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Query: UNIPROT:P50583 (asymmetrical)
 12,197 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. Mechanisms of ion transport across the choroidal epithelium were investigated using an in vitro preparation of the frog choroid plexus.2. Sodium was actively transported across the plexus from the vascular to the ventricular surface by an ouabain sensitive electrically silent pump. As in other epithelial membranes the rate of sodium transport was stimulated by the presence of bicarbonate ions in the Ringer solutions. Chloride and bicarbonate ions accompany the net flux of sodium across this tissue.3. Some experiments suggest that potassium is actively transported from the ventricular to the serosal surface, and that the rate of transport is a function of the extracellular potassium concentration.4. No evidence was obtained to suggest that calcium is actively transported across this tissue in either direction.5. Diamox, ethoxyzolamide, pitocin, pitressin, hydrocortisone, amiloride, spironolactone and anoxia all failed to influence sodium transport.6. The sequence of passive ion permeation across the plexus was P(Rb) approximately P(K) > P(Cs) approximately P(Na) approximately P(Cl) approximately P(HCO3) > P(Li) as deduced from diffusion potential measurements. At least for Na, K and Cl there was a good correlation between the permeability coefficients derived from unidirectional flux measurements and from electrical parameters. This indicates that exchange diffusion is unimportant as a mechanism for passive ion transport.7. The instantaneous current-voltage curves were linear in both symmetrical and asymmetrical salt solutions and the choroid plexus conductance was found to be directly proportional to the external salt concentration. These and other lines of evidence suggest that the major route of passive ion permeation across this epithelium is via the tight junction route and not through the cell interior.8. These results are discussed in relation to the in vivo studies of c.s.f. secretion and the mechanisms of active and passive ion transport across other epithelial membranes such as the gall-bladder, intestine and renal proximal tubule.
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PMID:Mechanisms of ion transport across the choroid plexus. 453 45

The permeability of frog skin under the influence of urea hyperosmolarity has been studied. Flux ratio asymmetry has been demonstrated again for tracer mannitol. The inhibitors DNP, CN(-), and ouabain have been used to eliminate active sodium transport and it was found that urea hyperosmolarity produces asymmetrical mannitol fluxes on frog skins having no short-circuit current. These findings suggest that flux ratio asymmetry is due to solute interaction and is unrelated to sodium transport. Studies with a synthetic membrane show clearly that bulk flow of fluid can produce a "solvent drag" effect and change flux ratios. When bulk flow is blocked and solute gradients allowed their full expression, then solute interaction "solute drag" is easily demonstrable in a synthetic system.
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PMID:Further observations on asymmetrical solute movement across membranes. 564 71

The internal pH (pHi) of chick muscle cells is determined by the transmembrane Na+ gradient. Li+, but not K+, Rb+ or Cs+, can substitute for Na+ for regulating the internal pH of chick muscle cells. Pharmacological evidence using amiloride and amiloride analogs has shown that the Na+/H+ exchange system is the membrane mechanism that couples the pHi to the transmembrane Na+ gradient. The pHi dependence of the amiloride-sensitive Na+/H+ exchange mechanism was defined. Internal H+ interacts cooperatively with the Na+/H+ exchange system, in contrast with external H+, thus indicating an asymmetrical behaviour of this exchanger. The half-maximum effect for the activation by the internal H+ of the Na+ transporting activity of the amiloride-sensitive Na+/H+ exchange was observed at pH 7.4. The Hill coefficient of the H+ concentration dependence is higher than 3. Insulin was shown to have no effect on the pHi of chick muscle cells.
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PMID:The Na+-dependent regulation of the internal pH in chick skeletal muscle cells. The role of the Na+/H+ exchange system and its dependence on internal pH. 609 Jan 29

Single-channel current recordings were carried out on excised inside-out patches of baso-lateral plasma membrane from exocrine acinar cells. The mouse pancreas and submandibular gland as well as the pig pancreas were investigated. In the mouse pancreas the voltage-insensitive Ca2+-activated cation channel was studied. Single-channel current-voltage (i/v) relationships were studied in symmetrical Rb+-rich solutions and in asymmetrical Rb+/Na+ and Na+/Rb+ solutions. In all cases the i/v relations were linear and had the same slope representing a single-channel conductance of about 33 pS which is identical to that previously obtained with symmetrical Na+ solutions or asymmetrical Na+/K+ solutions. In the mouse submandibular gland and the pig pancreas the voltage and Ca2+-activated K+ channel was studied. The outward currents observed after depolarization in the presence of quasi-physiological Na+/K+ gradients were immediately abolished when all the K+ in the bath fluid was replaced by Rb+ (bath fluid in contact with inside of plasma membrane). This effect was immediately and fully reversible upon return to the high K+ solution. The voltage and Ca2+-activated K+ channel was also studied in asymmetrical K+/Rb+ and Rb+/K+ solutions. In the first case inward (K+) currents could be observed but not outward (Rb+) currents, while in the other case inward (Rb+) currents could not be seen whereas outward (K+) currents were measured. The current-voltage relationships were approximately linear and the null potential was close to 0 mV in both situations.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Patch-clamp study of rubidium and potassium conductances in single cation channels from mammalian exocrine acini. 609 Oct 24

The operation of the voltage-sensitive sodium gating system in the nerve membrane involves conformational changes that are accompanied by small asymmetrical displacement currents. The asymmetry current may be divided into a component that is inactivated by positive voltage-clamp pulses, and recovers from inactivation with exactly the same time course as the sodium conductance, and one that is not inactivated. A method is described for recording the two components separately with the aid of an inactivating prepulse. They appear to have a marked difference in their rising phases, that of the non-inactivating component being just about as fast as the imposed step in membrane potential, while the inactivating component requires some tens of microseconds to reach its peak.
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PMID:Fractionation of the asymmetry current in the squid giant axon into inactivating and non-inactivating components. 612 12

