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T Zeuthen

Publications and source records attributed to T Zeuthen.

At least 55 records · Page 3Linked to original sources
PubMed1987

Electrodiffusion of Cl- and K+ in epithelial membranes reconstituted into planar lipid bilayers.

An electrodiffusive permeability for Cl-, its activation by low extracellular Cl--concentrations and the interaction between electrodiffusive fluxes of Cl- and K+ are demonstrated in the ventricular membranes from the epithelium of the bovine choroid plexus. Membranes were fused into artificial lipid bilayers formed at the tip of micropipettes. What is thought to be the cytoplasmic side of the membrane (the trans-side or the inside of pipette) was clamped at negative potentials (0 to -90 mV). Under these conditions the current was discrete, fluctuating less than 2 pA. With Cl- as the only conducting ion on the two sides we observed a small electrodiffusive permeability which was reduced by bumetanide or furosemide by 62%. When the outside solution was rendered Cl--free then the permeability to Cl- increased by a factor of 2-5; this activation was reduced by bumetanide or furosemide by about 80%. We observed an interaction between inwards movements of K+ and outwards movements of Cl- via the activated permeability: The total current was smaller than the sum of the expected inward K+-current and the expected outward activated Cl--current. Bumetanide or furosemide increased the total current; apparently the loss of current carried by Cl- was smaller than the gain in current carried by K+. The presence of K+ on both sides of the membrane was a condition for this interaction.

Animals↗
PubMed1986

Potassium transport of the frog retinal pigment epithelium: autoregulation of potassium activity in the subretinal space.

The K+ transport of the isolated retinal pigment epithelium from the bull-frog was studied using micropuncture with double-barrelled ion-selective micro-electrodes. Transient changes of intracellular values of electrical potential and K+ activity were monitored in response to abrupt changes in the K+ concentration on the retinal side of the tissue. The data were interpreted in terms of a simple three-compartment model of the epithelium in which the retinal (or apical) and choroidal (or basal) membranes separate the cellular compartment from the retinal and choroidal compartments. K+ transport across the retinal membrane was described by an active ouabain-sensitive K+ influx in parallel with a passive electrodiffusive K+ efflux. In steady state under control conditions, the active K+ influx (pump rate) averaged 0.18 X 10(-9) mol cm-2 s-1. The electrodiffusive K+ efflux was described by a K+ permeability, which in steady state under control conditions averaged 1.7 X 10(-5) cm s-1. K+ transport across the choroidal membrane was described as purely electrodiffusive. In steady state under control conditions, the K+ permeability of the choroidal membrane averaged 0.6 X 10(-5) cm s-1. When the K+ concentration on the retinal side of the tissue was increased from its control value, the K+ permeability of the retinal membrane decreased and the K+ permeability of the choroidal membrane increased. This caused the epithelium to attain a new steady state in which the cells transported K+ away from the retinal compartment at a high rate. When the K+ concentration on the retinal side of the tissue was decreased from its control value, the K+ permeability of the retinal membrane increased and the pump rate decreased. This caused the epithelial cells to transport K+ from the cellular compartment into the retinal compartment. In effect, the K+ transport of the retinal pigment epithelium depends on the K+ concentration in the retinal compartment in such a way as to keep variations in this concentration at a minimum.

Animals↗
PubMed1983

Influx mechanisms for Na+ and Cl- across the brush border membrane of leaky epithelia: a model and microelectrode study.

This paper presents a numerical model for the movement of Na+, K+, Cl-, H+ and HCO3- in a leaky epithelium. The model describes the active transport of Na+ and K+ at the serosal membrane and electrodiffusive permeation across the mucosal, serosal and junctional pathways. The model accounts for H+ and HCO3- production in the cell. The influx of Na+ and Cl- is assumed to occur mainly via Na/H and Cl/HCO3 exchange. The behavior of the cell, with this influx mechanism, is compared to a cell with an obligatory neutral coupled influx of Na+ and Cl-. All parameters are obtained from the literature, primarily from studies utilizing the Necturus gallbladder. The analysis shows (i) that it is virtually impossible in steady-state experiments to distinguish between cells with Na/H - HCO3/Cl transport and cells with Na/Cl transport mechanisms. (ii) That nonsteady-state experiments can decide whether Na/H - HCO3/Cl or Na/Cl transport mechanisms mediate the influx of salt. A comparison between studies with ion-selective microelectrodes and the model predictions indicates that the influx of Na+ and Cl- is mediated by Na/H - HCO3/Cl transport when the external solutions contain CO2 and HCO3. (iii) The model also explains the diuretic effects of furosemide and carbonic anhydrase inhibitor, as well as the stimulatory effects on salt transport of elevated levels of HCO3- at a constant pH. (iv) The model fails to explain some experiments performed in HCO3/Co2-free media and some experiments using inhibitors.

