KO mice. Renal clearance experiments showed significantly higher urine flow in AQP1 KO mice, but there was no significant difference in either Na+ and K+ or [Formula: see text] excretion. Acid-base parameters of blood pH, Pco2, [Formula: see text], and urine pH were the same in both WT and KO mice. In model calculations, tubules whose tight junction (TJ) water permeability (Pf) was that assigned to the rat TJ, showed no difference in Jv between WT and KO, whereas TJ Pf set to 25% of the rat predicted Jv concordant with our observations from AQP1 KO. These results affirm the dominance of AQP1 in mediating isotonic water reabsorption by the mouse PT and demonstrate that flow-stimulated [Formula: see text] reabsorption is intact and independent of AQP1. With reference to the model, the findings also suggest that TJ water flux in the PT is less prominent in the mouse than in the rat kidney.NEW & NOTEWORTHY We found an absence of flow-dependent modulation of fluid absorption but no effect on either proximal tubule (PT) [Formula: see text] absorption or acid-base parameters in the aquaporin 1 (AQP1) knockout mouse. We affirmed the dominance of the water channel AQP1 in mediating isotonic water reabsorption by the mouse PT and demonstrated that flow-stimulated [Formula: see text] reabsorption is independent of AQP1. With reference to the model, the findings also suggest that tight junctional water flux in the PT is less prominent in the mouse than rat kidney.

A mathematical model of the rat kidney. IV. Whole kidney response to hyperkalemia.

The renal response to acute hyperkalemia is mediated by increased K+ secretion within the connecting tubule (CNT), flux that is modulated by tubular effects (e.g., aldosterone) in conjunction with increased luminal flow. There is ample evidence that peritubular K+ blunts Na+ reabsorption in the proximal tubule, thick ascending Henle limb, and distal convoluted tubule (DCT). Although any such reduction may augment CNT delivery, the relative contribution of each is uncertain. The kidney model of this laboratory was recently advanced with representation of the cortical labyrinth and medullary ray. Model tubules capture the impact of hyperkalemia to blunt Na+ reabsorption within each upstream segment. However, this forces the question of the extent to which increased Na+ delivery is transmitted past the macula densa and its tubuloglomerular feedback (TGF) signal. Beyond increasing macula densa Na+ delivery, peritubular K+ is predicted to raise cytosolic Cl- and depolarize macula densa cells, which may also activate TGF. Thus, although the upstream reduction in Na+ transport may be larger, it appears that the DCT effect is critical to increasing CNT delivery. Beyond the flow effect, hyperkalemia reduces ammoniagenesis and reduced ammoniagenesis enhances K+ excretion. What this model provides is a possible mechanism. When cortical [Formula: see text] is taken up via peritubular Na+-K+([Formula: see text])-ATPase, it acidifies principal cells. Consequently, reduced ammoniagenesis increases principal cell pH, thereby increasing conductance of both the epithelial Na+ channel and renal outer medullary K+ channel, enhancing K+ excretion. In this model, the effect of aldosterone on principal cells, diminished DCT Na+ reabsorption, and reduced ammoniagenesis all provide relatively equal and additive contributions to renal K+ excretion.NEW & NOTEWORTHY Hyperkalemia blunts Na+ reabsorption along the nephron, and increased CNT Na+ delivery facilitates K+ secretion. The model suggests that tubuloglomerular feedback limits transmission of proximal effects past the macula densa, so that it is DCT transport that is critical. Hyperkalemia also reduces PCT ammoniagenesis, which enhances K+ excretion. The model suggests a mechanism, namely, that reduced cortical ammonia impacts CNT transport by raising cell pH and thus increasing both ENaC and ROMK conductance.

Sex difference in kidney electrolyte transport III: Impact of low K intake on thiazide-sensitive cation excretion in male and female mice.

