Electrogenic Active Transport Research Articles

Relationships between body mass index and short‐circuit current in human duodenal and colonic mucosal biopsies

Retrospectively, to investigate the relationship between body mass index (BMI) and basal electrogenic transport as measured by short-circuit current (SCC) in human duodenal and colonic mucosal biopsies. The study included biopsies from mucosa of normal appearance in the sigmoid colon and/or distal duodenum. Patients were referred for routine endoscopy (predominantly for monosymptomatic abdominal pain) and had normal endoscopic findings. Biopsies were mounted in miniaturized Ussing chambers and basal SCC was recorded. Patients were divided into two groups on the basis of BMI (≤25 and >25 kg m⁻²). Statistical significance was assessed by the unpaired t-test or Wilcoxon rank-sum test. Correlation coefficients were calculated by Pearson product moment correlation. In colonic biopsies, basal SCC (mean ± standard deviation) was significantly higher in 59 biopsies from 30 patients with low BMI than in 32 biopsies from 23 patients with high BMI (45 ± 29 μA cm⁻² vs. 27 ± 21 μA cm⁻², P = 0.016). In duodenal biopsies, mean basal SCC was numerically lower in 38 biopsies from 15 patients with low BMI than in 46 biopsies from 19 patients with high BMI (54 ± 26 μA cm⁻² vs. 74 ± 39 μA cm⁻², P = 0.069). The correlation coefficient between BMI and SCC was -0.26 (P = 0.06) in colonic biopsies and +0.44 (P = 0.001) in duodenal biopsies. Basal intestinal active electrogenic transport is related to BMI and this relationship may differ in different segments of the intestinal tract.

Cysteine-scanning mutagenesis study of the sixth transmembrane segment of the Na,K-ATPase α subunit

The accessibility of the residues of the sixth transmembrane segment (TM) of the Bufo marinus Na,K-ATPase α subunit was explored by cysteine scanning mutagenesis. Methanethiosulfonate reagents reached only the two most extracellular positions (T803, D804) in the native conformation of the Na,K-pump. Palytoxin induced a conductance in all mutants, including D811C, T814C and D815C which showed no active electrogenic transport. After palytoxin treatment, four additional positions (V805, L808, D811 and M816) became accessible to the sulfhydryl reagent. We conclude that one side of the sixth TM helix forms a wall of the palytoxin-induced channel pore and, probably, of the cation pathway from the extracellular side to one of their binding sites.

Recombinant Human Interleukin-11 Modulates Ion Transport and Mucosal Inflammation in the Small Intestine and Colon

Human recombinant interleukin 11 (rhIL-11) is a cytokine that suppresses the clinical signs of colitis in animal models of inflammatory bowel disease (IBD) and may be an effective therapeutic agent in the treatment of IBD. The objective of the current study was to investigate whether rhIL-11 was capable of reversing abnormalities in secretomotor function associated with gut inflammation. We investigated the effects of rhIL-11 on epithelial electrogenic ion transport in the jejunum and colon. Application of rhIL-11 (10 to 10,000 ng/ml) at either the luminal or serosal side of mucosal sheets isolated from control rats induced a concentration-dependent reduction of transmural potential difference (PD) in the jejunum and decreased the short-circuit current (Isc), representative of active electrogenic transport, in the colon. To investigate the effect of rhIL-11 on an inflamed gut, we isolated jejunal and colonic tissue from HLA-B27 transgenic rats with active inflammation of the bowel that represents an animal model of IBD. In jejunum and colon isolated from HLA-B27 transgenic rats, basal electrogenic ion transport was significantly attenuated and, under these conditions, rhIL-11 caused no changes in either transmural PD or Isc. However, in HLA-B27 rats, pretreatment with subcutaneous doses of rhIL-11 suppressed the symptoms of diarrhea, normalized myeloperoxidase activity in the jejunum and colon and healed mucosal injury. In the jejunum from HLA-B27 rats, healing of the intestinal inflammatory response enhanced basal transmural PD and the rhIL-11–induced changes in mucosal ion transport resembled those seen in uninflamed controls. Conversely, in the colon, healing of the mucosa did not normalize basal active ion transport nor did it reverse the inhibition of rhIL-11–induced changes in colonic Isc. Our results suggest that endogenous IL-11 may act as a modulator of epithelial transport under physiologic conditions and may act as a potent anti-inflammatory cytokine during active intestinal inflammation.

