Impedance analysis in epithelia and the problem of gastric acid secretion

Helicobacter pylori impedes acid-induced tightening of gastric epithelial junctions

Gastric infection by Helicobacter pylori is the most common cause of ulcer disease and gastric cancer. The mechanism of progression from gastritis and inflammation to ulcers and cancer in a fraction of those infected is not definitively known. Significant acidity is unique to the gastric environment and is required for ulcer development. The interplay between gastric acidity and H. pylori pathogenesis is important in progression to advanced disease. The aim of this study was to characterize the impact of acid on gastric epithelial integrity and cytokine release and how H. pylori infection alters these responses. Human gastric epithelial (HGE-20) cells were grown on porous inserts, and survival, barrier function, and cytokine release were studied at various apical pH levels in the presence and absence of H. pylori. With apical acidity, gastric epithelial cells demonstrate increased barrier function, as evidenced by increased transepithelial electrical resistance (TEER) and decreased paracellular permeability. This effect is reduced in the presence of wild-type, but not urease knockout, H. pylori. The epithelial inflammatory response is also modulated by acidity and H. pylori infection. Without H. pylori, epithelial IL-8 release decreases in acid, while IL-6 release increases. In the presence of H. pylori, acidic pH diminishes the magnitude of the previously reported increase in IL-8 and IL-6 release. H. pylori interferes with the gastric epithelial response to acid, contributing to altered barrier function and inflammatory response. H. pylori diminishes acid-induced tightening of cell junctions in a urease-dependent manner, suggesting that local pH elevation promotes barrier compromise and progression to mucosal damage.

Specific and nonspecific effects of protein kinase C on the epithelial Na (+) channel

The Xenopus oocyte expression system was used to explore the mechanisms of inhibition of the cloned rat epithelial Na(+) channel (rENaC) by PKC (Awayda, M.S., I.I. Ismailov, B.K. Berdiev, C.M. Fuller, and D.J. Benos. 1996. J. Gen. Physiol. 108:49-65) and to determine whether human ENaC exhibits similar regulation. Effects of PKC activation on membrane and/or channel trafficking were determined using impedance analysis as an indirect measure of membrane area. hENaC-expressing oocytes exhibited an appreciable activation by hyperpolarizing voltages. This activation could be fit with a single exponential, described by a time constant (tau) and a magnitude (DeltaI (V)). A similar but smaller magnitude of activation was also observed in oocytes expressing rENaC. This activation likely corresponds to the previously described effect of hyperpolarizing voltage on gating of the native Na(+) channel (Palmer, L.G., and G. Frindt. 1996. J. Gen. Physiol. 107:35-45). Stimulation of PKC with 100 nM PMA decreased DeltaI(V) in hENaC-expressing oocytes to a plateau at 57.1 +/- 4.9% (n = 6) of baseline values at 20 min. Similar effects were observed in rENaC-expressing oocytes. PMA decreased the amiloride-sensitive hENaC slope conductance (g(Na)) to 21.7 +/- 7.2% (n = 6) of baseline values at 30 min. This decrease was similar to that previously reported for rENaC. This decrease of g (Na) was attributed to a decrease of membrane capacitance (C (m)), as well as the specific conductance (g(m)/C(m )). The effects on g(m)/C(m) reached a plateau within 15 min, at approximately 60% of baseline values. This decrease is likely due to the specific ability of PKC to inhibit ENaC. On the other hand, the decrease of C(m) was unrelated to ENaC and is likely an effect of PKC on membrane trafficking, as it was observed in ENaC-expressing as well as control oocytes. At lower PMA concentrations (0.5 nM), smaller changes of C(m) were observed in rENaC- and hENaC-expressing oocytes, and were preceded by larger changes of g(m ) and by changes of g(m)/C(m), indicating specific effects on ENaC. These findings indicate that PKC exhibits multiple and specific effects on ENaC, as well as nonspecific effects on membrane trafficking. Moreover, these findings provide the electrophysiological basis for assessing channel-specific effects of PKC in the Xenopus oocyte expression system.