Animals↗
PubMed1983

Ion activities in the lateral intercellular spaces of gallbladder epithelium transporting at low external osmolarities.

The ion activities in the lateral spaces of the unilateral preparation of the gallbladder of Rana catesbiana were measured by double-barrelled ion-selective microelectrodes. The bladders were bathed in a saline solution with a low osmolarity (62 mOsm) containing, in mM: 27 Na+, 27 Cl-, 2 K+, 1 Ca++, 4 HCO3-. Working at reduced osmolarities had the advantage of an increased volume transport and of widened intercellular spaces. The reference barrel recorded an electrical potential of +2.7 mV in the spaces; they contained a solution similar to the external solution. The electrodes recorded a Na+ concentration of 27 mM, a K+ concentration of 1.7 mM, a Ca++ concentration of 0.69 mM and a Cl- concentration of 28.5 mM. In the spaces there was a lower resistance between the tip of the electrode and the serosal bath than that recorded with the tip in the lumen, and injection of fluorescent dye (11 A diameter) via the electrodes did not stain the cells. The concentrations in the secretion were similar to those in the spaces. The intracellular compartment had an apparent K+ concentration of 95 mM, and the concentrations of Na+ and Cl- were both about 5 mM. These data indicate that when the gallbladder is bathed with hypotonic solutions and is transporting fluid at approximately three or four times the normal rate, there are no significant osmotic gradients between the lumen and the lateral spaces. It is suggested that transcellular transport of water is implemented by a combination of high osmotic permeabilities across both mucosal and serosal cell membranes and low reflection coefficients (for K+ salts) at the serosal cell membranes.

Animals↗
PubMed1982

Cl- transport by gastric mucosa: cellular Cl- activity and membrane permeability.

The mechanism of Cl- secretion in the isolated, resting (i.e. cimetidine-treated) gastric mucosa of Necturus has been investigated with radioisotopic and electrophysiological techniques. Measurement of transepithelial 36Cl- fluxes (mucosal to serosal (M leads to S), Jms Cl-; S leads to M, Jsm Cl-) during control conditions show that at open circuit, when the transepithelial potential difference psi ms = 20 mV (S ground), Jms Cl- = Jsm Cl-, i.e. Jnet Cl- = 0, but during short-circuit current conditions Jnet Cl- = I sc = 2 mu equiv cm-2 h. Experiments with low [Cl-] solutions indicate that Cl- exchange diffusion does not contribute significantly to either Jms Cl- or Jsm Cl-. Double-barrelled, Cl- -selective microelectrodes showed that in open circuit, the cellular (C) chemical potential for Cl-, psi c Cl- = 31 mV (apparent [Cl-] = 29 mM), the electrical potential across the M membrane, psi m = -34 mV (mucosa ground) while that across the S membrane, psi s = -52 mV (serosa ground). During short-circuit current conditions, psi m = psi s = -49 mV and [Cl-]c = 30 mM. The permeability of the M membrane to Cl- (Pm Cl-) was calculated both from the tracer experiments and the electrode measurements by using the constant-field equation. Short-term (45 s) uptake of 36Cl- at [Cl]m = 96 mM during short circuit conditions gave Pm Cl- = 2.6 x 10(-5) cm s-1. Measurement of [Cl-]c by means of the electrodes when [Cl-]m was changed from 96 to 2 mM or from 2 to 96 mM gave Pm Cl- = 2.9-5.7 x 10(-5) cm s-1. Our results indicate that during open circuit conditions Cl- is accumulated across the S membrane into gastric cells in an energy-requiring step, but since Jnet Cl- = 0, Cl- must leak back into the S solution at a rate equal to the entry rate. When the tissue is short-circuited, Cl- secretion occurs (Jnet Cl- = Isc) owing to the same energy-requiring accumulation of Cl- by the cells and a passive (apparently electrodiffusive) movement across the mucosal membrane.

