Distal urinary acidification from Homer Smith to the present

(Pro)renin receptor is required for prorenin-dependent and -independent regulation of vacuolar H+-ATPase activity in MDCK.C11 collecting duct cells

Prorenin binding to the prorenin receptor [(P)RR] results in nonproteolytic activation of prorenin but also directly (i.e., independent of angiotensin generation) activates signal transduction cascades that can lead to the upregulation of profibrotic factors. The (P)RR is an accessory protein of vacuolar-type H+-ATPase (V-ATPase) and is required for V-ATPase integrity. In addition, in collecting duct cells, prorenin-induced activation of Erk depends on V-ATPase activity. However, whether prorenin binding to the (P)RR directly regulates V-ATPase activity is as yet unknown. Here, we studied the effect of prorenin on plasma membrane V-ATPase activity in Madin-Darby canine kidney clone 11 (MDCK.C11) cells, which resemble intercalated cells of the collecting duct. Prorenin increased V-ATPase activity at low nanomolar concentrations, and the V-ATPase inhibitor bafilomycin A1, but not the angiotensin II type 1 and 2 receptor blockers irbesartan and PD-123319, prevented this. Increased, but not basal, V-ATPase activity was abolished by small interfering RNA depletion of the (P)RR. Unexpectedly, the putative peptidic (P)RR blocker handle region peptide also increasedV-ATPase activity in a (P)RR-dependent manner. Finally, [Arg8]-vasopressin-stimulated V-ATPase activity and cAMP production were also abolished by (P)RR depletion. Our results show that in MDCK.C11 cells, the (P)RR is required for prorenin-dependent and -independent regulation of V-ATPase activity.

Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney

Hypokalemia is a common electrolyte disorder that increases renal ammonia metabolism and can cause the development of an acid-base disorder, metabolic alkalosis. The ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), are expressed in the distal nephron and collecting duct and mediate critical roles in acid-base homeostasis by facilitating ammonia secretion. In the current studies, the effect of hypokalemia on renal Rhbg and Rhcg expression was examined. Normal Sprague-Dawley rats received either K(+)-free or control diets for 2 wk. Rats receiving the K(+)-deficient diet developed hypokalemia and metabolic alkalosis associated with significant increases in both urinary ammonia excretion and urine pH. Rhcg expression increased in the outer medullary collecting duct (OMCD). In OMCD intercalated cells, hypokalemia resulted in more discrete apical Rhcg expression and a marked increase in apical plasma membrane immunolabel. In principal cells, in the OMCD, hypokalemia increased both apical and basolateral Rhcg immunolabel intensity. Cortical Rhcg expression was not detectably altered by immunohistochemistry, although there was a slight decrease in total expression by immunoblot analysis. Rhbg protein expression was decreased slightly in the cortex and not detectably altered in the outer medulla. We conclude that in rat OMCD, hypokalemia increases Rhcg expression, causes more polarized apical expression in intercalated cells, and increases both apical and basolateral expression in the principal cell. Increased plasma membrane Rhcg expression in response to hypokalemia in the rat, particularly in the OMCD, likely contributes to the increased ammonia excretion and thereby to the development of metabolic alkalosis.

Effects of ischemia-reperfusion injury on renal ammonia metabolism and the collecting duct

