P2Y(2) receptor-mediated inhibition of amiloride-sensitive short circuit current in M-1 mouse cortical collecting duct cells

Extracellular nucleotides modulate renal ion transport. Our previous results in M-1 cortical collecting duct cells indicate that luminal and basolateral ATP via P2Y2 receptors stimulate luminal Ca2+-activated Cl- channels and inhibit Na+ transport. Here we address the mechanism of ATP-mediated inhibition of Na+ transport. M-1 cells had a transepithelial voltage (V(te)) of -31.4 +/- 1.3 mV and a transepithelial resistance (R(te)) of 1151 +/- 28 Omegacm(2). The amiloride-sensitive short circuit current (I(sc)) was -28.0 +/- 1.1 microA/cm2. The ATP-mediated activation of Cl- channels was inhibited when cytosolic Ca2+ increases were blocked with cyclopiazonic acid (CPA). Without CPA the ATP-induced [Ca2+](i) increase was paralleled by a rapid and transient R(te) decrease (297 +/- 51 Omegacm(2)). In the presence of CPA, basolateral ATP led to an R(te) increase by 144 +/- 17 Omegacm(2) and decreased V(te) from -31 +/- 2.6 to -26.6 +/- 2.5 mV. Isc dropped from -28.6 +/- 2.4 to -21.6 +/- 1.9 microA/cm2. Similar effects were observed with luminal ATP. In the presence of amiloride, ATP was without effect. This reflects ATP-mediated inhibition of Na+ absorption. Lowering [Ca2+](i) by removal of extracellular Ca2+ did not alter the ATP effect. PKC inhibition or activation were without effect. Na+ absorption was activated by pH(i) alkalinization and inhibited by pH(i) acidification. ATP slightly acidified M-1 cells by 0.05 +/- 0.005 pH units, quantitatively not explaining the ATP-induced effect. In summary this indicates that extracellular ATP via luminal and basolateral P2Y2 receptors inhibits Na+ absorption. This effect is not mediated via [Ca2+](i), does not involve PKC and is to a small part mediated via intracellular acidification.

Na(+)/H(+)exchangers: linking osmotic dysequilibrium to modified cell function

The Na(+)/H(+) exchangers (NHEs) are among the major ion transporters involved in cell volume regulation. NHE activation leads to a cellular influx of Na(+) ions and extrusion of H(+) ions, which are readily replenished from intracellular buffers. This will result in a net import of Na(+). In many systems NHE operates in parallel to Cl(-)/ HCO3(-) exchange, resulting in cellular uptake of NaCl. The influx of osmotically obliged water will consequently lead to cell swelling. This makes NHEs suitable to serve as powerful mechanisms for increasing cell volume (CV). The low volume threshold for NHE activation enables the cells to respond to very minute reductions of the CV. By the coupling to the export of H(+) ions cell volume regulatory NHE activation may lead to changes in intracellular pH. On the other hand NHEs are activated by a broad variety of ligands and by intracellular acidosis, which, in turn, may consequently lead to cell swelling. In addition, NHEs are linked to other intracellular proteins and structures, like e.g. the cytoskeleton, which themelves are involved in the regulation of numerous cellular processes. Therefore NHEs link CV regulation to a diversity of cellular functions, both in physiological and pathophysiological conditions. Six isoforms of the Na(+)/H(+) exchanger, termed NHE1--6, have been cloned so far. NHE 1--5 are located in the plasma membrane, whereas NHE6 is sorted to the mitochondrial membrane. NHE1 and NHE6 are the ubiquitously expressed isoforms. The expression of the isoforms NHE2 to NHE5 is restricted to specific tissues and the pattern of their expression, as well as their subcellular localization indicate that they fulfill specialized functions. Cell shrinkage induced activation has been shown for NHE1,2 and 4. In contrast, NHE3 is inhibited by cell shrinkage. In many cells several isoforms are present and assigned to specific membrane domains where they may serve a functional crosstalk between the different ion transporters.

The promoter for constitutive expression of the human ICln gene CLNS1A

The ICln protein is expressed ubiquitously in mammals. Experiments designed to knock down the ICln protein in NIH 3T3 fibroblasts as well as in epithelial cells led to the conclusion that this protein is crucially involved in volume regulation after cytoplasmic swelling. Reconstitution of the ICln protein in lipid bilayers revealed the ion channel nature of ICln. Here we describe a new human promoter sequence, composed of 89 nucleotides, which is responsible for a highly constitutive expression of the ICln protein. The promoter sequence lacks a TATA box, and the transcription can be effected at multiple sites. In addition to the starting sites, upstream sequence elements are mandatory for an efficient transcription of the ICln gene (CLNS1A). These new nucleotide elements were defined by site-directed mutagenesis.