Contribution of electrogenic ion transport to impedance of the algae Valonia utricularis and artificial membranes

The cell membrane of Valonia utricularis contains an electrogenic carrier system for chloride (Wang et al., Biophys J. 59:235-248 (1991)). The electrical impedance of V. utricularis was measured in the frequency range between 1 Hz and 50 kHz. The analysis of the impedance spectra from V. utricularis and its comparison with equivalent circuit models showed that the transport system created a characteristic contribution to the impedance in the frequency range between 10 Hz and 5 kHz. The fit of the impedance spectra with the formalism derived from the theory of carrier-mediated transport allowed the determination of the kinetic parameters of chloride transport through the cell membrane of V. utricularis, and its passive electrical properties. Simultaneous measurements of the kinetic parameters with the charge pulse method demonstrated the equivalence of both experimental approaches with respect to the evaluation of the translocation rate constants of the free and the charged carriers and the total density of carriers within the membrane. Moreover, the impedance spectra of the protonophor-mediated proton transport by FCCP (carbonylcyanide p-trifluoromethoxyphenyl-hydrazone) were measured in model membranes. The carrier system made a substantial contribution to the impedance of the artificial membranes. The analysis of the spectra in terms of a simple carrier system (Benz and McLaughlin, 1983, Biophys. J. 41:381-398) allowed the evaluation of the kinetic and equilibrium parameters of the FCCP-mediated proton transport. The possible application of the measurement of impedance spectra for the study of biological transport systems is discussed.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

Whole-cell currents in isolated resting Necturus gastric oxynticopeptic cells

1. Necturus gastric mucosa secretes Cl- actively across the gastric glands which are composed almost entirely of acid- and enzyme-secreting oxynticopeptic cells. Single channel studies on Necturus oxynticopeptic cells have shown that the basolateral membrane possesses multiple K(+)-selective channels but no observable Cl- channels while the apical membrane has Cl- channels but no observable K+ channels. To relate these channel properties to the conductance of the whole cell we have investigated the macroscopic membrane currents with conventional whole-cell patch-clamp techniques. 2. When bathed in amphibian Ringer solution, gastric oxynticopeptic cells had a membrane resistance of 47.8 +/- 2.8 M omega and a membrane capacitance of 75.5 +/- 2.7 pF (n = 82). This gave a specific membrane resistance of 3260 +/- 160 omega cm2 (n = 82). Reversal potentials of the oxynticopeptic cells were -13.8 +/- 1.2 mV (n = 45) for an intracellular Cl- concentration ([Cl-]i) of 42 mM and were significantly more negative -24.4 +/- 3.1 mV (n = 31, P < 0.001) for [Cl-]i = 22 mM. 3. In the absence of ATP in the pipette solution, there was an 80% reduction of the whole-cell current with a typical half-time (t1/2) of 5 min. The run-down was not observed when the pipette solution contained 4 mM ATP. 4. A slow and voltage-independent inhibition of 80% of the whole-cell currents occurred after addition of NPPB (35 microM). Ba2+ (10 mM) produced a reversible inhibition of 20% of the total current. Together, 35 microM NPPB and 10 mM Ba2+ eliminated 95% of the whole-cell currents. These data suggest that in the resting oxynticopeptic cells Cl- carried the major fraction of the current while K+ ions carried only a small fraction. 5. Total replacement of Cl- in the pipette and bath solution by gluconate- increased the membrane resistance to 751 +/- 104 M omega (n = 53) and shifted the reversal potential to -38.1 +/- 2.8 mV (n = 53). 6. Increasing the bath K+ concentration from 6 to 91 mM activated a current which had a high selectivity for K+ over choline+, Li+, Na+, Rb+ and Cs+ and was independent of Cl-. The activation of this K+ current (IK*) by high external K+ was not seen with ATP-free pipette solution. 7. Ba2+ or Cs+ had a voltage-dependent blocking effect of this inward K+ current. Ouabain (1 mM) or SCH 28080 (200 microM), specific inhibitors of the Na+,K(+)-ATPase and H+,K(+)-ATPase, had no effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane capacitance increases induced by histamine and cyclic AMP in single gastric acid-secreting cells of the guinea pig

During acid secretion, gastric parietal cells undergo profound morphological changes including formation of the apical secretory membrane. To examine the mechanism of histamine-induced increases in the apical membrane area at the single cell level, we monitored the membrane capacitance by applying a time-resolved phase-sensitive detection method to singly isolated parietal cells of guinea pig. A real-time increase in the membrane capacitance was detected within several min after stimulation with histamine. An H2-blocker (cimetidine), but not an H1-blocker (pyrilamine), inhibited the histamine response. Dibutyryl cyclic AMP mimicked the histamine effect. The capacitance response to histamine was sensitive to cytosolic Ca2+, temperature and N-ethylmaleimide. The histamine response was inhibited by intracellular application of a non-hydrolyzable ATP analog (AMP-PNP) and an isoquinolinesulfonamide derivative that works as an inhibitor of protein kinase A (H-8). These results indicate that in parietal cells, elevation of intracellular cyclic AMP induces exocytotic insertion of intracellular membranes into the plasma membrane, presumably by activating protein kinase A.

Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.

Role of paracellular pathway in nonelectrolyte permeation across rat colon epithelium enhanced by sodium caprate and sodium caprylate

The enhancing effects of 0.25% sodium caprate (C10) and sodium caprylate (C8) on the paracellular permeation of seven water-soluble nonelectrolytes (inulin, polyethylene glycol 900, mannitol, erythritol, glycerol, thiourea, and urea) across the isolated rat colonic epithelium were examined using the Ussing-type chamber technique. The paracellular changes were also measured by impedance analysis. In both the presence and the absence of enhancers, the permeation clearances (Pm) for inulin (12-15 A in molecular radius) to erythritol (3.2 A) increased linearly with the increase in their free diffusion coefficients (Dfr), showing the existence of a paracellular shunt pathway unrestricted to any molecular size. Glycerol (2.9 A), thiourea (2.6 A), and urea (2.3 A) had higher clearances than the expected linear values, showing the existence of a restricted paracellular or transcellular pathway. Both C10 and C8 increased the permeabilities in the two pathways, but C10 was more effective than C8. The increase in the permeabilities via the shunt pathway caused by the enhancers was greater than that via the restricted pathway, and thus, the two-phase pattern in the relationship of Pm and Dfr was similar to that in the absence of enhancers. The transcellular permeabilities for urea and thiourea, which were obtained from the efflux experiments, were increased by the enhancers. However, the relative increase caused by C10 was smaller than that of the paracellular-restricted permeabilities. The paracellular changes probably were due to the increase in pore area per unit diffusive path length.(ABSTRACT TRUNCATED AT 250 WORDS)

Redistribution of membranes and cytoskeletal proteins in chicken oxyntic cells during the HCl secretory cycle: ultrastructural and immunofluorescence study

Changes in ultrastructure and cytoskeletal organization by avian oxyntic cells, at the onset of HCl secretion, were analysed. Cells in resting state, induced by fasting and cimetidine, were compared with histamine stimulated secreting cells. Ultrastructural studies were done by transmission electron microscopy; the distribution of prekeratin, myosin, and filamin-like protein, by immunofluorescence; and that of F-actin using FITC-phalloidin. Resting cells show short pericellular clefts. These are increasingly deepened in secreting cells by a reorganization of the lateral cell borders involving displacement of the junctional complexes toward the cell base and incorporation of the tubular system to the luminal plasma membrane. In secreting cells, the processes of the secretory surface are concentrated in a pericellular groove. Histamine stimulation induces a drastic redistribution of cytoskeletal proteins. In chicken oxyntic cells, in addition to the F-actin cytoskeleton associated with the membranes of the secretory surface, there is a cytoskeletal ring containing F-actin, myosin, and a filamin-like protein, located at the level of the junctional complexes. In resting cells, filaments and masses of cytoskeletal matrix are associated with the zonula adherens. In secreting cells, the junctional complexes maintain their association with the filamentous ring, while the amorphous matrix is replaced by microfilaments that support the processes of the luminal surface. Intermediate filaments form a peripheral ring probably associated with the zonula adherens, and project from the ring toward the cell cytoplasm. Thus, with the onset of HCl secretion, the apical cytoskeletal ring of resting cells displaces toward the cell base. A role for this cytoskeletal ring in the changes in shape parallel to HCl secretion is discussed.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

Changes in membrane conductances and areas associated with bicarbonate secretion in turtle bladder