Animals↗
PubMed1982

Relations between intracellular ion activities and extracellular osmolarity in Necturus gallbladder epithelium.

The interactions between ion and water fluxes have an important bearing on osmoregulation and transepithelial water transport in epithelial cells. Some of these interactions were investigated using ion-selective microelectrodes in the Necturus gallbladder. The intracellular activities of K+ and Cl- in epithelial cells change when the epithelium is adapted to transport in solutions of a low osmolarity. In order to achieve new steady states at low osmolarities, cells lost K+, Cl- and some unidentified anions. Surprisingly, the apparent K+ concentration remained high: at an external osmolarity of 64 mOsm the intracellular K+ concentration averaged 95 mM. This imbalance was sensitive to anoxia and ouabain. The effects of abrupt changes in the external osmolarities on the intracellular activities of Na+, K+ and Cl- were also investigated. The gradients were effectuated by mannitol. The initial relative rates of change of the intracellular activities of Na+ and Cl- were equal. The data were consistent with Na+ and Cl- ions initially remaining inside the cell and a cell membrane Lp of 10(-3) cm sec-1 osm-1, which is close to the values determined by Spring and co-workers (K.R. Spring, A. Hope & B.E. Persson, 1981. In: Water Transport Across Epithelia. Alfred Benzon Symposium 15. pp. 190-200. Munskgaard, Copenhagen). The initial rate of change of the intracellular activity of K+ was only 0.1-0.2 times the change observed in Na+ and Cl- activities, and suggests that K+ ions leave the cell during the osmotically induced H2O efflux and enter with an induced H2O influx. The coupling is between 98 and 102 mmoles liter-1. Various explanations for the anomalous behavior of intracellular K+ ions are considered. A discussion of the apparent coupling between K+ and H2O, observed in nonsteady states, and its effects on the distribution of K+ and H2O across the cell membrane in the steady states, is presented.

Animals↗
PubMed1982

Structure of Necturus gallbladder epithelium during transport at low external osmolarities.

Gallbladders transport isotonically over a wide range of osmolarities. This ability has been assumed to depend on the geometry of the lateral intercellular spaces. We report that this geometry in the Necturus gallbladder varies extensively with the external osmolarity and depends in vitro on the integrity of the subepithelial tissues. The structure of the living epithelium was studied by Nomarski light microscopy while ultrastructural effects were revealed by electron microscopy. The short-term effects (less than 60 min) of low external osmolarities were: 1) the cells became bell-shaped with an increased cell height measured centrally, 2) lateral intercellular spaces lost their convoluted character; and 3) numerous membrane-bound cavities appeared in the cells. Furthermore, long-term exposure to the low external osmolarities caused an uneven density of epithelial cells. With subepithelial tissues intact, blistering of the epithelium cell layer was evident. Qualitative electron-microscopic data indicate that the membrane of the cavities was recruited from the basolateral cell membrane. This agrees well with light-microscopic observation that the cavities were initiated as invaginations of this cell membrane.

Animals↗
PubMed1982

Steady states and the effects of ouabain in the Necturus gallbladder epithelium: a model analysis.

A simple numerical model for the Necturus gallbladder epithelium is presented. K+, Na+ and Cl- cross the mucosal and serosal membranes as well as the junctions by means of electrodiffusion; furthermore the mucosal membrane contains a neutral entry mechanism for NaCl and the serosal membrane contains an active pump for K+ and Na+. The values which have been used for the model are taken from the literature. The model can only attain steady states if the resistance of the serosal membrane is lower than 1000 omega cm2. Values reported in the literature for the resistance of this membrane vary from about 3000 to about 100 omega cm2. We shall argue, however, that the higher estimates are in error because they are derived from a model of the tissue in which each membrane and the junction are modeled by a resistor; this procedure is invalid because the resistance of the lateral intercellular space relative to the resistance of the tight junctions is neglected and consequently the resistance of the serosal membrane is overestimated by a factor of about four. Apart from predicting a realistic steady state at normal external concentrations the model can predict quantitatively several experimental results obtained from the living epithelium. We have focused on the experiments which test the permeabilities of the serosal membrane and the properties of the pump: i) Replacement of serosal Cl- by an impermeant ion. ii) Replacement of serosal K+ by Na+. iii) Inhibiting the (Na+, K+)-pump. The best correspondence between model and experiments is obtained when the pump is assumed to be electrogenic (or rheogenic) with a ratio of coupling between Na+ and K+ of 3:2. In this case both model and direct experiments (also presented in this paper) show an initial abrupt depolarization of 6 to 7 mV. The model also shows that it cannot be concluded from i and ii that the Cl- permeability of the serosal membrane is low. The model explains, even with high passive Cl- permeabilities, why the intracellular Cl- concentration is relatively unaffected by paracellular currents, a fact which in other epithelia has been taken as an implication of a low Cl- permeability of the serosal membranes.

