Electrophysiological analysis of rat renal sugar and amino acid transport. I. Basic phenomena

Modulation of luminal L-alanine transport in proximal tubular cells of frog kidney induced by low micromolar Cd2+ concentration

The kidneys are recognized as a major target of cadmium-induced toxicity. However, all mechanisms that are involved in the early stages of cadmium nephrotoxicity, particularly considering low micromolar concentrations of cadmium ions (Cd²⁺) are still unknown. Therefore, the aim of this study was to investigate the effects of peritubular acute exposure to micromolar Cd²⁺ concentration (2.3 μmol/L) on the rapid depolarization and the rate of slow repolarization of peritubular membrane potential difference (PD), induced by luminal application of L-alanine in proximal tubular cells of frog kidney. The results showed that the luminal application of L-alanine rapidly depolarized the peritubular membrane PD of -42.00 ± 11.68 mV by 23.89 ± 4.15 mV with an average rate of slow repolarization of 5.64 ± 0.81 mV/min. Additionally, peritubular acute exposure to Cd²⁺ induced change in rapid depolarization of peritubular membrane PD of -53.33 ± 13.01 mV by 18.78 ± 3.31 mV (P < 0.01) after luminal application of L-alanine. Also, peritubular acute exposure to Cd²⁺ led to statistically significant decrease in the rate of slow repolarization of peritubular membrane PD (3.53 ± 0.35 mV/min; P < 0.05). In conclusion, these results suggest that peritubular acute exposure to low micromolar Cd²⁺ concentration decreased the rapid depolarization and the rate of slow repolarization of peritubular membrane PD induced by luminal application of L-alanine. This is followed by reduced luminal sodium-coupled transport of L-alanine and this change may be one of the possible mechanisms involved in the early stages of Cd²⁺-induced nephrotoxicity.

Stationary and Nonstationary Ion and Water Flux Interactions in Kidney Proximal Tubule: Mathematical Analysis of Isosmotic Transport by a Minimalistic Model

Our mathematical model of epithelial transport (Larsen et al. Acta Physiol. 195:171–186, 2009) is extended by equations for currents and conductance of apical SGLT2. With independent variables of the physiological parameter space, the model reproduces intracellular solute concentrations, ion and water fluxes, and electrophysiology of proximal convoluted tubule. The following were shown:

Quantifying the relative contributions of different solute carriers to aggregate substrate transport

Determining the contributions of different transporter species to overall cellular transport is fundamental for understanding the physiological regulation of solutes. We calculated the relative activities of Solute Carrier (SLC) transporters using the Michaelis-Menten equation and global fitting to estimate the normalized maximum transport rate for each transporter (V_max). Data input were the normalized measured uptake of the essential neutral amino acid (AA) L-leucine (Leu) from concentration-dependence assays performed using Xenopus laevis oocytes. Our methodology was verified by calculating Leu and L-phenylalanine (Phe) data in the presence of competitive substrates and/or inhibitors. Among 9 potentially expressed endogenous X. laevis oocyte Leu transporter species, activities of only the uniporters SLC43A2/LAT4 (and/or SLC43A1/LAT3) and the sodium symporter SLC6A19/B⁰AT1 were required to account for total uptake. Furthermore, Leu and Phe uptake by heterologously expressed human SLC6A14/ATB^0,+ and SLC43A2/LAT4 was accurately calculated. This versatile systems biology approach is useful for analyses where the kinetics of each active protein species can be represented by the Hill equation. Furthermore, its applicable even in the absence of protein expression data. It could potentially be applied, for example, to quantify drug transporter activities in target cells to improve specificity.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Micropuncture of the kidney: a primer on techniques

Techniques to evaluate renal function at the single nephron level have been instrumental and indispensible in furthering our understanding of the mammalian kidney. Techniques that were first introduce in the 1920s, and later refined in the 1950s and 1960s, permit sophisticated interrogation of glomerular filtration and hemodynamics, and tubular epithelial transport activity. Much of what we know about the physiology and pathophysiology of the kidney has been produced or, to some degree, confirmed by renal micropuncture. While micropuncture is perhaps not as widely employed as before, it remains an essential tool for comprehensive evaluation of kidney function, particularly in this age of genetically pliable experimental models. This review aims to provide a introduction to common methodologies and approaches used to conduct micropuncture experiments. Topics covered include instrumentation and equipment, pipet fabrication techniques, animal preparation, and experimental procedures for evaluating single nephron hemodynamics and tubular function.

Tight junction claudins and the kidney in sickness and in health

The epithelial cell tight junction has several functions including the control of paracellular transport between epithelial cells. Renal paracellular transport has been long recognized to exhibit unique characteristics within different segments of the nephron, functions as an important component of normal renal physiology and has been speculated to contribute to renal related pathology if functioning abnormally. The discovery of a large family of tight junction associated 4-transmembrane spanning domain proteins named claudins has advanced our understanding on how the paracellular permeability properties of tight junctions are determined. In the kidney, claudins are expressed in a nephron-specific pattern and are major determinants of the paracellular permeability of tight junctions in different nephron segments. The combination of nephron segment claudin expression patterns, inherited renal diseases, and renal epithelial cell culture models is providing important clues about how tight junction claudin molecules function in different segments of the nephron under normal and pathological conditions. This review discusses early observations of renal tubule paracellular transport and more recent information on the discovery of the claudin family of tight junction associated membrane proteins and how they relate to normal renal function as well as diseases of the human kidney.