Transepithelial impedance-analysis studies were performed in turtle bladder epithelium in order to measure changes in the different epithelial membranes resulting from stimulation of electrogenic bicarbonate secretion. Changes in membrane conductance relate to changes in ionic permeability, whereas changes in membrane capacitance relate to changes in membrane area, since most biological membranes exhibit a specific capacitance of approximately 1 muF/cm2. The results of this investigation are summarized as follows: (i) cAMP and carbachol, agents which have been shown previously to stimulate electrogenic bicarbonate secretion, result in increases in apical-membrane conductance and capacitance; (ii) these changes occur concomitantly with the observed change in transport (measured using the short-circuit-current technique), thereby suggesting that bicarbonate secretion may be regulated in part by changes in the chloride conductance of the apical membrane; (iii) the increase in conductance does not reflect an increase in the membrane's specific conductance, thereby indicating that it results from the addition of membrane possessing similar ionic permeability as the existing apical membrane; (iv) the magnitude of the changes in capacitance indicate that a minor cell population (beta-type carbonic-anhydrase-rich cells) increase their apical-membrane area by several-fold; (v) a lack of transport-associated changes in the basolateral-membrane parameters suggest that transport is not regulated by alterations in basolateral-membrane ionic conductance or area; (vi) a lack of colchicine sensitivity, coupled with the magnitude of the changes in apical-membrane capacitance, indicate that the membrane remodeling processes are different from those involved in the regulation of proton secretion in a different cell population (alpha-type carbonic-anhydrase-rich cells).

Quantitative subcellular study of apical pole membranes from chicken oxyntic cells in resting and HCl secretory state

Vertebrate oxyntic cells, responsible for gastric HCl production, undergo a remarkable morphological reorganization in relation to their secretory cycle. In resting state, the luminal surface of the cells is smooth; a peculiar system of endocellular membranes, the tubular system, occupies the luminal cytoplasm. Actin filaments frame a cortical network between the tubular system and the luminal plasma membrane. With the onset of HCl secretion, the tubular system becomes incorporated into the luminal plasma membrane. Villous processes containing microfilaments fill the secretory surface. This morphological reorganization of membranes and cytoskeletal matrix could regulate HCl secretion by translocation of membranes containing the proton pump from the endocellular compartment to the secretory surface. In this paper, we describe the isolation of membranes that selectively belong to the tubular system or to the cytoplasmic processes of the secretory surface of chicken oxyntic cells. Chicken oxyntic cells are the main cellular component of the proventricular glands. A resting state was obtained after cimetidine treatment, whereas the HCl-secretory state was induced by histamine. We present a comparative analysis of resting and stimulated chicken gastric glands by quantitative subcellular fractionation. The HCl secretory state was related to specific modifications in membrane fractions derived from the secretory pole of oxyntic cells. Morphological and functional reorganization of oxyntic cells was closely correlated with changes in: the sedimentation pattern of the marker enzyme of the apical pole membrane (K-NPPase), the total activity of K-NPPase and nonmitochondrial Mg-ATPase, the valinomycin dependence of K-ATPase, and polypeptides that cosediment in purified membrane fractions. Changes in the distribution pattern of K-NPPase after fractionation of histamine-stimulated glands were consistent with the replacement of the small vesicles typical of resting glands by dense membrane profiles, analogous to the luminal processes of stimulated oxyntic cells. SDS-PAGE showed that, in purified membrane fractions of stimulated glands, the concentration of 28-, 43-, and 200-kD polypeptides increased while that of 95- and 250-kD polypeptides decreased. The present results define the tubular system of oxyntic cells as an organelle with properties different from those of endoplasmic reticulum, mitochondria, and plasma membrane. The biochemical and physico-chemical properties of this membraneous system changed when the organization of the membranes and the cytoskeletal matrix of the apical pole was modified by the onset of HCl secretion.

Physiological regulation of transepithelial impedance in the intestinal mucosa of rats and hamsters