A quantitative comparison between the voltage dependence of the inactivating component of the asymmetrical charge transfer in the squid giant axon and that of the sodium conductance indicates that activation of the sodium system involves either three subunits operating in parallel or a three-step series mechanism. This is confirmed by an examination of the relative timing of the flow of asymmetry and ionic currents during the opening and closing of the sodium channels. In agreement with previous suggestions, inactivation is coupled sequentially to activation. The evidence appears to argue against a trimeric system with three wholly independent subunits and favours a monomeric system that undergoes a complex sequence of conformational changes.
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PMID:The relationship between the inactivating fraction of the asymmetry current and gating of the sodium channel in the squid giant axon. 612 13

Using 3H-ouabain autoradiography, Na+-K+-ATPase has been localized on the basolateral membranes of ciliated and nonciliated cells in the oviduct (pars recta, p. convoluta I, II, III) of the European fire salamander, Salamandra salamandra. The mucous and seromucous gland cells of the p. convoluta I, II, III, however, do not show any significant labelling. An asymmetrical distribution of ouabain binding sites is a main feature of transporting epithelia.
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PMID:Na-pump sites in the oviduct of Salamandra salamandra (L.) (Amphibia, Urodela). 626 38

Voltage clamp experiments were made on ezymically isolated and internally perfused rat cardiac cells. The effect of a diethylamine analog of ethmozine (DAAE) on sodium current (INa) was tested when the drug was applied inside or outside the cell. It was found that the effect of DAAE (8 X 10(-6) g/ml) on INa was asymmetrical: after DAAE addition outside the cell, the amplitude of INa was effectively suppressed. Thus, 5 minutes after DAAE action the maximal value of INa in a voltage-current relationship was 20% of the control value without significant changes in the kinetics of INa. When the DAAE was added inside the cell preferentially, the inactivation time constant was increased without significant changes in the amplitude of the maximal INa. The same results were obtained with pronase (1 mg/ml) added inside the cell. It was supposed that as compared to ethmozine, the DAAE possesses a supplementary binding site on the cardiac cell membrane possibly linked to the structures responsible for inactivation processes.
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PMID:[Effect of diethylamine analog of ethmozine on parameters of sodium current in isolated rat cardiomyocytes]. 629 14

Diadenosine tetraphosphatase, an enzyme splitting diadenosine tetraphosphate to AMP and ATP, has been purified to apparent homogeneity from a permanent cell line derived from a leukemic child. The purification procedure consisted of fractionation by ammonium sulfate precipitation, followed by Sephacryl 200 and DEAE-cellulose chromatography, and finally a differential membrane filtration. The enzyme is a single polypeptide chain of Mr = 17,500 as determined by gel electrophoresis in the presence of sodium dodecyl sulfate. The apparent molecular weight of the native enzyme was calculated as 20,000 from gel filtration data. The apparent Km for Ap4A was 0.5 microM as determined by two independent kinetic assays. None of the following compounds were substrates of the enzyme: diadenosine triphosphate, NAD, nucleoside 5'-phosphates (AMP, ATP, GDP, GTP, and UTP). The enzyme had optimal activity in the presence of 1 mM Mg2+, showing no activity in the presence of EDTA.
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PMID:Diadenosine tetraphosphatase from human leukemia cells. Purification to homogeneity and partial characterization. 630 76

The electrophysiological properties of the dorsal and ventral canine lingual epithelium are studied in vitro. The dorsal epithelium contains a special ion transport system activated by mucosal solutions hyperosmotic in NaCl or LiCl. Hyperosmotic KCl is significantly less effective as an activator of this system. The lingual frenulum does not contain the transport system. In the dorsal surface it is characterized by a rapid increase in inward current and can be quantitated as a second component in the time course of either the open-circuit potential or short-circuit current when the mucosal solution is hyperosmotic in NaCl or LiCl. The increased inward current (hyperosmotic response) can be eliminated by amiloride (10(-4) M). The specific location of this transport system in the dorsal surface and the fact that it operates over the concentration range characteristic of mammalian salt taste suggests a possible link to gustatory transduction. This possibility is tested by recording neural responses in the rat to NaCl and KCl over a concentration range including the hyperosmotic. We demonstrate that amiloride specifically blocks the response to NaCl over the hyperosmotic range while affecting the KCl response significantly less. The results suggest that gustatory transduction for NaCl is mediated by Na entry into the taste cells via the same amiloride-sensitive pathway responsible for the hyperosmotic response in vitro. Further studies of the in vitro system give evidence for paracellular as well as transcellular current paths. The transmural current-voltage relations are linear under both symmetrical and asymmetrical conditions. After ouabain treatment under symmetrical conditions, the short-circuit current decays to zero. The increase in resistance, though significant, is small, which suggests a sizeable shunt pathway for current. Flux measurements show that sodium is absorbed under symmetrical conditions. Mucosal solutions hyperosmotic in various sugars also induce an amiloride-sensitive inward current. In summary, this work provides evidence that the sodium taste receptor is most probably a sodium transport system, specifically adapted to the dorsal surface of the tongue. The transport paradigm of gustation also suggests a simple model for electric taste and possible mechanisms for sweet taste.
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PMID:The active ion transport properties of canine lingual epithelia in vitro. Implications for gustatory transduction. 633 Feb 75


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