We compared the regulation of the NaCl cotransporter (NCC) in adaptation to a low-K (LK) diet in male and female mice. We measured hydrochlorothiazide (HCTZ)-induced changes in urine volume (UV), glomerular filtration rate (GFR), absolute (ENa, EK), and fractional (FENa, FEK) excretion in male and female mice on control-K (CK, 1% KCl) and LK (0.1% KCl) diets for 7 days. With CK, NCC-dependent ENa and FENa were larger in females than males as observed previously. However, with LK, HCTZ-induced ENa and FENa increased in males but not in females, abolishing the sex differences in NCC function as observed in CK group. Despite large diuretic and natriuretic responses to HCTZ, EK was only slightly increased in response to the drug when animals were on LK. This suggests that the K-secretory apparatus in the distal nephron is strongly suppressed under these conditions. We also examined LK-induced changes in Na transport protein expression by Western blotting. Under CK conditions females expressed more NCC protein, as previously reported. LK doubled both total (tNCC) and phosphorylated NCC (pNCC) abundance in males but had more modest effects in females. The larger effect in males abolished the sex-dependence of NCC expression, consistent with the measurements of function by renal clearance. LK intake did not change NHE3, NHE2, or NKCC2 expression, but reduced the amount of the cleaved (presumably active) form of γENaC. LK reduced plasma K to lower levels in females than males. These results indicated that males had a stronger NCC-mediated adaptation to LK intake than females.

ENaC and ROMK channels in the connecting tubule regulate renal K+ secretion.

We measured the activities of epithelial Na channels (ENaC) and ROMK channels in the distal nephron of the mouse kidney and assessed their role in the process of K+ secretion under different physiological conditions. Under basal dietary conditions (0.5% K), ENaC activity, measured as amiloride-sensitive currents, was high in cells at the distal end of the distal convoluted tubule (DCT) and proximal end of the connecting tubule (CNT), a region we call the early CNT (CNTe). In more distal parts of the CNT (aldosterone-sensitive portion [CNTas]), these currents were minimal. This functional difference correlated with alterations in the intracellular location of ENaC, which was at or near the apical membrane in CNTe and more cytoplasmic in the CNTas. ROMK activity, measured as TPNQ-sensitive currents, was substantial in both segments. A mathematical model of the rat nephron suggested that K+ secretion by the CNTe predicted from these currents provides much of the urinary K+ required for K balance on this diet. In animals fed a K-deficient diet (0.1% K), both ENaC and ROMK currents in the CNTe decreased by ∼50%, predicting a 50% decline in K+ secretion. Enhanced reabsorption by a separate mechanism is required to avoid excessive urinary K+ losses. In animals fed a diet supplemented with 3% K, ENaC currents increased modestly in the CNTe but strongly in the CNTas, while ROMK currents tripled in both segments. The enhanced secretion of K+ by the CNTe and the recruitment of secretion by the CNTas account for the additional transport required for K balance. Therefore, adaptation to increased K+ intake involves the extension of robust K+ secretion to more distal parts of the nephron.

Restoration of proximal tubule flow-activated transport prevents cyst growth in polycystic kidney disease.

Flow-activated Na+ and HCO3- transport in kidney proximal tubules (PT) underlies relatively constant fractional reabsorption during changes in glomerular filtration rate (GFR) or glomerulotubular balance (GTB). In view of hypothesized connections of epithelial cilia to flow sensing, we examined flow-activated transport in 3 polycystic kidney disease-related mouse models based on inducible conditional KO of Pkd1, Pkd2, and Kif3a. PTs were harvested from mice after gene inactivation but prior to cyst formation, and flow-mediated PT transport was measured. We confirm that higher flow increased both Na+ and HCO3- absorption in control mice, and we observed that this flow effect was preserved in PTs of Pkd1-/- and Kif3a-/-mice. However, flow activation was absent in Pkd2+/- and Pkd2-/- PT. In heterozygous (Pkd2+/-) mice, a dopamine receptor 1 (DA1) antagonist (SCH23390) restored transport flow sensitivity. When given chronically, this same antagonist reduced renal cyst formation in Pkd2-/-, as evidenced by reduced kidney weight, BUN, and the cystic index, when compared with untreated mice. In contrast, SCH23390 did not prevent cyst formation in Pkd1-/- mice. These results indicate that Pkd2 is necessary for normal GTB and that restoration of flow-activated transport by DA1 antagonist can slow renal cyst formation in Pkd2-/- mice.