Isolated intestinal segments from rats or hamsters were recirculated with balanced salt solutions containing fluorocarbon emulsion to provide 6 vpc oxygen. The lumen contained an axial Ag-AgCl electrode, and the serosal surface was surrounded by a cylindrical shell of Ag-AgCl. Transmural impedances were measured at frequencies from 0.01-30 kHz before and after removal of the mucosal epithelium. The resistance of intercellular junctions, RJ, the distributed resistance of the lateral spaces, RL, and the distributed membrane capacitance, CM, were computed from the relations between frequency and impedance. Activation of Na-coupled solute transport by addition of glucose, 3-0-methyl glucose, alanine or leucine caused two- to threefold decreases of transepithelial impedance. Typical changes induced by glucose in hamster small intestine were RJ 30----13 omega, RL 23----10 omega, and CM 8----20 microF (per cm length of segment). Half maximal response occurred at a glucose concentration of 2-3 mM. The area per unit path length of the junctions (Ap/delta chi = specific resistance divided by RJ) in glucose activated epithelium was 3.7 cm in hamster midgut and 6.8 cm in rat. These values are close to the 4.3 cm estimated independently from coefficients of solvent drag and hydrodynamic conductance in glucose-activated rat intestine in vivo. The transepithelial impedance response to Na-coupled solute transport was reversibly dependent upon oxygen tension. It is proposed that activation of Na-coupled solute transport triggers contraction of circumferential actomyosin fibers in the terminal web of the microvillar cytoskeletal system, thereby pulling apart junctions and allowing paracellular absorption of nutrients by solvent drag as described in the previous accompanying paper. Anatomical evidence in support of this hypothesis is presented in the following second accompanying paper.

Epithelial impedance analysis in experimentally induced colon cancer

Epithelial impedance analysis was used to measure the alterations in resistance of the large bowel in a murine model of large bowel cancer. The technique was able to resolve the epithelial resistance from the total resistance of the bowel wall. A progressive decrease in resistance of the bowel epithelium occurs during carcinogenesis induced with dimethyhydrazine. About a 21% decrease in epithelial resistance from 22.0 +/- 1.3 omega.cm-2 to 17.5 +/- 1.1 omega cm-2 (p less than 0.025) was observed after 20 wk of carcinogen administration. The sensitivity of the technique in detecting altered epithelial resistance in premalignant bowel mucosa was improved by examining the impedance profile in a sodium-free Ringer's solution where the epithelium of control colons had a resistance of 24.4 +/- 1.8 omega.cm-2 compared with 19.0 +/- 1.1 omega.cm-2 (p less than 0.02) in colons from animals treated for only 4 wk with the carcinogen. Epithelial impedance analysis would seem to be a sensitive technique capable of identifying changes in the electrical properties or the large bowel early in disease states.

Evidence for a high and specific concentration of (Na+,K+)ATPase in the plasma membrane of the osteoclast

During bone resorption, the osteoclast actively acidifies a limited extracellular compartment. We hypothesized that, like other cells engaged in ion transport and proton translocation, the osteoclast's membrane might be highly enriched in sodium pumps. Using monoclonal antibodies to both the alpha and the beta subunits, immunoblot analysis, and [3H]ouabain binding, we have demonstrated that the osteoclast plasma membrane is both highly and specifically enriched in (Na+,K+)ATPase, compared with other bone cells, monocytes, macrophages, and other blood and bone marrow cells. The density of binding sites on the osteoclast is equivalent to that of kidney tubule cells. This observation is consistent with the hypothesis that the (Na+,K+)ATPase plays a role in the mechanism of bone resorption, possibly coupled with secondary active calcium and/or proton transport. Monoclonal antibodies against the (Na+,K+)ATPase can therefore be used as specific markers for the osteoclast in bone and bone marrow preparations.

Membrane electrical parameters in turtle bladder measured using impedance-analysis techniques

Equivalent-circuit impedance analysis experiments were performed on the urinary bladders of freshwater turtles in order to quantify membrane ionic conductances and areas, and to investigate how changes in these parameters are associated with changes in the rate of proton secretion in this tissue. In all experiments, sodium reabsorption was inhibited thereby unmasking the electrogenic proton secretion process. We report the following: transepithelial impedance is represented exceptionally well by a simple equivalent-circuit model, which results in estimates of the apical and basolateral membrane ionic conductances and capacitances; when sodium transport is inhibited with mucosal amiloride and serosal ouabain, the apical and basolateral membrane conductances and capacitances exhibit a continual decline with time; this decline in the membrane parameters is most likely caused by subtle time-dependent changes in cell volume, resulting in changes in the areas of the apical and basolateral membranes; stable membrane parameters are obtained if the tissue is not treated with ouabain, and if the oncotic pressure of the serosal solution is increased by the addition of 2% albumin; inhibition of proton secretion using acetazolamide in CO2 and HCO3- -free bathing solutions results in a decrease in the area of the apical membrane, with no significant change in its specific conductance; stimulation of proton transport with CO2 and HCO3- -containing serosal solution results in an increase in the apical membrane area and specific conductance. These results show that our methods can be used to measure changes in the membrane electrophysiological parameters that are related to changes in the rate of proton transport. Notably, they can be used to quantify in the live tissue, changes in membrane area resulting from changes in the net rates of endocytosis and exocytosis which are postulated to be intimately involved in the regulation of proton transport.