Functional reconstitution of ICln in lipid bilayers

Reconstitution of purified ICln in lipid bilayer leads to functional ion channels showing varying rectification. The reconstituted single channels have a conductance of approximately equal to 3 pS and their open probability is sensitive to nucleoside analogues. Mutation of a putative nucleotide binding site identified at the predicted extracellular mouth of the ICln channel protein leads to the reduction of the nucleoside-analogue sensitivity. Reconstituted ICln channels can be permeated both by cations and anions. The relative permeability of cations over anions depends on the presence of calcium. In the presence of calcium reconstituted ICln channels are more permeable to bromide than chloride, and more permeable to potassium than sodium. Similarly in NIH3T3 fibroblasts, the relative permeability of cations over anions of swelling-dependent chloride channels depends on extracellular calcium. Site-directed mutagenesis revealed the calcium-binding site responsible for the shift of the selectivity from cations towards anions of reconstituted ICln channels. Additional indirect structural information has been obtained by mutating a histidine in the predicted pore region of ICln. This histidine seems to have access to the ion-conducting tunnel of the pore. Our experiments show that ICln can act as an ionic channel, which does not exclude additional functions of the protein in regulatory mechanisms of the cell. Since knocking down the ICln protein in fibroblasts and epithelial cells leads to an impaired regulatory volume decrease (RVD) after cytoplasmic swelling and reconstituted ICln channels show several biophysical features of ion channels activated after swelling, ICln is a molecular candidate for these channels.

ICln, an ion channel-forming protein associated with cell volume regulation

It is not resolved whether the anionic channel involved in volume regulation after cell swelling comprises one or more subunits. Moreover, it remains to be determined which of the different proteins cloned so far, for which an involvement in cell volume regulation has been postulated, is the ideal candidate. In this review, we consider the role of the ICln protein, cloned from MDCK cells, in cell volume regulation.

Characterization of the human gene coding for the swelling-dependent chloride channel ICln at position 11q13.5-14.1 (CLNS1A) and further characterization of the chromosome 6 (CLNS1B) localization

Expression cloning revealed a chloride channel (ICln) that we found to be fundamental for the regulatory volume decrease in a variety of cells. The chromosomal localization of the human ICln-gene showed two loci, one at chromosome 11 in position q13.5-q14.1, termed CLNS1A, and a second one at chromosome 6 at position p12.1-q13, termed CLNS1B. In this study, we offer a detailed characterization of the CLNS1A gene and provide the exact position (6p12) and sequence data of CLNS1B, an intronless gene 91.3% homologous to the coding region of CLNS1A.

Chromosomal localization of the genes (CLNS1A and CLNS1B) coding for the swelling-dependent chloride channel ICln

ICln is a cloned chloride channel paramount for regulatory volume decrease. Two different loci that carry the coding region for ICln were identified in the human genome. By PCR strategies an intronless copy of the gene was located on chromosome 6 at position 6p12.1-6q13 (CLNS1B). By fluorescence in situ hybridization a copy carrying introns with a putative length of 19 kb was located at chromosome 11 on position 11q13.5-q14.1 (CLNS1A). The characterization and chromosomal localization of the ICln gene offer the opportunity to study the regulatory sites of this gene in greater detail and could be helpful in establishing linkages between ICln and potential human diseases.

The lysosomal Ca2+ pool in MDCK cells can be released by ins(1,4,5)P3-dependent hormones or thapsigargin but does not activate store-operated Ca2+ entry

In several cell types, Ca2+ release from intracellular Ca2+ stores by Ins(1,4,5)P3 elicits Ca2+ influx from the extracellular space into the cytoplasm, termed store-operated Ca2+ entry (SOCE). In MDCK cells, the Ins(1,4,5)P3-sensitive Ca2+ store giving rise to SOCE essentially overlaps with the thapsigargin (TG)-sensitive store. Recent evidence suggests that in MDCK cells lysosomes form a Ca2+ pool that is functionally coupled with the Ins(1,4,5)P3-sensitive Ca2+ store: Ca2+ can be selectively released from lysosomes by glycyl-L-phenylalanine naphthylamide, an agent inducing lysosomal swelling with subsequent and reversible permeabilization of the vesicular membranes. This compartment is also depleted by Ins(1,4,5)P3-dependent agonists or TG, indicating that it is part of a larger, Ins(1,4,5)P3-sensitive Ca2+ pool. Here we show that whereas SOCE is triggered by Ca2+ release from the entire Ins(1,4,5)P3-sensitive Ca2+ pool, selective Ca2+ release from lysosomes alone is unable to trigger SOCE. This finding is consistent with measurements of the store-operated cation current, a direct parameter for store-operated Ca2+ and Na+ entry into MDCK cells. Hence it is proposed that the Ins(1,4,5)P3-sensitive Ca2+ pool is composed of different intracellular compartments that do not uniformly stimulate Ca2+ entry into the cell.