Electrogenic property of Na +, K+ -ATPase through computer simulation

A computer simulation of the electrogenic nature of the membrane-bound Na+, K(+)-ATPase is presented. The model involves coupling two simulation systems for passive and active transports, using a minimum of empirical parameters, and studies the contribution of the pump to the membrane potential. The simulation results indicate that electrogenic active transport accelerates the restoration of the resting electrochemical gradients and contributes approximately 0.44-1.1 mV to the resting potential of the membrane, depending on the Na:K coupling ratio. The effect of membrane potential and the physical positioning of the enzyme from the passive transporting channel on the enzyme function is also presented. The validity of the model is checked by comparing our results with reported literature values.

Baroreflex control of jejunal blood flow and fluid transport in cats; effects of yohimbine, an alpha 2-adrenergic antagonist.

The aim of the study was to test the hypothesis that baroreceptor unloading increases jejunal fluid absorption rate via an alpha 2-adrenergic effect on electrogenic active transport. In 13 chloralose-anaesthetized cats, the carotid sinus baroreceptors were isolated and perfused with arterial blood, and we studied the effects of a graded decrease in carotid sinus pressure on intestinal vascular resistance, net fluid absorption rate and the potential difference between the intestinal lumen and the peritoneal cavity (PD). Experiments were performed in seven control animals and in six animals pretreated with yohimbine, an alpha 2-adrenergic antagonist, at a dose of 0.1 mg kg-1 i.v. Yohimbine per se had no significant effects on systemic arterial pressure, intestinal vascular resistance, net fluid absorption rate or PD. In the control animals, baroreceptor unloading induced an increase in systemic arterial pressure, intestinal vascular resistance and net fluid absorption rate, and a decrease in the PD. Yohimbine pretreatment did not significantly affect the systemic blood pressure response to baroreceptor unloading, but abolished the effect on intestinal vascular resistance and PD. After yohimbine treatment, decreases in carotid sinus pressure still enhanced net fluid absorption rate, but this response was observed in a higher range of carotid sinus pressures than in control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The dependence of pump-induced hyperpolarization on the tonicity of the surrounding medium and intracellular sodium concentration

1. 1. The dependence of pump-induced hyperpolarization of the membrane on the quantity of sodium ions injected into the cell and the tonicity of the surrounding medium was studied. 2. 2. The quantity of injected sodium ions necessary for saturation of the mechanism of active electrogenic transport varied from cell to cell ranging from 0.10 to 0.25 and from 0.18 to 0.37 nM in hypertonic and hypotonic solutions, respectively. 3. 3. At a low dose of sodium injection, much lower than the saturation region, the pump-induced hyperpolarization of the membrane was smaller in hypotonic solutions in comparison with that in hypertonic solutions. 4. 4. However, at a high dose of injected sodium ions when the mechanism of active electrogenic transport was saturated, the pump-induced hyperpolarization became larger in hypotonic solutions compared to that in hypertonic solutions.