Pig gastric mucus: a one-way barrier for H+

Gastric mucus is thought to protect the underlying mucosal cells from mechanical hazards and back-diffusion of luminal H+. In health, a pH gradient exists across the mucus layer from the variable low pH of the lumen to a pH approaching neutrality at the epithelial cell surface. By current hypotheses this gradient is maintained by the combined effects of an unstirred layer, restricted or slowed diffusion of H+ in the mucus, and the epithelial cell secretion of bicarbonate, which is confined to the cell surface by the mucus layer. These mechanisms do not explain how H+ is secreted through mucus in the first place. Using a modified diffusion chamber we have shown that pig gastric mucus facilitates a low-efficiency Na+/H+ exchange--a property that helps to clarify some previously unexplained components of H+ secretion. When a solution containing Na+ was separated by a layer of fresh pig gastric mucus from a solution of similar pH containing a much lower concentration of sodium, the sodium-rich solution was electrically negative relative to the sodium-poor solution and its pH decreased significantly with time. A similar pH gradient developed when the barrier was a synthetic cation-exchange membrane, and one of opposite sign when it was an anion exchanger; no pH gradient developed across neutral barriers. It is suggested that similar electrical coupling of H+ diffusion to active Na+ transport might in vivo ensure that secreted H+ moves into the gastric lumen.

Steady state voltages in the frog lens

Electro-chemical steady state in the lens depends on the transport properties of its various constituent cells. These transport properties, at a minimum, include the active transport of Na/K and the passive leak of Na, K and Cl through membrane channels. The work of Kinsey and Reddy (1), first localized active Na/K transport to the anterior surface cell membranes. In this paper, we estimate that the pump current density is 2 to 4 mu amp/cm2 of surface membrane, by measuring the change in intracellular voltage when the lens is exposed to 100 microM ouabain. Our impedance data suggest the passive leak of K is mostly across the membranes of surface cells, but whether these are anterior or posterior cells is not yet known. Membranes of the fiber cells throughout the volume of the lens appear to have channels that are selective for Na and Cl but few K channels. A simple model of electro-chemical steady state is derived to relate localized transport properties to the resting voltages in the lens. The above described localization of properties predicts radially circulating currents at steady state and spatial gradients in the intracellular and extracellular voltages. These predictions are compared to our measurements of steady state voltages and we find good agreement.

Na+-H+ exchange in gastric glands as measured with a cytoplasmic-trapped, fluorescent pH indicator

We have used the pH-sensitive, fluorescent, cytoplasmic-trapped dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) to identify Na+-H+ exchange in gastric glands isolated from rabbit stomachs by high-pressure perfusion and collagenase digestion. The fluorescence of BCECF-loaded glands was calibrated in terms of cytosolic pH (pHc) by permeabilizing the cell membranes and titrating the extracellular solution to different pH values. In one set of experiments in Cl--free solutions, glands were treated with 0.1 mM ouabain for 45 min to increase cellular cytosolic molar sodium ion concentration [( Na+]c) to high levels. Subsequent suspension of these cells in a Na+-free Ringer's solution (to generate [Na+]c greater than [Na+]o) caused cells to acidify rapidly (t1/2 approximately equal to 60 sec) from pHc approximately equal to 7.15 to pHc approximately equal to 6.55. Subsequent addition of 100 mM Na+ or Li+, but not K+, caused cells rapidly to increase pHc (t1/2 approximately equal to 30 sec) toward the control value. These changes of pHc were blocked when ouabain-treated glands had been preequilibrated for 10 min with 1 mM amiloride, and this block was overcome by adding 10 microM monensin (an ionophore that artificially exchanges Na+ for H+). In another set of experiments in Cl--containing Ringer's solution, glands were acid-loaded by treatment with 30 mM NH4Cl for 4 min, followed by washing the NH4Cl from the solutions. Under these conditions, pHc decreased from 7.02 to approximately equal to 6.5; subsequent alkalinization of cells back to control pHc was stimulated by Na+ (t1/2 approximately equal to 60 sec), but not K+, and was inhibited by 1 mM amiloride. This amiloride block also was overcome by further addition of 10 microM monensin. We conclude that gastric glands contain a Na+-H+ exchanger that appears independent of Cl-, not activated by K+, and blocked by 1 mM amiloride. This exchanger is likely localized to the serosal membrane of gland cells. Na+-H+ exchange may play an important role in regulation of pHc in oxyntic and chief cells exposed to high luminal acidity, where back diffusion of H+ into cells may occur at rapid rates.