ICln: a chloride channel paramount for cell volume regulation

Cell volume regulation is a ubiquitous cell regulatory mechanism based on meticulously controlled ion transport mechanisms. Keeping the absolute volume constant seems to be of the highest priority for most cells and is achieved at the expense of altered intracellular ion concentrations. We have been able to demonstrate that ICln, a chloride channel cloned from epithelial cells, is paramount for the ability of swollen cells to regulate their volume back to that under resting conditions. A unique feature of ICln is the distinct sensitivity of these channels for nucleotides and nucleoside analogues added to the extracellular fluid. In addition, cromolyn sodium and nedocromil sodium, drugs used by patients with asthma, are able to impede the function of these channels.

Fluorescence-optical measurements of chloride movements in cells using the membrane-permeable dye diH-MEQ

Fluorescence-optical measurements of the intracellular chloride concentration facilitate identification of chloride movements across the cell membrane of living cells. The two main dyes used for this purpose are 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-quinolyl acetoethyl ester (MQAE). The use of both substances is impaired by their poor membrane permeability and therefore limited loading of the cells to be studied. Here we report the use of 6-methoxy-N-ethylquinolinium iodide (MEQ), a chloride-sensitive dye for which a membrane-permeable form is easily prepared. This makes the loading procedure as easy as with the acetoxymethyl (AM) forms of other dyes for sensing intracellular ions. In addition, the original method, which described absolute concentration measurements of chloride in the cytosol, was modified in so far as only relative measurements were made. This avoids the known limitations of single wavelength excitation and emission dyes with respect to exact concentration measurements. Moreover, to enhance the signal-to-noise ratio the driving force for chloride was considerably increased by changing the original direction of the anion flux in the cells under investigation. We verified the method by using fibroblasts and activating ICln, a putative chloride channel cloned from epithelial cells and of paramount importance in the regulatory volume decrease in these cells. In the presence of SCN- the MEQ quench measured in NIH 3T3 fibroblasts is dramatically enhanced in hypotonically challenged cells compared with cells under isotonic conditions. Antisense oligodeoxynucleotides sensing ICln considerably impeded the swelling-induced chloride current (ICl) in NIH 3T3 fibroblasts. Accordingly, the chloride movement measured by the SCN- quench of the MEQ signal was significantly reduced. Similar results can be obtained in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) or 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), two known blockers of chloride transport in the plasma membrane of a variety of cells. In conclusion, fluroscence-optical measurements using MEQ as the chloride-sensitive dye provide a reliable and easy-to-use method for measuring changes of the chloride flux across the cell membrane of living cells.

Blockade of swelling-induced chloride channels by phenol derivatives

1. In NIH3T3 fibroblasts, the chloride channel involved in regulatory volume decrease (RVD) was identified as ICln, a protein isolated from a cDNA library derived from Madin Darby canine Kidney (MDCK) cells. ICln expressed in Xenopus laevis oocytes gives rise to an outwardly rectifying chloride current, sensitive to the extracellular addition of nucleotides and the known chloride channel blockers, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid). We set out to study whether substances structurally similar to NPPB are able to interfere with RVD. 2. RVD in NIH3T3 fibroblasts and MDCK cells is temperature-dependent. 3. RVD, the swelling-dependent chloride current and the depolarization seen after reducing extracellular osmolarity can be blocked by gossypol and NDGA (nordihydroguaiaretic acid), both structurally related to NPPB. 4. The cyclic AMP-dependent chloride current elicited in CaCo cells is less sensitive to the two substances tested while the calcium-activated chloride current in fibroblasts is insensitive. 5. The binding site for the two phenol derivatives onto ICln seems to be distinct but closely related to the nucleotide binding site identified as G x G x G, a glycine repeat located at the predicted outer mouth of the ICln channel protein.

The lysosomal compartment as intracellular calcium store in MDCK cells: a possible involvement in InsP3-mediated Ca2+ release

To test for a possible role of lysosomes in intracellular Ca2+ homeostasis, the effects of glycyl-L-phenylalanine-beta-naphthylamide (GPN), known to permeabilize these organelles by osmotic swelling, were studied in single MDCK cells. Fluorescence of acridine orange, rhodol green dextran, lysotracker green and FITC-dextran indicated that GPN (0.2 mmol/l) elicited a reversible permeabilization of lysosomes. Cytosolic Ca2+ ([Ca2+]i) as determined by Fura-2 fluorescence increased from 60 +/- 11 to 534 +/- 66 nmol/l (n = 41) in the presence of GPN. Whereas only a single intracellular Ca2+ release could be induced by GPN in a Ca(2+)-free perfusate, repetitive release could be evoked in Ca2+ containing solutions suggesting reuptake of Ca2+ into lysosomal stores. GPN-induced Ca2+ release was blunted after pretreatment with thapsigargin (TG), an inhibitor of Ca(2+)-ATPase, or repeated applications of ATP inducing Ca2+ release from inositol trisphosphate (InsP3) sensitive Ca2+ stores. The effect of ATP on Ca2+ release was, however, not abolished by preceding GPN treatment. GPN-induced Ca2+ release from lysosomes was independent of InsP3 formation or Ca(2+)-induced Ca2+ release, since it was unaffected by the phospholipase C inhibitor U-73, 122 or by caffeine and ruthenium red. These results suggest that Ca2+ largely accumulates in lysosomal vesicles. Moreover, these organelles seem to be part or functionally coupled with InsP3-sensitive Ca2+ stores.