Epithelial barrier characteristics and expression of cell adhesion molecules in proximal tubule-derived cell lines commonly used for in vitro toxicity studies

Recent studies indicate that the actions of several nephrotoxic substances involve alterations in the function of cell adhesion molecules and changes in the paracellular permeability of the proximal tubule. In light of these findings, there is a need for appropriate in vitro model systems to study these phenomenae in greater detail. In the present study, the transepithelial resistance (TER), paracellular permeability of 14C-mannitol and immunofluorescent labeling of cell adhesion molecules (E-cadherin, N-cadherin, ZO-1, occludin, and claudins-2 and -7) were evaluated in several proximal tubule-derived cell lines that have been commonly used as model systems for in vitro toxicity studies. The cell lines studied included: LLC-PK1, OK, NRK-52E and HK-2, along with commercially available primary cultures of human renal proximal tubule epithelial cells (HRPTE). LLC-PK1 cells developed the highest TER followed by the OK cells and NRK-52E cells. The other cell lines failed to develop a TER even after 2 weeks in culture. There was a direct correlation between TER and ability to restrict paracellular movement of 14C-mannitol. Labeling studies showed that the LLC-PK1 and NRK-52E cells expressed high levels of E-cadherin while the HRPTE cells expressed lower levels. OK cells expressed moderate levels of N-cadherin while LLC-PK1 and NRK-52E cells expressed lower levels in isolated patches of cells. All cell lines expressed moderate-high levels of ZO-1. LLC-PK1 also expressed the tight-junction proteins occludin and claudin-7; OK cells also expressed moderate levels of occludin. All other cell lines had weak claudin-7 and occludin labeling. None of the cell lines expressed claudin-2. These results show that the LLC-PK1, OK and NRK-52E cell lines exhibit characteristics that most closely resembled those of the proximal tubule in vivo, and they indicate that these cell lines would be appropriate models for studying the effects of toxicants on cell-cell junctions and cell adhesion molecules.

Novel renal amino acid transporters

Reabsorption of amino acids, similar to that of glucose, is a major task of the proximal kidney tubule. Various amino acids are actively transported across the luminal brush border membrane into proximal tubule epithelial cells, most of which by cotransport. An important player is the newly identified cotransporter (symporter) B0AT1 (SLC6A19), which imports a broad range of neutral amino acids together with Na+ across the luminal membrane and which is defective in Hartnup disorder. In contrast, cationic amino acids and cystine are taken up in exchange for recycled neutral amino acids by the heterodimeric cystinuria transporter. The basolateral release of some neutral amino acids into the extracellular space is mediated by unidirectional efflux transporters, analogous to GLUT2, that have not yet been definitively identified. Additionally, cationic amino acids and some other neutral amino acids leave the cell basolaterally via heterodimeric obligatory exchangers.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Chloride transport in the renal proximal tubule

The renal proximal tubule is responsible for most of the renal sodium, chloride, and bicarbonate reabsorption. Micropuncture studies and electrophysiological techniques have furnished the bulk of our knowledge about the physiology of this tubular segment. As a consequence of the leakiness of this epithelium, paracellular ionic transport--in particular that of Cl(-)--is of considerable importance in this first part of the nephron. It was long accepted that proximal Cl(-) reabsorption proceeds solely paracellularly, but it is now known that transcellular Cl(-) transport also exists. Cl(-) channels and Cl(-)-coupled transporters are involved in transcellular Cl(-) transport. In the apical membrane, Cl(-)/anion (formate, oxalate and bicarbonate) exchangers represent the first step in transcellular Cl(-) reabsorption. A basolateral Cl(-)/HCO(3)(-) exchanger, involved in HCO(3)(-) reclamation, participates in the rise of intracellular Cl(-) activity above its equilibrium value, and thus also contributes to the creation of an outwardly directed electrochemical Cl(-) gradient across the cell membranes. This driving force favours Cl(-) diffusion from the cell to the lumen and to the interstitium. In the basolateral membrane, the main mechanism for transcellular Cl(-) reabsorption is a Cl(-) conductance, but a Na(+)-driven Cl(-)/HCO(3)(-) exchanger may also participate in Cl(-) reabsorption.

Regulation of tight junction permeability with switch-like speed

PURPOSE OF REVIEW

The case is made that tight junctions can undergo large reversible conductance changes in a matter of seconds and yet preserve their permselectivity. The diuretic peptide leucokinin transforms (renal) Malpighian tubules of the yellow fever mosquito from a moderately tight epithelium to a leaky epithelium by increasing the chloride-conductance of the paracellular shunt pathway. The nine-fold increase in the paracellular chloride-conductance brings about a non-selective stimulation of transepithelial sodium chloride and potassium chloride secretion, as expected from a conductance increase in the pathway taken by the counterion of sodium and potassium.