Acute renal injury induces metabolic acidosis, but its specific effects on the collecting duct, the primary site for urinary ammonia secretion, the primary component of net acid excretion, are incompletely understood. We induced ischemia-reperfusion (I/R) acute renal injury in Sprague-Dawley rats by clamping the renal pedicles bilaterally for 30 min followed by reperfusion for 6 h. Control rats underwent sham surgery without renal pedicle clamping. I/R injury decreased urinary ammonia excretion significantly but did not persistently alter urine volume, Na+, K+, or bicarbonate excretion. Histological examination demonstrated cellular damage in the outer and inner medullary collecting duct, as well as in the proximal tubule and the thick ascending limb of the loop of Henle. A subset of collecting duct cells were damaged and/or detached from the basement membrane; these cells were present predominantly in the outer medulla and were less frequent in the inner medulla. Immunohistochemistry identified that the damaged/detached cells were A-type intercalated cells, not principal cells. Both TdT-mediated dUTP nick-end labeling (TUNEL) staining and transmission electron microscopic examination demonstrated apoptosis but not necrosis. However, immunoreactivity for caspase-3 was observed in the proximal tubule, but not in collecting duct intercalated cells, suggesting that mechanism(s) of collecting duct intercalated cell apoptosis differ from those operative in the proximal tubule. We conclude that I/R injury decreases renal ammonia excretion and is associated with intercalated cell-specific detachment and apoptosis in the outer and inner medullary collecting duct. These effects likely contribute to the metabolic acidosis frequently observed in acute renal injury.

Proton pump-driven cutaneous chloride uptake in anuran amphibia

Krogh introduced the concept of active ion uptake across surface epithelia of freshwater animals, and proved independent transports of Na(+) and Cl(-) in anuran skin and fish gill. He suggested that the fluxes of Na(+) and Cl(-) involve exchanges with ions of similar charge. In the so-called Krogh model, Cl(-)/HCO(3)(-) and Na(+)/H(+) antiporters are located in the apical membrane of the osmoregulatory epithelium. More recent studies have shown that H(+) excretion in anuran skin is due to a V-ATPase in mitochondria-rich (MR) cells. The pump has been localized by immunostaining and H(+) fluxes estimated by pH-stat titration and mathematical modelling of pH-profiles in the unstirred layer on the external side of the epithelium. H(+) secretion is voltage-dependent, sensitive to carbonic-anhydrase inhibitors, and rheogenic with a charge/ion-flux ratio of unity. Cl(-) uptake from freshwater is saturating, voltage independent, and sensitive to DIDS and carbonic-anhydrase inhibitors. Depending on anuran species and probably on acid/base balance of the animal, apical exit of protons is coupled to an exchange of Cl(-) with base (HCO(3)(-)) either in the apical membrane (gamma-type of MR cell) or in the basolateral membrane (alpha-type MR cell). The gamma-cell model accounts for the rheogenic active uptake of Cl(-) observed in several anuran species. There is indirect evidence also for non-rheogenic active uptake accomplished by a beta-type MR cell with apical base secretion and basolateral proton pumping. Several studies have indicated that the transport modes of MR cells are regulated via ion- and acid/base balance of the animal, but the signalling mechanisms have not been investigated. Estimates of energy consumption by the H(+)-ATPase and the Na(+)/K(+)-ATPase indicate that the gamma-cell accomplishes uptake of NaCl in normal and diluted freshwater. Under common freshwater conditions with serosa-positive or zero V(t), the K(+) conductance of the basolateral membrane would have to maintain the inward driving force for Na(+) uptake across the apical membrane. With the K(+) equilibrium potential across the basolateral membrane estimated to -105 mV, this would apply to external Na(+) concentrations down to 40-120 micromol/l. NaCl uptake from concentrations down to 10 micromol/l, as observed by Krogh, presupposes that the H(+) pump hyperpolarizes the apical membrane, which would then have to be associated with serosa-negative V(t). In diluted freshwater, exchange of cellular HCO(3)(-) with external Cl(-) seems to be possible only if the proton pump has the additional function of keeping the external concentration of HCO(3)(-) low. Quantitative considerations also lead to the conclusion that with the above extreme demand, at physiological intracellular pH of 7.2, the influx of Cl(-) via the apical antiporter and the passive exit of Cl(-) via basolateral channels would be possible within a common range of intracellular Cl(-) concentrations.