血管条細胞内電位に関する実験的研究 か牛中央階外側壁構造内における直流電位分布

It is important to learn the generating mechanism of the EP, and the distribution of DC potentials in the stria vascularis of the guinea pig cochlea was investigated by inserting a recording microelectrode into the scara media through the stria vascularis.The microelectrode was driven at 3 microns per second by a special hydraulic autodrive micromanipulator and the position of the electrode tip was identified by the electrophoretic dye-injection (marking method).Several positive DC steps were always recorded from various parts of the stria vascularis, and the experimental results obtained by marking methods suggested that the source of these positive DC steps was the intracellular potential of the strial cell. The data substantiated that the intracellular potentials of the strial cells are positive, whereas it is common knowledge that the intracellular potential should be negative. In most cases, 3 steps of the positive DC potential could be recorded corresponding to the layers of the strial cells. It must be presumed that the positive DC steps were recorded at the moment of pushing through the strial cell membrane by a recording microelectrode, and the largest 3rd positive step arose from the marginal cell membrane facing the intermediate cell. In the marginal cells, the positivity of the intracellular DC potential reached as high as +80mV, which is still a few millivolts less than the positivity of the EP.It has been well known that the negative potential is generally recorded from inside the cell, as the membrane is much more permeable to K-ions than to Na-ions are extruded from inside the cell by active sodium ion transport. Suppose that the positive component of the EP is the positive secretion potential generated by the electrogenic potassium ion pump located in the strial cell membrane as having been postulated by Kuijpers, it could be assumed that the strial cell membrane is selectively permeable to Na-ions and not to K-ions. On this assumption, it might be expected that the positive intracellular DC potentials in the strial cells are composed of both the sodium ion concentration gradient between the interior and the exterior of the strial cell and the electrogenic active transport of K-ions in the strial cell membrane.

Bicarbonate assimilation by fresh-water charophytes and higher plants: I. Membrane transport of bicarbonate ions is not proven

Although it is generally believed thatChara and some fresh-water angiosperms transport bicarbonate ions inwards across their plasma membranes, there has been no direct demonstration of such transport in these plants. The (indirect) arguments for their transporting HCO 3 − are arguments against the inward diffusion of CO2 at the observed rates. They rest on calculations of the equilibrium concentration of CO2 or of the maximum rate at which CO2 might be produced from HCO 3 − at the pH of the medium outside the cells. SinceChara acidifies the medium over about half the cell surface during C assimilation, these calculations have been based on questionable premises. We propose a model forChara in which the acidification is attributed to active efflux of H+, and we calculate that both the equilibrium concentration of CO2 and its rate of production outside the cell can be high enough to support the observed rates of C assimilation, without postulating transport of the species HCO 3 − or H2CO3. Calculations are presented also for alternative models in which there is membrane transport of HCO 3 − . The first includes symport of H+ with HCO 3 − , again dependent on active H+ efflux. In the second, there is active electrogenic transport of HCO 3 − . In this case the low pH in the medium outside the cell is caused by the dissociation of H2CO3 produced by hydration of CO2 which leaks from the cell cytoplasm. All three models are consistent with the observations to date, but the first is more economical of postulates. It can also explain the apparent transport of HCO 3 − by fresh-water angiosperms such asEgeria.

A kinetic model for determining the consequences of electrogenic active transport in cardiac muscle

When active transport is electrogenic in a tissue that is continuously active, such as cardiac muscle, the active transport current is as important in the generation of the action potential as are the passive currents. A thermodynamically constrained kinetic model of electrogenic active transport of sodium and potassium ions has been developed in which the influences of voltage and chemical composition are explicitly defined. This model is coupled to a system of passive permeabilities, of the minimum degree of complexity, to simulate the integrated activity of active and passive ion transport in the generation of the cardiac action potential. Results of preliminary simulations indicate that electrogenic active transport provides a mechanism for slowly changing currents both within the time scale of an action potential as well as of many action potentials. The presence of active transport also complicates the interpretation of isotopic flux measurements and the separation of currents.

Basolateral membrane potential of a tight epithelium: ionic diffusion and electrogenic pumps.