RECENT FINDINGS

The leucokinin signaling pathway consists in part of a receptor coupled G-protein, phospholipase C, inositol-1,4,5-trisphosphate, and increased intracellular calcium concentration that bring about the increase in the paracellular, tight junction chloride-conductance. As the conductance of the tight junction pathway increases it becomes more selective for the transepithelial passage of chloride.

SUMMARY

Epithelial cells in Malpighian tubules taper to tight junctions at their lateral edges exposing them directly to apical and serosal solutions. Furthermore, evolutionary pressures to excrete salt and water at high rates without the aid of glomerular filtration have led to powerful mechanisms of tubular secretion, capable of diuresis when the mosquito is challenged with the volume expansion of a blood meal. The tubular diuresis is mediated in part by increasing the paracellular chloride conductance. Thus, anatomical and physiological specializations in Malpighian tubules combine to yield the evidence for the dynamic hormonal regulation of the tight junction pathway.

Guanylin, uroguanylin, and heat-stable euterotoxin activate guanylate cyclase C and/or a pertussis toxin-sensitive G protein in human proximal tubule cells

Membrane guanylate cyclase C (GC-C) is the receptor for guanylin, uroguanylin, and heat-stable enterotoxin (STa) in the intestine. GC-C-deficient mice show resistance to STa in intestine but saluretic and diuretic effects of uroguanylin and STa are not disturbed. Here we describe the cellular effects of these peptides using immortalized human kidney epithelial (IHKE-1) cells with properties of the proximal tubule, analyzed with the slow-whole-cell patch clamp technique. Uroguanylin (10 or 100 nm) either hyperpolarized or depolarized membrane voltages (V(m)). Guanylin and STa (both 10 or 100 nm), as well as 8-Br-cGMP (100 microm), depolarized V(m). All peptide effects were absent in the presence of 1 mm Ba(2+). Uroguanylin and guanylin changed V(m) pH dependently. Pertussis toxin (1 microg/ml, 24 h) inhibited hyperpolarizations caused by uroguanylin. Depolarizations caused by guanylin and uroguanylin were blocked by the tyrosine kinase inhibitor, genistein (10 microm). All three peptides increased cellular cGMP. mRNA for GC-C was detected in IHKE-1 cells and in isolated human proximal tubules. In IHKE-1 cells GC-C was also detected by immunostaining. These findings suggest that GC-C is probably the receptor for guanylin and STa. For uroguanylin two distinct signaling pathways exist in IHKE-1 cells, one involves GC-C and cGMP as second messenger, the other is cGMP-independent and connected to a pertussis toxin-sensitive G protein.

Role of KCNE1-dependent K+ fluxes in mouse proximal tubule

The electrochemical gradient for K+ across the luminal membrane of the proximal tubule favors K+ fluxes to the lumen. Here it was demonstrated by immunohistochemistry that KCNE1 and KCNQ1, which form together the slowly activated component of the delayed rectifying K+ current in the heart, also colocalize in the luminal membrane of proximal tubule in mouse kidney. Micropuncture experiments revealed a reduced K+ concentration in late proximal and early distal tubular fluid as well as a reduced K+ delivery to these sites in KCNE1 knockout (-/-), compared with wild-type (+/+) mice. These observations would be consistent with KCNE1-dependent K+ fluxes to the lumen in proximal tubule. Electrophysiological studies in isolated perfused proximal tubules indicated that this K+ flux is essential to counteract membrane depolarization due to electrogenic Na+-coupled transport of glucose or amino acids. Clearance studies revealed an enhanced fractional urinary excretion of fluid, Na+, Cl-, and glucose in KCNE1 -/- compared with KCNE1 +/+ mice that may relate to an attenuated transport in proximal tubule and contribute to volume depletion in these mice, as indicated by higher hematocrit values.

Cl- and membrane potential dependence of amino acid transport across the rat renal brush border membrane

The relative roles of the anion present and the membrane potential in the operation of each of the seven amino acid transport systems in the renal tubular brush border membrane were explored by manipulating transmembrane potential and chemical gradients across the membrane. The effect of various external anions with different permeabilities of the membrane and of valinomycin-generated K+ diffusion potential on Na+-coupled amino acid accumulation by rat renal brush border membrane vesicles was examined. Accumulation of all amino acids examined, except for cystine, was membrane potential dependent. The highest voltage dependence was observed for taurine (equivalent to glucose) and l-methionine. Addition of taurine uptake values obtained under each electrical gradient (inside negative) and a chemical gradient (100 mM NaCl out) condition yielded markedly lower values than under conditions where there was a combined electrochemical gradient. Cl- gradient rather than merely imposing a voltage gradient was a specific mediator of Na+-coupled transport of l-proline, taurine, l-glutamic acid, and glycine across the brush border membrane. Cl- gradient alone under Na+-equilibrated conditions could energize an overshoot of taurine accumulation by vesicles providing evidence that taurine is energetically activated by and coupled to Cl- transport. These data suggest that Na+-linked transport of most amino acids across the tubular luminal membrane is an electrogenic positive process and for proline, taurine, glutamic acid, and glycine, a Cl--requiring process. A negative intracellular potential combined with luminal chloride is required for optimal Na+-coupled transport of these amino acids across the luminal membrane of the proximal tubule. The coupling of Cl- to the transport of these osmoprotective amino acids may enhance their volume regulatory effect in kidney cells and other mammalian cells.