Ultrapure polymerized bovine hemoglobin improves structural and functional integrity of the isolated perfused rat kidney

Since it became evident that organ dysfunctions after acute hemolysis are not induced by hemoglobin per se, but by stroma-contaminated hemoglobin, solutions of ultrapure stroma-free hemoglobins were regarded to be possible substitutes for blood in transfusion medicine. We tested one of the recently developed modified bovine hemoglobins (Ultrapure polymerized bovine hemoglobin 1; UPPBHb1) in the isolated perfused rat kidney (IPRK) model, using a recirculating system. Control kidneys were perfused with a substrate-enriched Ringer solution containing hydroxyethyl starch (HES) to produce isoncotic conditions. In the experimental group HES was substituted in part by UPPBHb1 (34 g/l). For determination of functional parameters, the kidneys were perfused for 180 min. A separate set of kidneys of both groups was perfusion fixed after 80 min of perfusion which is the period of optimal function. Light and electron microscopic analysis revealed major alterations only for the outer medulla of HES kidneys. Only these suffered from a considerable extent of proximal tubular S3 damage, exhibiting condensed tubular epithelia. In the inner stripe of the outer medulla, which is the zone of greatest sensitivity to damage in the isolated perfused kidney, severe hydropic degeneration, cell detachment, and necrotic destruction of the medullary thick ascending limb were seen in the HES-perfused group, too. In the UPPBHb1 group, the medullary thick ascending limb was well preserved, and S3 showed only a minor degree of damage. UPPBHB1 kidneys were further characterized by the occurrence of intracapillary and interstitial precipitates of UPPBHb1 in inner stripe of the outer medulla and inner medulla. The glomerular filtration rate was significantly higher in UPPBHb1-perfused kidneys (870 +/- 80 vs. 630 +/- 55 microliters/min/g kidney weight for HES). Absolute reabsorption of sodium paralleled the behavior of the glomerular filtration rate. The values for renal perfusate flow and urinary flow rate did not differ significantly between both groups. Renal autoregulation was better preserved in UPPBHb1-perfused kidneys (74 +/- 6% of full autoregulatory response) than in HES-perfused controls (42 +/- 4%). Our results suggest that perfusion of isolated rat kidneys with UPPBHb1 improves kidney function and morphology, providing better oxygenation than in control kidneys. UPPBHb1 does not exert additional nephrotoxic effects on the IPRK that will exceed the noxious potential of the method itself. Thus, it must be concluded that UPPBHb1 may be an oxyphoretic blood substitute with nephroprotective characteristics when compared with nonoxyphoretic substitutes. At least, UPPBHb1 seems to be a promising candidate as oxyphoretic additive to perfusates for the IPRK model.

Renal hemodynamic response to intravenous and oral amino acids in animals

Oral or parenteral application of amino acids leads to marked hyperfiltration and increased of renal plasma flow. Amino acids stimulate the release of glucagon, which increases hepatic production and release of cyclic adenosine monophosphate (cAMP). In the kidney, the combined effect of cAMP and glucagon increases glomerular filtration rate (GFR), possibly by reducing NaCl concentration at the macula densa and depression of the tubuloglomerular feedback. Vasopressin-dependent urea recycling and delivery to the thick ascending limb could similarly reduce NaCl concentration at the macula densa. Beyond that, amino acids may trigger a hepatorenal reflex or directly interfere with renal function. Mechanisms invoked include dopamine from renal nerves, prostaglandins, nitric oxide (NO), and angiotensin II. At this point, it is not clear to which extent the described mechanisms participate in, permit, or fully account for the hyperfiltrative effect of amino acids.