Animals↗
PubMed1981

Extracellular ion concentrations during spreading depression and ischemia in the rat brain cortex.

We compared interstitial ion concentrations in rat brain cortex during two conditions where pronounced changes are observed: spreading depression and ischemia. Initially, during the two phenomena, an increase of [K+]e from 3 to approximately 10 mM were observed, but only small changes of the other ion concentrations. Hereafter, [K+]e exhibited a rapid increase (2-3 s) to 55 mM, whereas [Na+]e rapidly decreased to 60 mM, [Cl-]e to 75 mM, and [Ca++]e to 0.08 mM. The changes were accompanied by a rapid negative shift in the local electrical potential. However, there were differences in the ionic events during the two phenomena. In spreading depression, the initial [K+]e increase took place in 5-10 s, but in ischemia it lasted 1-2 min. The ionic perturbations were spontaneously reverted in SD, but in ischemia they proceeded further and reached after 5 min (mM): [K+]e 75, [Na+]e 50, [Cl-]e 72, and [Ca++]e 0.06. The similar chain of ionic events during spreading depression and ischemia suggests a common mechanism for the ionic changes, probably involving changes of ionic permeability of brain cells.

Animals↗
PubMed1980

Carbon dioxide, membrane potential and intracellular potassium activity in frog skeletal muscle.

1. The membrane potential of isolated frog muscle fibres has been measured in absence and in presence of CO2, at constant external pH. 2. At a normal external Cl concentration, CO2 (PCO2 = 97 mmHg; pH = 7.0) applied for 10 min caused a highly variable depolarization, the average potential change being 8 mV after 5 min. The effect was reversible 3. In Cl-free solutions, CO2 (PCO2 = 97 mmHg; pH = 7.0) caused a biphasic depolarization of 20 mV after 5 min. The effect was fully reversible on CO2 removal. 4. The same effect appeared at a lower partial pressure (PCO2 = 38 mmHg; pH 7.3) in the presence of tetrodotoxin (10(-7) M). 5. In order to investigate the cause of the CO2-induced depolarization, membrane potential and intracellular K activity (ai/K,)( of surface muscle fibres were measured with voltage and ion-sensitive micro-electrodes. 6. At a normal external Cl concentration, CO2(PCO2 approximately equal to 97 mmHg; pH = 7.0) decreased ai/K by 5 mM after 5 min. 7. The same effect appeared at low external Cl concentration (11 mM). 8. At high partial pressure (PCO2 approximately equal to 588 mmHg; pH = 6.8), CO2 reduced ai/K by 19 mM in 10 min. 9. In long-term experiments performed over 4 h with a normal external Cl concentration, CO2 (PCO2 approximately equal to 97 mmHg; pH 5.8 or 7) changed practically neither membrane potential, nor ai/K. 10. It is concluded that increasing the PCO2 when keeping the external pH constant causes an early depolarization of muscle. This effect is particularly marked in the absence of chloride. It can be explained partly, in surface muscle fibres, by a decrease of the intracellular K activity.

Animals↗
PubMed1980

Ba2+ inhibition of electrogenic Cl- secretion in vitro frog and piglet gastric mucosa.