cGMP-dependent and -independent inhibition of a K+ conductance by natriuretic peptides: molecular and functional studies in human proximal tubule cells

In immortalized human kidney epithelial (IHKE-1) cells derived from proximal tubules, two natriuretic peptide receptors (NPR) were identified. In addition to NPR-A, which is bound by atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and urodilatin (URO), a novel form of NPR-B that might be bound by C-type natriuretic peptide (CNP) was identified using PCR. This novel splice variant of NPR-B (NPR-Bi) was also found in human kidney. Whereas ANP, BNP, and URO increased intracellular cGMP levels in IHKE-1 cells in a concentration-dependent manner, CNP had no effect on cGMP levels. To determine the physiologic responses to these agonists in IHKE-1 cells, the membrane voltage (Vm) was monitored using the slow whole-cell patch-clamp technique. ANP (10 nM), BNP (10 nM), and URO (16 nM) depolarized these cells by 3 to 4 mV (n = 47, 7, and 16, respectively), an effect that could be mimicked by 0.1 mM 8-Br-cGMP (n = 15). The effects of ANP and 8-Br-cGMP were not additive (n = 4). CNP (10 nM) also depolarized these cells, by 3+/-1 mV (n = 28), despite the absence of an increase in cellular cGMP levels, indicating a cGMP-independent mechanism. In the presence of CNP, 8-Br-cGMP further depolarized Vm significantly, by 1.6+/-0.3 mV (n = 5). The depolarizations by ANP were completely abolished in the presence of Ba2+ (1 mM, n = 4) and thus can be related to inhibition of a K+ conductance in the luminal membrane of IHKE-1 cells. The depolarizations attributable to CNP were completely blocked when genistein (10 microM, n = 6), an inhibitor of tyrosine kinases, was present. These findings indicate that natriuretic peptides regulate electrogenic transport processes via cGMP-dependent and -independent pathways that influence the Vm of IHKE-1 cells.

Cross-talk and the role of KATP channels in the proximal tubule

Over the last few years it has become evident that an assortment of functionally-related, but diverse, KATP channels provide an important and physiologically-regulated determinant of the K conductive pathways in many, if not all, epithelial cells expressed along the nephron. As such, KATP plays central roles in regulating and maintaining a number of transport processes in concert with physiological demands of the kidney. In the renal proximal tubule, KATP channels and changes in the hydrolytic activity of the Na,K-ATPase permit ATP to act as a coupling modulator of parallel Na,K-ATPase-K recycling. The response insures that cell membrane potential, intracellular K activity and cell volume are protected in the face of physiological variations in transcellular ion transport. In addition to demonstrating the physiological relevance of KATP in renal epithelial, these studies have provided a long awaited answer to the underlying mechanism of pump-leak coupling, a universal and essential homeostatic mechanism observed in nearly all salt translocating epithelia.

Effects of cell differentiation on ion conductances and membrane voltage in LLC-PK1 cells

LLC-PK1 cells serve as a widely used model for the renal proximal tubule. Until now, little has been found out about their membrane voltage (Vm) and ionic conductances (g). Several studies have shown changes in cell properties during differentiation and ageing. The aim of this study was to examine the relationship between Vm or g and the age of these cells. Therefore, we investigated single cells, subconfluent and confluent monolayers of LLC-PK1 cells aged 1-8 days with the slow-whole-cell patch-clamp technique. The Vm of all cells was -34 +/- 2 mV (n = 75) and the membrane conductance (gm) was 2.3 +/- 0.3 nS (n = 30). Vm in cells aged up to 2 days was -24 +/- 3 mV (n = 22) whereas Vm in cells aged 5-8 days was -50 +/- 3 mV (n = 15). An increase of extracellular K+ from 3.6 to 18.6 mmol/l led to a depolarization in all cells of 4 +/- 1 mV (n = 31) and an increase of gm by 17 +/- 13% (n = 15). Complete replacement of extracellular Na+ by N-methyl-D-glucamine (NMDG) led to a hyperpolarization of 19 +/- 2 mV (n = 38) and gm was lowered by 27 +/- 14% (n = 17). A reduction in extracellular Cl- from 147 to 32 mmol/l showed no significant effect on Vm (n = 16) or gm (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Classical and channel-like urate transporters in rabbit renal brush border membranes

The precise mechanism by which urate is transported across rabbit renal proximal tubule luminal membranes has not been defined. To determine whether urate flux across this membrane represents simple diffusion or transport on specific carriers, urate uptake was examined in brush border membrane vesicles that were prepared by a Mg+(+)-aggregation technique and then exposed to CuCl2. Na(+)-independent, voltage sensitive urate transport was demonstrated in these Cu+(+)-exposed vesicles. Transport was trans-stimulated by urate and cis inhibited by pyrazinoic acid and oxonate. A small fraction of transported urate and urate in the extravesicular fluid was oxidized to allantoin. Kinetic analysis revealed the presence of two kinetically distinct transporters; a channel-like carrier that was inhibited by pyrazinoic acid and oxonate, and a high-affinity, classical, saturable carrier that was inhibited by higher concentrations of oxonate. These studies provide the first direct evidence for carrier-mediated urate transport in rabbit renal brush-border membranes and demonstrate that the rabbit transporter(s) share a number of properties with the urate uniporter in rat proximal tubule cell membranes.