Ultrapure polymerized bovine hemoglobin (UPPBHb) improves integrity of the isolated perfused rat kidney (IPRK): effects on function and structure

To test the oxyphoretic properties and potential nephrotoxic side-effects of polymerized hemoglobin solutions, isolated rat kidneys were perfused in a recirculating system for 180 min. Group I was perfused with a substrate enriched Ringer solution containing hydroxyethylstarch (HES) to produce isoncotic conditions. In group II HES was substituted in part by UPPBHb (34 g/l) with a high portion of low molecular weight molecules (= UPPBHb1). In group III 34 g/l of UPPBHb containing an increased fraction of high molecular weight polymers (= UPPBHb2) was used. Only UPPBHb2-perfused kidneys showed a reduced renal perfusate flow (RPF, 13.3 +/- 1.1 ml/min g kw), when compared to HES-perfused controls (15.5 +/- 0.8) and UPPBHb1 (15.1 +/- 1.2). Glomerular filtration rate (GFR) was significantly higher in UPPBHb1-perfused kidneys (902 +/- 107 vs 633 +/- 55 microliters/min g kw for HES). This difference became even more pronounced in the third hour of perfusion (474 +/- 125 vs. 103 +/- 33). In contrast, UPPBHb2 produced low initial GFR levels of 385 +/- 25, which had only a minor tendency to decline with time. Parallel to GFR, absolute reabsorption of sodium (TNa) andoxygen consumption (QO2) showed values of 110 +/- 16 and 5.46 +/- 0.33 mumol/min g kw in UPPBHb1-kidneys vs 83 +/- 6 and 5.09 +/- 0.27 in controls and vs 53 +/- 4 and 3.66 +/- 0.12 in UPPBHb2-kidneys. Fractional excretion of sodium (FENa), of potassium (FEK), and of water (FEH2O) in UPPBHb1 and UPPBHb2-perfused kidneys were not significantly different from HES-perfused controls at any time of perfusion. Urinary flow rate (UFR) was similar in UPPBHb1- and HES-kidneys. Nevertheless, control kidneys tended to render oliguric during the third hour of perfusion (UFR 19.9 +/- 4.1 microliters/min g kw), whereas UPPBHb1 preserved urinary flow in a better way (83.7 +/- 32.4). UFR of UPPBHb2-kidneys was significantly reduced initially (30.2 +/- 5.1 vs. 105 +/- 33 for HES), but increased steadily up to 67 +/- 23. In the UPPBHb1 and HES group, all functional parameters determined declined dramatically within the third hour of perfusion, whereas UPPBHb2 produced functional stability. The in vivo reaction pattern of renal autoregulation was better preserved in UPPBHb-perfused kidneys than in HES-perfused controls: 74 +/- 6 vs. 59 +/- 5 vs. 42 +/- 4% (of full autoregulatory response) for UPPBHb1, UPPBHb2, and HES kidneys, respectively. Light- and electron microscopic analysis revealed major alterations only for the outer medulla of HES-kidneys.(ABSTRACT TRUNCATED AT 400 WORDS)

Damaging effects of high energy shock waves on cultured Madin Darby canine kidney (MDCK) cells

Shock wave lithotripsy (ESWL) has become an almost non-invasive standard treatment modality for urolithiasis. Several investigations, however, demonstrated that ESWL is not completely free of side effects. Among others alteration of renal tubular function has been reported. To study the effect of shock waves on tubular cells directly an in-vitro model with cultured Madin Darby Canine Kidney (MDCK) cells was established. Suspensions of MDCK cells (7 groups of 6 containers each) were exposed to 0, 16, 32, 64, 128, 256 shock waves (Dornier HM4, 18 kV). Before and 0, 1, 3, 6, 9, 12, 24 h after ESWL the following parameters were measured in the nutrient medium: lactate dehydroxygenase (LDH), glutamate oxalacetate transaminase (GOT), electrolytes. LDH and GOT increased depending on the number of shock waves indicating a membrane damage of MDCK cells. The MDCK model seems suitable for further studies on the effect of shock waves on renal tubular cells.

[Renal handling of alcohol and its tubular effects]

The renal handling of ethanol comprises glomerular filtration and tubular reabsorption. Due to its high permeability alcohol concentration in the tubular fluid approaches that of peritubular fluid and under steady state conditions alcohol concentration in the final urine is almost the same as in serum water. Even in high concentrations alcohol does not significantly interfere with kidney cell function. This seems to be due to the fact that renal tissue is almost free from alcohol dehydrogenase. Thus, acetaldehyde, the cytotoxic intermediate of alcohol metabolism, is not accumulated in effective doses. If applied directly in micropuncture experiments alcohol is without distinct effects while acetaldehyde inhibits the main parameters of cellular vitality as measured by electrical membrane potentials and intracellular ion activities.

The influence of intracellular sodium activity on the transport of glucose in proximal tubule of frog kidney