The contribution of specific ions to the conductance and potential of the basolateral membrane of the rabbit urinary bladder has been studied with both conventional and ion-specific microelectrode techniques. In addition, the possibility of an electrogenic active transport process located at the basolateral membrane was studied using the polyene antibiotic nystatin. The effect of ion-specific microelectrode impalement damage on intracellular ion activities was examined and a criterion set for acceptance or rejection of intracellular activity measurements. Using this criterion, we found (K+) = 72 mM and (Cl-) = 15.8 mM. Cl- but not K+ was in electrochemical equilibrium across the basolateral membrane. The selective permeability of the basolateral membrane was measured using microelectrodes, and the data analyzed using the Goldman, Hodgkin-Katz equation. The sodium to potassium permeability ratio (PNa/PK) was 0.044, and the chloride to potassium permeability ratio (PCl/PK) was 1.17. Since K+ was not in electrochemical equilibrium, intracellular (K+) is maintained by active metabolic processes, and the basolateral membrane potential is a diffusion potential with K+and C1- the most permeable ions. After depolarizing the basolateral membrane with high serosal potassium bathing solutions and eliminating the apical membrane as a rate limiting step for ion movement using the polyene antibiotic nystatin, we found that the addition of equal aliquots of NaCl to both solutions caused the basolateral membrane potential to hyperpolarize by up to 20mV (cell interior negative). This potential was reduced by 80% within 3 min of the addition of ouabain to the serosal solution. This hyperpolarization most probably represents a ouabain sensitive active transport process sensitive to intracellular Na+. An equivalent electrical circuit for Na+ transport across rabbit urinary bladder is derived, tested, and compared to previous results. This circuit is also used to predict the effects that microelectrode impalement damage will have on individual membrane potentials as well as time-dependent phenomena; e.g., effect of amiloride on apical and basolateral membrane potentials.

Ionic basis of electrical activity in cardiac tissues

Cardiac electrical events are described in terms of membrane physiology. The concept that cardiac membranes possess specific ionic channels controlled by gates bearing electrical charges is discussed. When open, these channels permit ions to cross the membrane, giving rise to passive inward (depolarizing) and outward (repolarizing) currents. Two different inward and four or five different outward currents appear to be responsible for the development of cardiac electrical activity; both inward currents appear to be controlled by activation and inactivation variables, whereas outward currents are essentially controlled by activation variables and/or inward-going rectifiers. The potential range in which the different currents activate and inactivate (or are limited by inward-going rectification), and the kinetics of activation and inactivation processes explain the development of electrical activity in normal cardiac tissues and in partially depolarized fibers. In addition to passive ionic currents, electrogenic active transport participates in the development of electrical phenomena. The conductance of the membrane for potassium ions and the electrical coupling between cardiac cells depend on the intracellular concentration of calcium ions.

Mechanism of the Effect of Acetate on Frog Ventricular Muscle

Substitution of acetate for external Cl produced a large persistent increase in the resting membrane potential (R.M.P.) of frog ventricle and a somewhat steeper relation between membrane potential (M.P.) and [K]o (external K concentration). An increased K conductance or reduced permeability to other ions could account for most of these results, but not for hyperpolarizations as great as −110 mV. Potentials of this size suggested a contribution from an active electrogenic transport system, but they were unaffected by several treatments including exposure to ouabain (10−7 M − 5 × 10−6 M), dinitrophenol (10−6 M, 10−5 M) or 30 mM tetraethylammonium.Acetate caused a prolongation of the action potential (A.P.) and a change in its configuration. Acetate also enhanced twitch tension and increased the rate of tension development. Similar changes are produced by removal of [K]o. The effects of both acetate and K removal on A.P. configuration were prevented by a reduced rate of stimulation.When acetate-induced hyperpolarization was reversed by raising [K]o to 10–15 mM, the configuration of the A.P. resembled that of controls and twitch tension did not increase. Thus, acetate-induced changes in the shape of the A.P. and in twitch tension appeared to be secondary to the increase in R.M.P. However, the relationship does not seem to be direct because these changes were temporary, whereas hyperpolarization was persistent.The character of the acetate-induced changes in A.P. configuration, and the dependence on stimulation rate and [Ca]o (external Ca concentration), suggested a raised [Ca]i (internal Ca concentration) and a possible increase in Ca influx. However, addition of Mn to the acetate solution did not prevent initial acetate-induced changes in the shape of the A.P. plateau and in twitch tension. Also in the absence of [Ca]o, disappearance of twitch tension was slowed by acetate. But acetate decreased the contracture tension produced in response to either increased [K]o or Na removal. Acetate may cause a redistribution of Ca within the cell.