The value of in vivo electrophysiological measurements for monitoring functional adaptation after massive small bowel resection in the rat

The process of functional adaptation after extensive small bowel resection is complex and imprecisely understood. In vivo electrophysiological measurements for monitoring the functional adaptive process after massive small bowel resection in Brown-Norway rats were evaluated. Rats underwent either a sham operation (SH) or a 90% small bowel resection (SB). Standard rat chow was fed in unlimited quantities. At three or 10 weeks after operation, jejunal and ileal transepithelial potential differences (PD, mV) were determined. Electrogenic ion transport in the villus was measured after glucose (sodium coupled active glucose absorption; PD-glu) and in the crypt, after theophylline infusion (theophylline stimulated chloride secretion; PD-theo). Biopsies were taken simultaneously. Each experimental group consisted of three to five animals. At three weeks the PD-theo and PD-glu in SB rats were significantly lower than in SH rats in both jejunal and ileal segments. At 10 weeks PD-theo and PD-glu were significantly diminished in the jejunal segment of the SB rats compared with the SH rats. The values of PD-theo and PD-glu in the ileal segments were, however, no longer different between the two groups. Three and 10 weeks after operation the length of the villi in the SB group was increased significantly compared with the SH controls. These results indicate that in the early phase of adaptation in vivo electrophysiological variables do not correlate with histological changes in the SB rats. This might be due to cell immaturity resulting from an increased rate of cell turnover or lack of intercellular tight junctions. This hypothesis is supported by a recovery of PD responses in the ileum 10 weeks after resection.

Effect of bicarbonate on potassium conductance of isolated perfused rat pancreatic ducts

The aim of this study was to investigate the role of the K+ conductance in unstimulated and stimulated pancreatic ducts and to see how it is affected by provision of exogenous HCO3-/CO2. For this purpose we have applied electrophysiological techniques to perfused pancreatic ducts, which were dissected from rat pancreas. The basolateral membrane potential PDbl of unstimulated duct cells was between -60 mV and -70 mV, and the cells had a relatively large K+ conductance in the basolateral membrane as demonstrated by (a) 20-22 mV depolarization of PDbl in response to increase in bath K+ concentration from 5 mmol/l to 20 mmol/l and (b) the effect of a K+ channel blocker, Ba2+ (5 mmol/l), which depolarized PDbl by 30-40 mV. These effects on unstimulated ducts were relatively independent of bath HCO3-/CO2. The luminal membrane seemed to have no significant K+ conductance. Upon stimulation with secretin or dibutyryl cyclic AMP, PDbl depolarized to about -35 mV in the presence of HCO3-/CO2. Notably, the K+ conductance in the stimulated ducts was now only apparent in the presence of exogenous HCO3-/CO2 in the bath solutions. Upon addition of Ba2+, PDbl depolarized by 13 +/- 1 mV (n = 7), the fractional resistance of the basolateral membrane, FRbl increased from 0.66 to 0.78 (n = 6), the specific transepithelial resistance, Rte, increased from 52 +/- 13 omega cm2 to 59 +/- 15 omega cm2 (n = 11), and the whole-cell input resistance, Rc, measured with double-barrelled electrodes, increased from 20 M omega to 26 M omega (n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical study of a rat membrane protein which induces a selective potassium permeation: its localization in the apical membrane portion of epithelial cells

We previously reported a novel rat membrane protein that exhibits a voltage-dependent potassium channel activity on the basis of molecular cloning combined with an electrophysiological assay. This protein, termed IsK protein, is small and different from the conventional potassium channel proteins but induces selective permeation of potassium ions on its expression in Xenopus oocytes. In this investigation, we examined cellular localization of rat IsK protein by preparing three different types of antibody that specifically reacts with a distinct part of rat IsK protein. Immunohistochemical analysis using these antibody preparations demonstrated that rat IsK protein is confined to the apical membrane portion of epithelial cells in the proximal tubule of the kidney, the submandibular duct and the uterine endometrium. The observed tissue distribution of rat IsK protein was consistent with that of the IsK protein mRNA determined by blot hybridization analysis. In epithelial cells, the sodium, potassium-ATPase pump in the basolateral membrane generates a sodium gradient across the epithelial cell and allows sodium ions to enter the cell through the apical membrane. Thus, taking into account the cellular localization of the IsK protein, together with its electrophysiological properties, we discussed a possible function of the IsK protein, namely that this protein is involved in potassium permeation in the apical membrane of epithelial cells through the depolarizing effect of sodium entry.