Inhibition of basolateral Na+/K+ ATPase by ouabain eventually abolishes transport of glucose. The present study was performed to test, if this effect is due to a dissipation of the electrochemical gradient for sodium or due to a regulatory inhibition of sodium-coupled glucose entry across the luminal membrane at increasing intracellular sodium activity. To this end, proximal convoluted tubules of the doubly perfused isolated frog kidney were perfused alternatively with solutions containing either 5 mmol/l glucose or raffinose. The potential difference across the peritubular cell membrane (PDpt) and across the epithelium (PDte) has been recorded with conventional and across the peritubular cell membrane with ion selective microelectrodes (PDpt). In the absence of luminal glucose PDpt is (+/- SEM) -54.0 +/- 2.4 mV, PDte = -1.2 +/- 2.0 mV and PDNapt = -96 +/- 5 mV. The electrochemical gradient for sodium (mu Na+) amounts to 95 mV and intracellular sodium activity to 14 mmol/l (extracellular sodium activity is 74 mmol/l). Luminal application of glucose leads to a rapid depolarisation of PDpt (delta PDpt = 8.6 +/- 0.9 mV and PDNapt (delta PDNapt = 11.1 +/- 3.0 mV) and to hyperpolarisation of PDte (delta PDte = -0.8 +/- 0.2 mV). The peritubular application of ouabain leads to a gradual, reversible and proportional decline of PDpt, PDNapt and mu Na+. Glucose induced delta PDpt and delta PDNapt decrease in parallel to PDpt and PDNapt, resp. In a separate series, the lumped conductance (Gm) of the luminal and basolateral cell membrane has been determined, which amounts to 2.4 +/- 0.3 microS/mm (tubule length). Gm decreases 23 +/- 4%, when PDpt is decreased to half.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of ouabain on intracellular activities of K+, Na+, Cl-, H+ and Ca2+ in proximal tubules of frog kidneys

Using conventional and ion selective microelectrodes, the effect of ouabain (10(-4) mol/l) on peritubular cell membrane potential (PDpt), on intracellular pH (pHi) as well as on the intracellular ion activities of Cl- (Cli-), K+ (Ki+), Na+ (Nai+) and Ca2+ (Ca2i+) was studied in proximal tubules of the isolated perfused frog kidney. In the absence of ouabain (PDpt = -57.0 +/- 1.9 mV), the electrochemical potential difference of chloride (apparent mu Cl- = -22 +/- 2 mV) and of potassium (mu K+ = +24 +/- 3 mV) is directed from cell to bath, of H+ (mu H+ = -42 +/- 5 mV), of Na+ (mu Na+ = -102 +/- 4 mV) and of Ca2+ (mu Ca2+ = -148 +/- 6 mV) from bath to cell. Ouabain leads to a gradual decline of PDpt, which is reduced to half (PDpt, 1/2) within 31 +/- 4.6 min (in presence of luminal glucose and phenylalanine), and to a decline of the absolute values of apparent mu Cl+, of mu H+, mu Na+ and mu Ca2+. In contrast, an increase of mu K+ is observed. At PDpt, 1/2 apparent Cl-i increases by 6.2 +/- 1.0 mmol/l, pHi by 0.13 +/- 0.03, Ca2+i by 185 +/- 21 nmol/l, and Nai+ by 34.2 +/- 4.6 mmol/l, whereas Ki+ decreases by 37.7 +/- 2.2 mmol/l. The results suggest that the application of ouabain is followed by a decrease of peritubular cell membrane permeability to K+, by an accumulation of Ca2+, Na+ and HCO3- in the cell and by a dissipation of the electrochemical Cl- gradient.

Evidence for an amiloride sensitive Na+ pathway in the amphibian diluting segment induced by K+ adaptation

The effect of amiloride on cell membrane potentials and intracellular Na activity (Nai) was tested in early distal tubules of the isolated perfused kidney of control and of K-adapted (high-K diet) Amphiuma. Conventional and Na-sensitive liquid ion-exchanger microelectrodes were employed to measure the peritubular cell membrane potential (PDpt), the transepithelial potential difference (PDte) and the Na electrochemical gradient across the peritubular cell membrane (ENapt), in the absence and the presence of amiloride (1 X 10(-4) mol X 1(-1] in both groups of animals. Amiloride did not affect PDpt and ENapt in control animals but depolarized PDpt and ENapt by about 8 mV in K-adapted animals. Nai (11.0 +/- 0.6 mmol X 1(-1) in early distal cells of control animals) did not change significantly by this maneuver. However, Nai decreased to extremely low values (2.3 +/- 0.2 mmol X 1(-1] when the luminal cotransport system for Na, Cl and K was inhibited by the luminal application of furosemide (5 X 10(-5) mol/l) and when the luminal cell membrane was exposed simultaneously to amiloride. The amiloride-induced effects on PDpt, ENapt and Nai occurred within seconds and were fully reversible. We conclude that high-K diet (K adaptation) induces an amiloride-sensitive pathway in the luminal cell membrane of early distal cells of Amphiuma which exists in parallel with the furosemide-sensitive cotransport system located in this cell barrier. The results suggest a luminal amiloride-sensitive Na/H exchange mechanism which regulates the luminal K permeability.