A mathematical model of the rat kidney. III. Ammonia transport.

Ammonia generated within the kidney is partitioned into a urinary fraction (the key buffer for net acid excretion) and an aliquot delivered to the systemic circulation. The physiology of this partitioning has yet to be examined in a kidney model, and that was undertaken in this work. This involves explicit representation of the cortical labyrinth, so that cortical interstitial solute concentrations are computed rather than assigned. A detailed representation of cortical vasculature has been avoided by making the assumption that solute concentrations within the interstitium and peritubular capillaries are likely to be identical and that there is little to no modification of venous composition as blood flows to the renal vein. The model medullary ray has also been revised to include a segment of proximal straight tubule, which supplies ammonia to this region. The principal finding of this work is that cortical labyrinth interstitial ammonia concentration is likely to be several fold higher than systemic arterial ammonia. This elevation of interstitial ammonia enhances ammonia secretion in both the proximal convoluted tubule and distal convoluted tubule, with uptake by Na+-K+-ATPases of both segments. Model prediction of urinary ammonia excretion was concordant with measured values, but at the expense of greater ammoniagenesis, with high rates of renal venous ammonia flux. This derives from a limited capability of the model medulla to replicate the high interstitial ammonia concentrations that are required to drive collecting duct ammonia secretion. Thus, renal medullary ammonia trapping appears key to diverting ammonia from the renal vein to urine, but capturing the underlying physiology remains a challenge.NEW & NOTEWORTHY This is the first mathematical model to estimate solute concentrations within the kidney cortex. The model predicts cortical ammonia to be several fold greater than in the systemic circulation. This higher concentration drives ammonia secretion in proximal and distal tubules. The model reveals a gap in our understanding of how ammonia generated within the cortex is channeled efficiently into the final urine.

A mathematical model of the rat kidney. II. Antidiuresis.

Kidney water conservation requires a hypertonic medullary interstitium, NaCl in the outer medulla and NaCl and urea in the inner medulla, plus a vascular configuration that protects against washout. In this work, a multisolute model of the rat kidney is revisited to examine its capacity to simulate antidiuresis. The first step was to streamline model computation by parallelizing its Jacobian calculation, thus allowing finer medullary spatial resolution and more extensive examination of model parameters. It is found that outer medullary NaCl is modestly increased when transporter density in ascending Henle limbs from juxtamedullary nephrons is scaled to match the greater juxtamedullary solute flow. However, higher NaCl transport produces greater CO2 generation and, by virtue of countercurrent vascular flows, establishment of high medullary Pco2. This CO2 gradient can be mitigated by assuming that a fraction of medullary transport is powered anaerobically. Reducing vascular flows or increasing vessel permeabilities does little to further increase outer medullary solute gradients. In contrast to medullary models of others, vessels in this model have solute reflection coefficients close to zero; increasing these coefficients provides little enhancement of solute profiles but does generate high interstitial pressures, which distort tubule architecture. Increasing medullary urea delivery via entering vasa recta increases inner medullary urea, although not nearly to levels found in rats. In summary, 1) medullary Na+ and urea gradients are not captured by the model and 2) the countercurrent architecture that provides antidiuresis also produces exaggerated Pco2 profiles and is an unappreciated constraint on models of medullary function.

Accurate measurement of fast endocytic recycling kinetics in real time.

The fast turnover of membrane components through endocytosis and recycling allows precise control of the composition of the plasma membrane. Endocytic recycling can be rapid, with some molecules returning to the plasma membrane with a half time <5 min. Existing methods to study these trafficking pathways utilize chemical, radioactive or fluorescent labeling of cell surface receptors in pulse-chase experiments, which require tedious washing steps and manual collection of samples. Here, we introduce a live-cell endocytic recycling assay based on a newly designed cell-impermeable fluorogenic ligand for HaloTag, Janelia Fluor 635i (JF635i, where i indicates impermeant), which allows real-time detection of membrane receptor recycling at steady state. We used this method to study the effect of iron depletion on transferrin receptor (TfR) recycling using the chelator desferrioxamine. We found that this perturbation significantly increases the TfR recycling rate. The high temporal resolution and simplicity of this assay provides a clear advantage over extant methods and makes it ideal for large scale cellular imaging studies. This assay can be adapted to examine other cellular kinetic parameters such as protein turnover and biosynthetic trafficking.