A simple method for multiple fluid exchange

A simple device for fluid exchange is described, which allows the exchange of an unlimited number of solutions at low (10-1000 nl/min) constant perfusion rate. The applicability of the system has been tested in microperfusion experiments of rat distal tubules. At a luminal perfusion rate of 40 nl/min, the lag time was some 20 sec and 80% fluid exchange time some 3 sec. Simple modification allows further reduction of the lag time. Under control conditions, the potential difference across the late distal tubule (PDte) approaches -19.4 +/- 2.5 mV (n = 27). Increase of luminal potassium concentration from 5.4 to 40 mmol/l hyperpolarizes PDte to -29.9 +/- 4.3 mV (n = 8). Amiloride (10 mumol/l) leads to a reversible depolarization to -3.2 +/- 1.0 mV (n = 19), barium (1 mmol/l) to a reversible hyperpolarization to -25.8 +/- 2.6 mV (n = 19). As expected, PDte is largely created by amiloride sensitive sodium channels and is partially blunted by barium sensitive potassium channels.

Sodium-coupled amino acid transport in renal tubule

Amino acids are reabsorbed from the tubular lumen by a saturable, carrier-mediated, concentrative transport mechanism driven by a Na+ electrochemical gradient across the luminal membrane. This process is followed by efflux mainly via carrier-mediated, Na+-independent facilitated diffusion across the basolateral membrane. Individual amino acids may have two or more Na+-dependent transport systems with different kinetic characteristics along the luminal membrane of the proximal tubule, thereby enabling very efficient amino acid reabsorption. Dual Na+-coupled transport pathways for some amino acids located in both the luminal and the peritubular membranes may operate in concert to provide the tubular epithelial cell with essential nutrients. One or more Na+ ions, H+, Cl- and in the case of acidic amino acids, K+ ion, may be involved in the translocation of the carrier complex. For most amino acids this process is electrogenic positive, favored by a negative cell interior. At least seven distinct, but largely interacting, Na+-dependent amino acid transport systems have been identified in the brush border membrane. A diet-induced adaptation in Na+-coupled taurine transport and acidosis-induced adaptive response in Na+-dependent glutamine transport are expressed at the luminal and the basolateral membrane surfaces, respectively. The aminoaciduria of early life may be related to a rapid dissipation of the Na+ electrochemical gradient necessary for amino acid reabsorption.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Computer analysis reveals changes in renal Na+-glucose cotransporter in diabetic rats

A novel, computer-assisted program was developed to analyze the time course of Na+-glucose cotransport by rat renal cortical brush-border membrane vesicles (BBMV). Transporter characteristics can be measured, which routine kinetic analyses fail to distinguish: cotransporter membrane density is derived from the picomoles of D-glucose bound per milligram of protein. Binding is stereospecific, blocked by phlorizin, and supported equally well by Na+ or K+ (but not Cs+). Quasi-first-order influx and efflux rate constants for the composite Na+-driven influx and the (presumed) Na+-independent efflux processes were highly dependent on glucose concentration. Either two Na+-glucose transporters exist in proximal tubules or a single mechanism abruptly changes rate when glucose falls to low levels. The major operation mode is slow, has a high capacity but low affinity, and may have a 2 Na+:2 glucose stoichiometry (Hill coefficient is unity). The minor system is a fast, smaller-capacity, higher-affinity operation with a 2 Na+:1 glucose stoichiometry that was not distinguishable when the same data were analyzed in conventional kinetic plots. Results with streptozocin-induced diabetic rats illustrate the method's utility. Low-glucose-affinity cotransporters were upregulated in hyperglycemic, but not in cachectic, ketoacidotic animals. Rate constants, especially for efflux, were decreased in diabetes.

Ca2+-activated K+ conductance causes membrane hyperpolarizations in a monkey kidney cell line (JTC-12)

We have previously reported hyperpolarizing membrane potential changes in a monkey kidney cell line (JTC-12) which has characteristics resembling proximal tubular cells. These hyperpolarizations could be observed spontaneously or evoked by mechanically touching adjacent cells. In this report, we have shown further evidence that these hyperpolarizations are elicited by an increase in membrane conductance to K+ which is caused by an increase in cytosolic Ca2+ concentration. In addition, we have found another type of hyperpolarization which is evoked by applying flow of extracellular fluid to the cell. Intracellular injection of Ca2+ and Sr2+ evoked hyperpolarizations, while intracellular injection of Mn2+ and Ba2+ did not. Intracellular injection of EGTA suppressed both spontaneous and mechanically evoked hyperpolarizations. In Ca2+-free medium, both spontaneous and flow-evoked hyperpolarizations were not observed, while mechanical stimuli consistently evoked hyperpolarization. In Na+-free medium, the incidence of cells showing the spontaneous or flow-evoked hyperpolarization increased, and the amplitude and the duration of the mechanically evoked hyperpolarization became greater. Quinidine inhibited all types of hyperpolarization. These data suggest that hyperpolarizations in JTC-12 cells are due to an increase in Ca2+-activated K+ conductance.

Role of cyclic AMP in steroid action in rat intestine

We have investigated the effect of mineralocorticoids on intestinal fluid and electrogenic glucose-linked Na+ transport across isolated sections of the rat small intestine. A rapid in vitro response is observed that contrast with the delay normally associated with steroid hormone responses. Cyclic AMP is known to affect intestinal glucose, water and Na+ transport and the effects of the steroids may be understood in terms of an inhibitory effect on intestinal cyclic AMP production. An inhibitory effect of the steroids on membrane-bound adenylate cyclase has been demonstrated and dose-response effects suggest the presence of specific membrane-bound glucocorticoid receptors.