Rapid determination of intraepithelial resistance barriers by alternating current spectroscopy. II. Test of model circuits and quantification of results

The impedance of Necturus gallbladder epithelium was measured with transepithelial and intracellular microelectrodes in different transport states. The data are analysed with five electrical equivalent circuits, which differ with respect to the configuration of the paracellular shunt path (lumped vs distributed model of the lateral space), and of the apical cell membrane (non-ideal capacitance or surface amplification by micro-tubular infoldings). Least square fits indicate: 1. that the lumped model cannot represent the epithelium properly, even under control conditions; 2. that the distributed model, which considers the lateral intracellular space separately as a cable-like structure, describes the data well, both under control conditions and during collapse of the lateral spaces; and 3. that the above indicated variations of the apical membrane configuration improve the fits, but have little effect on the magnitude of the calculated circuit parameters. Quantitatively the analysis of 214 measurements on 25 gallbladders under control conditions yields the following results: The resistances of the tight junctions, of the lateral intercellular space, and of the apical and basal cell membrane are Rj = 123, Rlis = 35.5, Ra approximately 3,500, and Rb = 225 (all in omega cm2), and the capacitances of the cell membranes are Ca = 4.95 and Cbl = 26.5 (mu F/cm2). In oxygen deficiency and after cessation of chamber perfusion transport decreased, the spaces collapsed, and Rlis increased to approximately 130 omega cm2. Although the accuracy of the estimated Ra values is still limited, the analysis shows that it is possible to determine Rj and Rlis separately and to follow their changes in response to experimental maneuvers.

Rapid determination of intraepithelial resistance barriers by alternating current spectroscopy. I. Experimental procedures

A method has been developed to determine rapidly and simultaneously the resistance of the tight junctions, the resistance of the lateral intercellular space, and the resistances and capacitances of the apical and basolateral cell membrane in leaky epithelia by alternating current spectroscopy. The present paper describes the experimental procedures. Multi-frequency sine-wave currents are simultaneously applied across the tissue and the transepithelial and intracellular voltage responses are recorded with shielded microelectrodes, digitized, and stored in a computer. Using Fast Fourier Transform techniques the frequency-dependent transepithelial impedance and an apparent basal cell membrane impedance are then calculated from the voltage and current waveform. By fitting appropriate model circuits to the data the above listed individual resistances can be deduced. The model calculations and the fit results are described in the subsequent paper [14]. In the present design the analysis covers the frequency range between 2.5 Hz and 12.5 kHz. The minimal measuring time is in the order of 1-2s.

Redistribution of gastric K+-NPPase in vertebrate oxyntic cells in relation to hydrochloric acid secretion: a cytochemical study

Gastric K+-NPPase represents a partial reaction of the (K+-H+)ATPase system, which is considered to be the proton pump in mammalian parietal cells. In the present paper, K+-NPPase activity was cytochemically studied by the method of Mayahara et al. (1980) in gastric glands of birds, amphibia, and mammals, either in the resting state induced by cimetidine or after stimulation of HCl secretion by histamine. The gastric K+-NPPase cytochemical reaction was localized only in oxyntic cells of the gastric mucosa in the three species tested. The subcellular distribution of the K+-NPPase reaction product drastically changes with the secretory state of HCl. In resting cells, the K+-NPPase staining is associated with the membranes of the endocellular tubular system while in HCl-secreting cells, it is associated with the plasma membrane of the elaborate secretory surface characteristic of this functional state. The above results demonstrate that the same enzymatic activity, which is associated with the gastric proton pump, is present in both membranous systems of the oxyntic cell secretory pole. This fact supports the proposal that the tubular system represents a membrane reserve that inserts the proton pump into the luminal plasma membrane in vertebrate oxyntic cells under the action of HCl secretagogues.