Inefficacy of bicarbonate infusions on the course of postischaemic acute renal failure in the rat

Since bicarbonate has been reported to elicit fast recovery from acute renal failure in man, clearance studies were performed to compare the effects of sodium bicarbonate and saline infusion on renal function in postischaemic renal failure in the rat. In a first set of experiments the left kidney and in a second both kidneys were clamped for a period of 45 min and renal function monitored up to 210 min after release of the clamp. Glomerular filtration rate (ml/min) decreased following clamping from (mean values +/- SEM) 1.33 +/- 0.09 to 0.12 +/- 0.02 (saline) or 1.43 +/- 0.1 to 0.08 +/- 0.01 (bicarbonate) in the unilaterally clamped kidney and from 2.94 +/- 0.20 to 0.41 +/- 0.10 (saline) or 2.81 +/- 0.17 to 0.22 +/- 0.03 (bicarbonate) when both kidneys were clamped. Fractional excretion of water and sodium increased to a similar extent in saline and bicarbonate treated animals. Plasma potassium decreased (from 3.37 +/- 0.10 to 2.95 +/- 0.07 [unilaterally clamped kidneys] or from 5.2 +/- 0.4 to 4.4 +/- 0.2 [bilaterally clamped kidneys]) in bicarbonate treated but remained constant in saline treated animals, an effect not related to altered renal potassium excretion. In conclusion, no evidence was found that bicarbonate improves renal function in postischaemic renal failure.

Renal handling of urate in healthy man in hyperuricaemia and renal insufficiency: circadian fluctuation, effect of water diuresis and of uricosuric agents

To differentiate between extrarenal and renal causes of hyperuricaemia and gout, clearances of urate and creatinine were monitored for 3 1/2 days in fifty-two individuals (seven with a history of gout) with no gross impairment of renal function (creatinine clearance 52-137 ml/min). Dietary purine intake was kept constant. Monophasic circadian fluctuations of fractional urate excretion (= urate clearance over creatinine clearance) were observed with peak values in the afternoon, about 50% higher than during the night. Circadian fluctuations of urinary flow rate were almost identical. However, enhancement of urinary flow rate due to water diuresis had no effect on urate clearance. Despite wide variation of plasma urate concentrations among different individuals (+/- 30% SD), daily urate excretion varied little (+/- 4% SD) and did not correlate with plasma urate (r = 0.03). Thus extrarenal factors appear not to account for the occurrence of hyperuricaemia in these patients. In contrast, a clearcut negative correlation was apparent between plasma urate concentration and fractional urate clearance (r = 0.72), which could fully account for the variations of plasma urate concentration. To elucidate further the mechanism responsible for antiuricosuria in hyperuricaemic patients, the effects of the uricosuric agents benzbromarone and probenecid were tested. A clearcut correlation was apparent between control fractional urate excretion and uricosuric effect of both benzbromarone and probenecid (r = 0.83 and 0.88, respectively), suggesting that anti-uricosuria was due to defective secretion. In an additional series, the uricosuric effect of probenecid was tested in ten patients with renal insufficiency. In these patients the uricosuric effect was clearly blunted, indicating that urate reabsorption is reduced in renal insufficiency.

[Additive phosphaturic action of parathyrin and calcitonin (author's transl)]

Parathyrin and calcitonin exert their effects on phosphate metabolism by influencing the functions of at least three organ systems, i. e. bone, gut and kidneys. To study the renal effects of these hormones under exclusion of systemic effects microinfusion studies were performed in anesthetised rats. After thyroparathyroidectomy radioactively labelled phosphate containing solutions were microinfused into single proximal convoluted tubules. The tracer recovery in the urine allowed calculation of phosphate reabsorption in the nephron segments beyond the micropuncture site. After a control period of 6 minutes the hormones were superfused to the nephron surface and tracer recovery measured during the following 36 minutes. Within few minutes both, parathyrin and calcitonin, clearly reduced phosphate reabsorption. Infusions of supramaximal doses of either hormone abolished the local action of this hormone but did not influence the effect of the other. Thus the phosphaturic actions of parathyrin and calcitonin are additive, indicating that the hormones involve different mechanisms and/or nephron sites.

Kinetics of L-proline reabsorption in rat kidney studied by continuous microperfusion

Renal tubular reabsorption of 3H and 14C labelled L-proline was measured in vivo et situ by continuous microperfusion of single proximal tubules of the rat. The reabsorption is shown to be saturable. Passive diffusion plays a relatively small role in the reabsorption. A maximum possible permeability coefficient of 25 micrometers 2.s-1 for proline was calculated. Two transport systems were found, one with a small affinity and a high capacity, the other with a very high affinity and a small capacity. The following values were estimated. Jmax 1 = 2.6 +/- 0.28 (SEM) nmol.m-1.S-1 Km1 = 11.8 +/- 1.7 (SEM) mmol.1-1 Jmax 2 = 9.6 +/- 1.92 (SEM) pmol.m-1.s-1 Km2 = 29.3 +/- 7.8 (SEM) mumol.1-1. Whereas the first system reabsorbs the bulk of the filtered load, the activity of the second system explains the extremely small amount of proline found in the final urine. Diisopropylphosphorofluoridate--a specific inhibitor of dipeptidyl peptidase IV--decreases the reabsorption of L-proline and L-alanine but has no influence on the reabsorption of the basic amino acid L-arginine and the acidic amino acid L-glutamic acid. This result correlates with a recent speculation that dipeptidyl peptidase IV is involved in proline and alanine reabosrption.