Kinetics of the Na+/alanine cotransporter in pancreatic acinar cells

Electric currents associated with Na+-coupled alanine transport in pancreatic acinar cells were investigated by the technique of tight-seal whole-cell recordings. In a previous study the observed concentration dependence of alanine-dependent currents was found to be consistent with a 'simultaneous' transport mechanism with 1:1 stoichiometry. In the present work the sidedness of the cotransporter was investigated by comparing inward (I") and outward currents (I') measured under mirror-symmetrical conditions. I' and I" were found to be nearly equal (within a factor of approx. 2) in a wide range of Na+ and alanine concentrations. The transport model was further tested by 'infinite-cis' experiments with fixed, saturating concentrations of Na+ and L-alanine on one side of the membrane and variable concentrations on the other. By measuring transmembrane currents as a function of Na+ and alanine concentrations, numerical values of the equilibrium dissociation constants of both substrates could be estimated.

On the nature of delayed repolarization during sustained sodium coupled transport in frog proximal tubules

In proximal tubules of the frog kidney, stimulation of coupled transport of sodium with phenylalanine leads to depolarization of the cell membrane, followed by repolarization within a few minutes. The repolarization is due to a delayed increase of potassium conductance at the peritubular cell membrane. The present study was designed to test for the role of depolarization, of calmodulin and of arachidonic acid metabolites for the delayed increase of potassium conductance. To this end, the potential difference across the peritubular cell membrane of proximal convoluted tubules (PDpt) has been recorded continuously during exposure of the lumen to phenylalanine or during galvanic current injection into a neighbouring cell. During control conditions, PDpt averages -68.6 +/- 1.0 mV (n = 45). Phenylalanine leads to a depolarization of the peritubular cell membrane by +31.5 +/- 1.3 mV (n = 20), followed by a repolarization by -12.9 +/- 1.1 mV (n = 20) within 3 min. Injection of currents from 10 to 80 nAmps leads to a depolarization by +0.83 +/- 0.01 mV/nAmps which is again followed by repolarization. A linear correlation is observed between the magnitude of depolarization (dep) and repolarization (rep) within 3 min: rep (mV) = -(0.24 +/- 0.01) dep (mV) +(2.45 +/- 0.12) mV (r = 0.90). Thus, depolarization is capable to trigger delayed repolarization. The extent of repolarization is a function of the magnitude of depolarization. The possible involvement of calmodulin or arachidonic acid metabolites has been tested for by inducing sodium coupled transport in the presence of 100 mumol/l mepacrine, 10 mumol/l indomethacin or 10 mumol/l trifluoperazine.

Measurement of Na+-K+ coupling ratio of Na+-K+-ATPase in rabbit proximal tubules

A combination of 23Na nuclear magnetic resonance (NMR) spectroscopy and a K+-selective electrode was used to make simultaneous measurements of net Na+ and K+ fluxes across plasma membranes of rabbit renal proximal tubules after an abrupt stimulation of Na+-K+-ATPase. After a step in extracellular K+ concentration ([K+]o) from low to higher concentration (0.1-0.3 mM to 0.5-5.2 mM) at 25 degrees C, net extrusion of Na+ and uptake of K+ were observed. These fluxes were completely inhibited by ouabain (10(-3) M). Because initial rates of K+ uptake in presence or absence of Ba2+ (a known inhibitor of plasma membrane K+ conductance) were indistinguishable, net K+ flux was virtually unidirectional. Because suspension buffers contained neither glucose nor amino acids and the ratio of net Na+ and K+ fluxes (JNa and JK, respectively) was constant over a wide range of transmembrane Na+ gradients and absolute values of the JNa and JK, it is likely that changes in electrogenic or passive net fluxes across plasma membranes were insignificant in the first 30-45 s after the [K+]o step. Thus the ratio of these initial net Na+ and K+ fluxes corresponds closely to the Na+-K+ coupling ratio of the Na+-K+-ATPase. In 12 experiments, the measured Na+-K+-ATPase coupling ratio was 1.54 +/- 0.07 (SE). The coupling ratio was constant over a wide range of intracellular Na+ content, intracellular sodium concentration, [K+]o and transmembrane Na+ gradient. The coupling ratio also remained constant over an eightfold range of Na+-K+-ATPase rates.