Studies of sodium channels in rabbit urinary bladder by noise analysis

Sodium channels in rabbit urinary bladder were studied by noise analysis. There are two components of short-circuit current (Isc) and correspondingly two components of apical Na+ entry, one amiloride-sensitive (termed IA and the A channel, respectively) and one amiloride-insensitive (IL and the leak pathway, respectively). The leak pathway gives rise to l/f noise, while the A channel in the presence of amiloride gives rise to Lorentzian noise. A two-state model of the A channel accounts well for how the corner frequency and plateau value of Lorentzian noise vary with amiloride concentration. The single-channel current is 0.64 pA, and the conducting channel density is on the order of 40 copies per cell. Triamterene blocks the A channel alone, and increasing external Na+ decreases the number but not the single-channel permeability of the A channel. Hydrostatic pressure pulses ("punching") increase the number of both pathways. Repeated washing of the mucosal surface removes most of the leak pathway without affecting the A channel. Properties of the A channel revealed by noise analysis of various tight epithelia are compared, and the mechanism of l/f noise is discussed. It is suggested that the A channel is synthesized intracellularly, stored in intracellular vesicles, transferred with or from vesicular membrane into apical membrane under the action of microfilaments, and degraded into the leak pathway, which is washed out into urine or destroyed. The A channel starts with PNa/PK approximately 30 and loses selectivity in stages until PNa/PK reaches the free-solution mobility ratio (approximately 0.7) for the leak pathway. This turnover cycle functions as a mechanism of repair and regulation for Na+ channels, analogous to the repair and regulation of most intracellular proteins by turnover. Vesicular delivery of membrane channels may be operating in several other epithelia.

Apical membrane area of rabbit urinary bladder increases by fusion of intracellular vesicles: An electrophysiological study

Mammalian urinary bladder undergoes, in a 24-hour period, a series of slow fillings and rapid emptying. In part the bladder epithelium accommodates volume increase by stretching the cells so as to eliminate microscopic folds. In this paper we present evidence that once the cells have achieved a smooth apical surface, further cell stretching causes an insertion of cytoplasmic vesicles resulting in an even greater apical surface area per cell and an enhanced storage capacity for the bladder. Vesicle insertion was stimulated by application of a hydrostatic pressure gradient which caused the epithelium to bow into the serosal solution. Using capacitance as a direct and nondestructive measure of area we found that stretching caused a 22% increase in area. Removal of the stretch caused area to return to within 8% of control. An alternate method for vesicle insertion was swelling the cells by reducing mucosal and serosal osmolarity. This perturbation resulted in a 74% increase in area over a 70-min period. Returning to control solutions caused area to decrease as a single exponential with an 11-min time constant. A microtubule blocking agent (colchicine) did not inhibit the capacitance increase induced by hypoosmotic solutions, but did cause an increase in capacitance in the absence of a decreased osmolarity. Microfilament disrupting agent (cytochalasin B, C.B.) inhibited any significant change in capacitance after osmotic challenge. Treatment of bladders during swelling with C.B. and subsequent return to control solutions increased the time constant of the recovery to control values (22 min). The Na+-transporting ability of the vesicles was determined and found to be greater than that of the apical membrane. Aldosterone increased the transport ability of the vesicles. We conclude that some constituent of urine causes a loss of apical membrane permeability. Using electrophysiological methods we estimated that the area of cytoplasmic vesicles is some 3.3 times that of the apical membrane area. We discuss these results in a general model for vesicle translocation in mammalian urinary bladder.

Turnover, membrane insertion, and degradation of sodium channels in rabbit urinary bladder

Noise analysis of rabbit bladder revealed two components: Lorentzian noise, arising from interaction of amiloride with the Na+ channel, and flicker noise (l/f, where f is frequency), as in other biological membranes. Hydrostatic pressure, which causes exchange between intracellular vesicular membrane and apical membrane, increases the number but not the single-channel current of the amiloride-sensitive channels. Flicker noise arises from degraded channels that have lost amiloride sensitivity and Na+ to K+ selectivity. The degraded channels were selectively removed by washing the mucosal surface. These results imply channel turnover by intracellular synthesis, transfer from vesicular to apical membrane, degradation, and elimination.