Maleic acid induced aminoaciduria, studied by free flow micropuncture and continuous microperfusion

The injection of 200 mg/kg BW maleic acid was found to be a suitable dose for exploring the experimental Fanconi syndrome by micropuncture techniques in rats. In clearance experiments, the fractional excretion of glycine, L-alanine, L-aspartate and taurine was measured. After intraperitoneal administration of maleic acid the excretion of these amino acids was increased in the range between the 20-fold and the 230-fold. Free flow micropuncture experiments showed that the reabsorption of these amino acids is reduced drastically along the whole proximal tubule. Continuous microperfusion experiments lead to the result that, in maleic acid pretreated rats, the reabsorption of 14C-glycine from the proximal convolution was strongly inhibited. It was found, furthermore, that after blocking the saturable glycine transport by L-phenylalanine, the remaining reabsorption of glycine (corresponding to passive diffusion) was exactly the same with and without maleic acid. Microinfusion experiments with 8 mumol.1(-1) L-3H-alanine into the early distal tubule showed a fractional recovery of 103 +/- 4.2% (S.D.) in the control and of 101 +/- 6.5% in presence of maleic acid. It is concluded that maleic acid inhibits the saturable reabsorption mechanism of amino acids along the proximal tubule. Passive permeability of the tubular membrane does not seem to be altered by maleic acid.

Renal handling of urate and oxalate: possible implications for urolithiasis

Both urate and oxalate are organic acids of considerable clinical interest, owing to their limited solubility. Calcium oxalate is the most frequent constituent of renal calculi and occasionally precipitates in body fluids. Urate precipitations are common in the kidney and in various other tissues. In this paper, a short outline of the present knowledge of renal handling of these substances will be followed by some conclusions as to the possible relevance of this knowledge for the understanding of urolithiasis and intrarenal precipitation. Direct (micropuncture) data are available for urate in the rat (1, 6, 7, 10, 21, 23, 28, 36, 42), rabbit (35), dog (34) and cebus monkey (33) and in the rat only for oxalate (11, 15, 20).

[Uptake of 2-C14 urate by erythrocytes in hyperuricemia and in gout (author's transl)]

The uptake of 2-C14 urate by erythrocytes was measured in 19 patients with primary hyperuricemia, 6 patients with secondary hyperuricemia, 17 patients with primary gout and 30 controls. The uptake of urate in patients with primary gout was significantly lower than in the controls. In contrast no such difference could be observed in patients with primary and secondary hyperuricemia. The uptake of labeled urate by erythrocytes from gouty patients is especially diminished in the early phase of the uptake kinetics. The possible relevance of this finding for the pathogenesis of urate precipitation in gout is discussed. Further, we consider the application of the tracer urate uptake by erythrocytes as an aid in the early diagnosis of gout.

Metabolic changes induced by acute phosphate loading in acutely thyroparathyroidectomized rats

Phosphate loading in acutely thyroparathyroidectomized rats is followed by a decline of phosphate reabsorption. In addition, a mixed respiratory and metabolic alkalosis and hypocalcemia develop. Normalization of plasma calcium concentration, unlike the reversal of the alkalosis, prevents the fall in phosphate reabsorption.

Factors affecting urate reabsorption in the rat kidney

1. Urate transport in the rat appears to be saturable. However, affinity of the transport system for urate is very low and transport far from saturated at physiological plasma concentrations. 2. Since increase of the nonionized fraction of uric acid by a factor of five failed to increase urate reabsorption, transport cannot be due to nonionic diffusion but rather involves ionized urate. 3. Increases in luminal flow rate markedly depress urate reabsorption in the loop of Henle, which results in wash out of medullary urate.

Handling of allantoin by the rat kidney. Clearance and micropuncture data

Renal excretion of allantoin was measured by tracer techniques. After injection of 2-C14 urate and H3 inulin, clearances of allantoin and inulin were measured and both proximal and distal tubules were micropunctured. In confirmation of earlier results 2-C14 urate injected into an intact animal is very rapidly converted to C14 allantoin: after 15 min more than 90% of urinary tracer is present as allantoin. It was further observed that 1) allantoin clearance is essentially identical with inulin clearance over a wide range of urine flows; 2) no net transport of allantoin occurs in either proximal or distal tubules. Clearly allantoin is handled by the rat kidney like inulin. The total excretion of filtered allantoin unlike that of filtered urate provides an easy and effective mechanisms for animals possessing the enzyme uricase to dispose of their purine loads.