Potassium conductance in straight proximal tubule cells of the mouse. Effect of barium, verapamil and quinidine

The present study has been performed to test for the influence of verapamil and quinidine on the potential difference across the basolateral cell membrane (PDbl) and on the basolateral potassium conductance of isolated perfused segments of the mouse proximal tubule. PDbl was recorded continuously with conventional microelectrodes during rapid alterations of bath or luminal perfusate composition. The contribution of the basolateral potassium conductance to the conductance of both cell membranes (tk) was estimated from the effects of altered bath potassium concentration on PDbl. Under control conditions tk approaches 0.8, i.e. the basolateral cell membrane is mainly conductive to potassium. Neither quinidine nor verapamil affect PDbl at concentrations below 10 mumol/l. At higher concentrations both substances depolarize the basolateral cell membrane mimicking the effect of 1 mmol/l barium. In the presence of 0.1 mmol/l verapamil tk is virtually abolished at 5 to 10 mmol/l bath potassium concentration but is almost unaffected at bath potassium concentrations between 20 and 40 mmol/l. 1 mumol/l ionophore A-23187 does not change the depolarizing effect of 0.1 mmol/l verapamil on cell membrane potential. In the presence of 0.1 mmol/l quinidine, tk is reduced to some 50%, irrespective of the bath potassium concentration. It is concluded that the potassium conductance in straight proximal tubules is inhibited not only by barium but as well by high concentrations of verapamil and quinidine. The effect is probably direct and not related to alterations in the intracellular calcium activity.

Kinetic transport model for cellular regulation of pH and solute concentration in the renal proximal tubule

An open circuit kinetic model was developed to calculate the time course of proximal tubule cell pH, solute concentrations, and volume in response to induced perturbations in luminal or peritubular fluid composition. Solute fluxes were calculated from electrokinetic equations containing terms for known carrier saturabilities, allosteric dependences, and ion coupling ratios. Apical and basolateral membrane potentials were determined iteratively from the requirements of cell electroneutrality and equal opposing transcellular and paracellular currents. The model converged to membrane potentials accurate to 0.05% in one to four iterations. Model variables included cell concentrations of Na, K, HCO3, glucose, pH (uniform CO2), volume, and apical and basolateral membrane potentials. The basic model contained passive apical membrane transport of Na/H, Na/glucose, H and K, basolateral transport of Na/3HCO3, K, H, and glucose, and paracellular transport of Na, K, Cl, and HCO3; apical H and basolateral 3Na/2K-ATPases were present. Apical Na/H and basolateral K transport were regulated allosterically by pH. Apical Na/H transport, basolateral Na/3HCO3 transport, and the 3Na/2K-ATPase were saturable. Model parameters were chosen from data in the rat proximal tubule. Model predictions for the magnitude and time course of cell pH, Na, and membrane potential in response to rapid changes in apical and peritubular Na and HCO3 were in excellent agreement with experiment. In addition, the model requires that there exist an apical H-ATPase, basolateral Na/3HCO3 transport saturable with HCO3, and electroneutral basolateral K transport.

Basolateral membrane Cl/HCO3 exchange in the rat proximal convoluted tubule. Na-dependent and -independent modes

To examine whether Cl-coupled HCO3 transport mechanisms were present on the basolateral membrane of the mammalian proximal tubule, cell pH was measured in the microperfused rat proximal convoluted tubule using the pH-sensitive, intracellularly trapped fluorescent dye (2',7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein. Increasing the peritubular Cl concentration from 0 to 128.6 meq/liter caused cell pH to decrease from 7.34 +/- 0.04 to 7.21 +/- 0.04 (p less than 0.001). With more acid extracellular fluid (pH 6.62), a similar increase in the peritubular Cl concentration caused cell pH to decrease by a similar amount from 6.97 +/- 0.04 to 6.84 +/- 0.05 (p less than 0.001). This effect was blocked by 1 mM SITS. To examine the Na dependence of Cl/HCO3 exchange, the above studies were repeated in the absence of luminal and peritubular Na. In alkaline Na-free solutions, peritubular Cl addition caused cell pH to decrease from 7.57 +/- 0.06 to 7.53 +/- 0.06 (p less than 0.025); in acid Na-free solutions, peritubular Cl addition caused cell pH to decrease from 7.21 +/- 0.04 to 7.19 +/- 0.04 (p less than 0.05). The effect of Cl on cell pH was smaller in the absence of luminal and peritubular Na than in its presence. To examine whether the previously described Na/(HCO3)n greater than 1 cotransporter was coupled to or dependent on Cl, the effect of lowering the peritubular Na concentration from 147 to 25 meq/liter was examined in the absence of ambient Cl. Cell pH decreased from 7.28 +/- 0.03 to 7.08 +/- 0.03, a response similar to that observed previously in the presence of Cl. The results demonstrate that Cl/HCO3 (or Cl/OH) exchange is present on the basolateral membrane. Most of Cl/HCO3 exchange is dependent on the presence of Na and may be coupled to it. The previously described Na/(HCO3)n greater than 1 cotransporter is the major basolateral membrane pathway for the coupling of Na and HCO3 and is not coupled to Cl.

Microscopic description of voltage effects on ion-driven cotransport systems

A microscopic model for the analysis of voltage effects on ion-driven cotransport systems is described. The model is based on the notion that the voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from the movement of an ion along the transport pathway, from the displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of overall transport rate is described by a set of dimensionless coefficients reflecting the dielectric distances over which charge is displaced in the elementary reaction steps. The dielectric coefficients may be evaluated from the shape of the experimental flux-voltage curve if sufficient information on the rate constants of the reaction cycle is available. Examples of flux-voltage curves which are obtained by numerical simulation of the transport model are given for a number of limiting cases.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.