Sex difference in kidney electrolyte transport II: impact of K+ intake on thiazide-sensitive cation excretion in male and female mice.

We studied sex differences in response to high K+ (HK) intake on thiazide-sensitive cation (Na+ and K+) excretion in wild-type (WT) and ANG II receptor subtype 1a (AT1aR) knockout (KO) mice. Renal clearance experiments were performed to examine Na+-Cl- cotransporter (NCC) activity on mice fed with control and HK (5% KCl, 7 days) diets. Hydrochlorothiazide (HCTZ)-induced changes in urine volume, glomerular filtration rate, absolute Na+ and K+ excretion, and fractional excretion were compared. HK-induced changes in NCC, Na+/H+ exchanger isoform 3 (NHE3), and ENaC expression were examined by Western blot analysis. In WT animals under the control diet, HCTZ-induced cation excretion was greater in female animals, reflecting larger increases in Na+ excretion, since there was little sex difference in HCTZ-induced K+ excretion. Under the HK diet, the sex difference in HCTZ-induced cation excretion was reduced because of larger increments in K+ excretion in male animals. The fraction of K+ excretion was 57 ± 5% in male WT animals and 36 ± 4% in female WT animals (P < 0.05), but this difference was absent in AT1aR KO mice. NCC abundance was higher in female animals than in male animals but decreased by similar fractions on HK diet. NHE3 abundance decreased, whereas cleaved forms of γ-ENaC increased, with HK in all groups; these changes were similar in male and female animals and were not significantly affected by AT1aR ablation. These results indicate that, with the HK diet, male animals display greater distal Na+ delivery and greater activation of K+ secretion mechanisms, all suggesting a more powerful male adaptation to HK intake.

Regulation of renal Na transporters in response to dietary K.

Changes in the expression of Na transport proteins were measured in the kidneys of mice with increased dietary K intake for 1 wk. The epithelial Na channel (ENaC) was upregulated, with enhanced expression of full-length and cleaved forms of α-ENaC and cleaved γ-ENaC. At the same time, the amount of the NaCl cotransporter NCC and its phosphorylated form decreased by ~50% and ~80%, respectively. The expression of the phosphorylated form of the Na-K-2Cl cotransporter NKCC2 also decreased, despite an increase in overall protein content. The effect was stronger in males (80%) than in females (40%). This implies that less Na+ is reabsorbed in the thick ascending limb of Henle's loop and distal convoluted tubule along with Cl-, whereas more is reabsorbed in the aldosterone-sensitive distal nephron in exchange for secreted K+. The abundance of the proximal tubule Na/H exchanger NHE3 decreased by ~40%, with similar effects in males and females. Time-course studies indicated that NCC and NHE3 proteins decreased progressively over 7 days on a high-K diet. Expression of mRNA encoding these proteins increased, implying that the decreased protein levels resulted from decreased rates of synthesis or increased rates of degradation. The potential importance of changes in NHE3, NKCC2, and NCC in promoting K+ excretion was assessed with a mathematical model. Simulations indicated that decreased NHE3 produced the largest effect. Regulation of proximal tubule Na+ transport may play a significant role in achieving K homeostasis.

Gender difference in kidney electrolyte transport. I. Role of AT1a receptor in thiazide-sensitive Na+-Cl- cotransporter activity and expression in male and female mice.