Mouse proton pump ATPase C subunit isoforms (C2-a and C2-b) specifically expressed in kidney and lung

The vacuolar-type H+-ATPases (V-ATPases) are multimeric proton pumps involved in a wide variety of physiological processes. We have identified two alternative splicing variants of C2 subunit isoforms: C2-a, a lung-specific isoform containing a 46-amino acid insertion, and C2-b, a kidney-specific isoform without the insert. Immunohistochemistry with isoform-specific antibodies revealed that V-ATPase with C2-a is localized specifically in lamellar bodies of type II alveolar cells, whereas the C2-b isoform is found in the plasma membranes of renal alpha and beta intercalated cells. Immunoprecipitation combined with immunohistological analysis revealed that C2-b together with other kidney-specific isoforms was selectively assembled to form a unique proton pump in intercalated cells. Furthermore, a chimeric yeast V-ATPase with mouse the C2-a or C2-b isoform showed a lower Km(ATP) and lower proton transport activity than that with C1 or Vma5p (yeast C subunit). These results suggest that V-ATPases with the C2-a and C2-b isoform are involved in luminal acidification of lamellar bodies and regulation of the renal acid-base balance, respectively.

Integrative physiology and functional genomics of epithelial function in a genetic model organism

Classically, biologists try to understand their complex systems by simplifying them to a level where the problem is tractable, typically moving from whole animal and organ-level biology to the immensely powerful "cellular" and "molecular" approaches. However, the limitations of this reductionist approach are becoming apparent, leading to calls for a new, "integrative" physiology. Rather than use the term as a rallying cry for classical organismal physiology, we have defined it as the study of how gene products integrate into the function of whole tissues and intact organisms. From this viewpoint, the convergence between integrative physiology and functional genomics becomes clear; both seek to understand gene function in an organismal context, and both draw heavily on transgenics and genetics in genetic models to achieve their goal. This convergence between historically divergent fields provides powerful leverage to those physiologists who can phrase their research questions in a particular way. In particular, the use of appropriate genetic model organisms provides a wealth of technologies (of which microarrays and knock-outs are but two) that allow a new precision in physiological analysis. We illustrate this approach with an epithelial model system, the Malpighian (renal) tubule of Drosophila melanogaster. With the use of the beautiful genetic tools and extensive genomic resources characteristic of this genetic model, it has been possible to gain unique insights into the structure, function, and control of epithelia.

Secretory-defect distal renal tubular acidosis is associated with transporter defect in H(+)-ATPase and anion exchanger-1

Recent progress in molecular physiology has permitted us to understand pathophysiology of various channelopathies at a molecular level. The secretion of H(+) from alpha-intercalated cells is mediated by apical plasma membrane H(+)-ATPase and basolateral plasma membrane anion exchanger-1 (AE1). Studies have demonstrated the lack of H(+)-ATPase immunostaining in the intercalated cells in a few patients with distal renal tubular acidosis (dRTA). Mutations in H(+)-ATPase and AE1 gene have recently been reported to cause dRTA. This study extends the investigation of the role of transporter defect in dRTA by using immunohistochemical methods. Eleven patients with hyperchloremic metabolic acidosis were diagnosed functionally to have secretory-defect dRTA: urine pH >5.5 during acidemia, normokalemia or hypokalemia, and urine-to-blood pCO(2) <25 mmHg during bicarbonaturia. Renal biopsy tissue was obtained from each patient, and immunohistochemistry was carried out using antibodies to H(+)-ATPase and AE1. For comparison, renal tissues from the patients who had no evidences of distal acidification defect by functional studies were used: four with glomerulopathy or tubulointerstitial nephritis (disease controls) and three from nephrectomized kidneys for renal cell carcinoma (normal controls). The H(+)-ATPase immunoreactivity in alpha-intercalated cells was almost absent in all of the 11 patients with secretory-defect dRTA. In addition, 7 of 11 patients with secretory-defect dRTA were accompanied by negative AE1 immunoreactivity. In both disease controls and normal controls, the immunoreactivity of H(+)-ATPase and AE1 was strong in alpha-intercalated cells. In conclusion, significant defect in acid-base transporters is the major cause of secretory-defect dRTA.