Gastric mucosae from frogs and newborn pigs were used for in vitro investigation of the effects of Ba2+ (10 microM to 7 mM) on transepithelial potential difference (PD), resistance and conductance (G), short-circuit current (Isc), H+ secretion, and transepithelial fluxes of 36Cl-. Ba2+ in the serosal, but not the mucosal, solution of both preparations caused PD, G, Isc, and Cl- secretion (JnetCl, Isc conditions) to decrease, while H+ secretion remained constant. Because the oxyntic cells were most likely the site of action for Ba2+, these cells must have the capacity to secrete Cl- in excess of H+ ions. The inhibitory effect of Ba2+ was not due to competition in the serosal membrane by Ba2+ for surface charges, Ca2+ sites, Na+ sites, or Cl- sites. When [K+] in both the mucosal and serosal solutions or in just the serosal solution ([K+]s) alone was increased to 10 mM, the inhibitory effects of low [Ba2+] were reduced; however, at higher [Ba2+], Isc was stimulated. At least part of the Ba2+ effect seems to be due to blockage of K+ channels in the serosal membrane of oxyntic cells. High [K+]s also caused decreased PD and Isc (but increased G) with no change in H+ secretion. It is proposed that during Isc conditions, JnetCl involves a neutral Na+-dependent accumulation of Cl- within oxyntic cells and a passive, conductive efflux fromthe cells into the mucosal solution. Ba2+ and high [K+] may alter this transport by depolarizing and, under certain conditions, hyperpolarizing intracellular voltage.

Animals↗
PubMed1979

Mechanism of the cerebrocortical vasodilatation during anoxia.

The possible role of cerebrocortical ion homeostasis, NAD/NADH redox state and of cortical oxygen tension was investigated in the initiation of hypoxic cortical vasodilatation. In addition, changes in cerebrocortical extracellular concentrations of Na+, K+, and Cl- during anoxia were studied. The results were as follows. a) The cerebrocortical reflectance decrease, e.g. cerebral vasodilatation, lagged behind the cortical pO2 decrease by 1-2 sec, but preceded the decrease of arterial blood pressure and ECoG as well as the extracellular Na+, K+, Cl- increases by 20-30 sec. Since the cortical pO2 decreased first and the ion changes lagged behind the onset of vasodilatation by 20-30 sec, it is suggested that the CBF increase in hypoxia is mediated via the cortical pO2 decrease. b) A significant NAD reduction was already present after 20 sec. of nitrogen breathing. Since the ECoG and MABP decreased, and K+ activity increased much later than this, it is presumed that the NAD reduction during the first 30-40 sec of anoxia indicates an increased rate of glycolysis, but not mitochondrial hypoxia. c) In the predepolarization phase a 17% K+, 4% Na+, 5% Cl- increase is probably the result of a reduction of the extracellular spaces caused by water movement and by the migration of Na+ and Cl- from the extracellular to the intracellular space. The large K+, Na+, Cl- changes during terminal depolarization can be interpreted as a result of the failure of the membrane bound Na+ -K+ pump and of the altered ion permeability of the cell membranes.

Action Potentials↗
PubMed1979

Effect of arterial hypoxia on the cerebrocortical redox state, vascular volume, oxygen tension, electrical activity and potassium ion concentration.

The effect of different degrees of arterial hypoxia on cerebrocortical NAD/NADH redox state, reflectance, oxygen tension, extracellular potassium ion concentration, ECoG and arterial blood pressure was investigated in rats. The results may be summarized as follows. a) The decrease of cortical pO2 preceded the dilatation of cortical vessels by 15-20 sec but the changes in cortical extracellular potassium ion concentration, ECoG and arterial blood pressure started later than the vasodilatation. These results give further support to the regulatory role of cortical pO2 decrease in the initiation of cerebrocortical vasodilatation during arterial hypoxia. b) Since the K+ concentration of the brain cortex and the ECoG did not change in mild arterial hypoxia, the significant NAD reduction obtained in this experimental group is likely to be of cytoplasmic origin. The same conclusion applies to the initial periods of severe arterial hypoxia. On the basis of the extent of NAD reduction during various degrees of arterial hypoxia it is concluded that about 30% of the NAD reduction occurring in anoxia is of cytoplasmic origin. c) When the animals were ventilated with a gas mixture containing 4-7% oxygen, the brain cortex became nearly anoxic, partly because of the gradual decrease of arterial blood pressure. Finally, the mechanism of potassium leakage is identical under prolonged severe arterial hypoxaemia and on anoxic terminal depolarization.

Action Potentials↗