We studied gender differences in Na+-Cl- cotransporter (NCC) activity and expression in wild-type (WT) and AT1a receptor knockout (KO) mice. In renal clearance experiments, urine volume (UV), glomerular filtration rate, absolute Na+ (ENa) and K+ (EK), and fractional Na+ (FENa) and K+ excretion were measured and compared at peak changes after bolus intravenous injection of hydrochlorothiazide (HCTZ; 30 mg/kg). In WT, females responded more strongly than males to HCTZ, with larger fractional increases of UV (7.8- vs. 3.4-fold), ENa (11.7- vs. 5.7-fold), FENa (7.9- vs. 4.9-fold), and EK (2.8- vs. 1.4-fold). In contrast, there were no gender differences in the responses to the diuretic in KO mice; HCTZ produced greater effects on male KO than on WT but similar effects on females. In WT, total (tNCC) and phosphorylated (pNCC) NCC protein expressions were 1.8- and 4.6-fold higher in females compared with males (P < 0.05), consistent with the larger response to HCTZ. In KO mice, tNCC and pNCC increased significantly in males to levels not different from those in females. There were no gender differences in the expression of the Na+/H+ exchanger (NHE3) in WT; NHE3 protein decreased to similar extents in male and female KO animals, suggesting AT1a-mediated NHE3 expression in proximal tubules. The resulting increase in delivery of NaCl to the distal nephron may underlie increased NCC expression and activity in mice lacking the AT1a receptor.

Regulation of glomerulotubular balance: flow-activated proximal tubule function.

The purpose of this review is to summarize our knowledge and understanding of the physiological importance and the mechanisms underlying flow-activated proximal tubule transport. Since the earliest micropuncture studies of mammalian proximal tubule, it has been recognized that tubular flow is an important regulator of sodium, potassium, and acid-base transport in the kidney. Increased fluid flow stimulates Na+ and HCO3- absorption in the proximal tubule via stimulation of Na/H-exchanger isoform 3 (NHE3) and H+-ATPase. In the proximal tubule, brush border microvilli are the major flow sensors, which experience changes in hydrodynamic drag and bending moment as luminal flow velocity changes and which transmit the force of altered flow to cytoskeletal structures within the cell. The signal to NHE3 depends upon the integrity of the actin cytoskeleton; the signal to the H+-ATPase depends upon microtubules. We have demonstrated that alterations in fluid drag impact tubule function by modulating ion transporter availability within the brush border membrane of the proximal tubule. Beyond that, there is evidence that transporter activity within the peritubular membrane is also modulated by luminal flow. Secondary messengers that regulate the flow-mediated tubule function have also been delineated. Dopamine blunts the responsiveness of proximal tubule transporters to changes in luminal flow velocity, while a DA1 antagonist increases flow sensitivity of solute reabsorption. IP3 receptor-mediated intracellular Ca2+ signaling is critical to transduction of microvillus drag. In this review, we summarize our findings of the regulatory mechanism of flow-mediated Na+ and HCO3- transport in the proximal tubule and review available information about flow sensing and regulatory mechanism of glomerulotubular balance.

Responses of distal nephron Na+ transporters to acute volume depletion and hyperkalemia.

We assessed effects of acute volume reductions induced by administration of diuretics in rats. Direct block of Na+ transport produced changes in urinary electrolyte excretion. Adaptations to these effects appeared as alterations in the expression of protein for the distal nephron Na+ transporters NCC and ENaC. Two hours after a single injection of furosemide (6 mg/kg) or hydrochlorothiazide (HCTZ; 30 mg/kg) Na+ and K+ excretion increased but no changes in the content of activated forms of NCC (phosphorylated on residue T53) or ENaC (cleaved γ-subunit) were detected. In contrast, amiloride (0.6 mg/kg) evoked a similar natriuresis that coincided with decreased pT53NCC and increased cleaved γENaC. Alterations in posttranslational membrane protein processing correlated with an increase in plasma K+ of 0.6-0.8 mM. Decreased pT53NCC occurred within 1 h after amiloride injection, whereas changes in γENaC were slower and were blocked by the mineralocorticoid receptor antagonist spironolactone. Increased γENaC cleavage correlated with elevation of the surface expression of the subunit as assessed by in situ biotinylation. Na depletion induced by 2 h of furosemide or HCTZ treatment increases total NCC expression without affecting ENaC protein. However, restriction of Na intake for 10 h (during the day) or 18 h (overnight) increased the abundance of both total NCC and of cleaved α- and γENaC. We conclude that the kidneys respond acutely to hyperkalemic challenges by decreasing the activity of NCC while increasing that of ENaC. They respond to hypovolemia more slowly, increasing Na+ reabsorptive capacities of both of these transporters.