Remodeling the cellular profile of collecting ducts by chronic carbonic anhydrase inhibition

Factors regulating the differentiated phenotype of principal cells (PC) and A- and B-intercalated cells (IC) in kidney collecting ducts are poorly understood. However, we have shown previously that carbonic anhydrase II (CAII)-deficient mice have no IC in their medullary collecting ducts, suggesting a potential role for this enzyme in determining the cellular composition of this tubule segment. We now report that the cellular profile of the collecting ducts of adult rats can be remodeled by inhibiting CA activity in rats by using osmotic pumps containing acetazolamide. The 31-kDa subunit of the vacuolar H(+)-ATPase, the sodium/hydrogen exchanger regulatory factor NHE-RF, and the anion exchanger AE1 were used to identify IC subtypes by immunofluorescence staining, while aquaporin 2 and aquaporin 4 were used to identify PC. In the cortical collecting ducts of animals treated with acetazolamide for 2 wk, the percentage of B-IC decreased significantly (18 +/- 2 vs. 36 +/- 4%, P < 0.01) whereas the percentage of A-IC increased (82 +/- 2 vs. 64 +/- 4%, P < 0.01) with no change in the percentage of total IC in the epithelium. In some treated rats, B-IC were virtually undetectable. In the inner stripe of the outer medulla, the percentage of IC increased in treated animals (48 +/- 2 vs. 37 +/- 3%, P < 0.05) and the percentage of PC decreased (52 +/- 2 vs. 63 +/- 3%, P < 0.05). Moreover, IC appeared bulkier, protruded into the lumen, and showed a significant increase in the length of their apical (20.8 +/- 0.5 vs. 14.6 +/- 0.4 microm, P < 0.05) and basolateral membranes (25.8 +/- 0.4 vs. 23.8 +/- 0.5 microm, P < 0.05) compared with control rats. In the inner medullary collecting ducts of treated animals, the number of IC in the proximal third of the papilla was reduced compared with controls (11 +/- 4 vs. 40 +/- 11 IC/mm(2), P < 0.05). These data suggest that CA activity plays an important role in determining the differentiated phenotype of medullary collecting duct epithelial cells and that the cellular profile of collecting ducts can be remodeled even in adult rats. The relative depletion of cortical B-IC and the relative increase in number and hyperplasia of A-IC in the medulla may be adaptive processes that would tend to correct or stabilize the metabolic acidosis that would otherwise ensue following systemic carbonic anhydrase inhibition.

Ganglioside GM2-activator protein and vesicular transport in collecting duct intercalated cells

This study describes the molecular characterization of an antigen defined by an autoantibody from a woman with habitual abortion as GM2-activator protein. The patient showed no disorder of renal function. Accidentally with routine serum screening for autoantibodies, an immunoreactivity was found in kidney collecting duct intercalated cells. Three distinct patterns of immunostaining of intercalated cells were observed: staining of the apical pole, basolateral pole, and diffuse cytoplasmic labeling. Ultrastructurally, the immunoreactivity was associated with "studs," which represent the cytoplasmic domain of the vacuolar proton pump in intercalated cells. This pump is subjected to a shuttling mechanism from cytoplasmic stores to the cell membrane, which exclusively occurs in intercalated cells. Peptide sequences of a 23-kD protein purified from rat kidney cortex showed complete identity with corresponding sequences of GM2-activator protein. In the brain, GM2-activator protein is required for hexosaminidase A to split a sugar from ganglioside GM2. Because neither ganglioside GM2 nor GM1 (its precursor) is present in significant amounts in the kidney, the previous finding that this tissue contains the highest level of activator protein in the body was confusing. In this study, a novel role for GM2-activator protein in intercalated cells is proposed, and possible roles in the shuttling mechanism are discussed.