Effects of noradrenaline on potassium reflux, membrane potential and electrolyte levels in tissue slices prepared from guinea-pig liver.

1. Some effects of noradrenaline on potassium efflux, electrolyte levels, membrane potential and current distribution in guinea-pig liver slices have been examined.2. The slices (thickness ca. 300 mum) were prepared from the median lobe of the liver and incubated at 38 degrees C in a mammalian Ringer fluid containing 2 mM pyruvate. After an initial recovery period, the ionic composition of the tissue remained stable for several hours.3. The steady-state contents of sodium, potassium and chloride were 296, 266 and 272 m-equiv/kg dry tissue respectively. The inulin space was 29 ml./100 g wet tissue.4. Most if not all of the tissue potassium was exchangeable. The rate constant for (42)K efflux was 0.019 min(-1).5. Noradrenaline (1 muM) markedly increased the efflux of (42)K and within 2 min caused tissue potassium to fall by 8%. At the same time the sodium content rose.6. Traverses of the slices with micro-electrodes showed many negative-going deflexions of 30-40 mV in amplitude. The evidence suggests that these correspond to the membrane potentials of the parenchymal cells.7. Noradrenaline (1 muM) caused a reversible hyperpolarization of about 10 mV. The response became larger on replacing external chloride by isethionate or methylsulphate, but was little affected by a reduction in external potassium.8. After slices had been bathed in potassium and chloride-free solutions for several min, restoration of external potassium caused the membrane potential to increase by up to 10 mV. This hyperpolarization, but not that caused by noradrenaline, was abolished by ouabain.9. Noradrenaline reduced the amplitude and quickened the time course of electrotonic potentials set up by current pulses from another microelectrode, suggesting that the membrane conductance had risen.10. Although certain mechanisms based on electrogenic active transport processes with unusual properties have not been excluded, the present findings are more simply explained by supposing that noradrenaline increases the potassium permeability of the parenchymal cell membrane.

The resting membrane potential of the somatic muscle cells of Ascaris lumbricoides.

1. The resting membrane potential of Ascaris muscle fibres, which is normally relatively insensitive to ion changes in the medium, has been measured under a wide variety of conditions.2. The results have been interpreted in terms of a form of the constant field equation containing additional terms for the contribution of ions and charged groups other than potassium, sodium and chloride.3. Normally the contribution of the additional terms is large and tends to outweigh the contributions of potassium, sodium and chloride.4. The contribution of the additional terms is considerably reduced in the absence of sodium and in the presence of gamma-amino butyric acid and acetylcholine.5. It is suggested that the additional terms may represent the contribution of an electrogenic active transport mechanism to the factors determining the membrane potential.

Some properties of unresponsive cells in the cerebral cortex.

Unresponsive cells in the pericruciate cortex of cats under pentobarbitone were distinguished from neurones by total absence of spontaneous or evoked spikes and synaptic responses. They had much greater membrane potentials (mean −62 mV) and resistances (mean 48 MΩ) than neurones (means −36 mV, 21 MΩ), and the potentials were unusually stable. They could be depolarized by external applications of K+, but were insensitive to internal injections of K+ or Cl−. Unlike neurones, they were not hyperpolarized by GABA or depolarized by L-glutamate, but large doses of GABA had a depolarizing effect, without reducing the membrane resistance. ACh also depolarized some neurones and unresponsive cells without a fall in membrane resistance; but nor-adrenaline and 5HT were ineffective on the few unresponsive cells tested. It is suggested that unresponsive cells are neuroglia; that the depolarizing actions of GABA and ACh indicate a change in electrogenic active transport, and that glia may remove transmitters from the extracellular space.