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  • 1
    Electronic Resource
    Electronic Resource
    Expression of amiloride-sensitive Na^+ channels of hen lower intestine in Xenopus oocytes: Electrophysiological studies on the dependence of varying NaCl intake
    Amsterdam : Elsevier
    Biochimica et Biophysica Acta (BBA)/Biomembranes 1111 (1992), S. 159-164 
    Details
    ISSN: 0005-2736
    Keywords: (Colon) ; (Coprodeum) ; (Hen intestine) ; Amiloride ; Current-voltage relationship ; Expression ; Oocyte ; Sodium ion channel
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Medicine , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0005-2736(92)90306-7
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  • 2
    Electronic Resource
    Electronic Resource
    Comparison of a Na^+/d-glucose contransporter from rat intestine expressed in oocytes of Xenopus laevis with the endogenous cotransporter
    Amsterdam : Elsevier
    Biochimica et Biophysica Acta (BBA)/Biomembranes 1063 (1991), S. 73-80 
    Details
    ISSN: 0005-2736
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Medicine , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0005-2736(91)90355-C
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  • 3
    Electronic Resource
    Electronic Resource
    The Ca2+-inactivated Cl− Channel at Work: Selectivity, Blocker Kinetics and Transport Visualization (1997)
    Springer
    The journal of membrane biology 155 (1997), S. 95 -104 
    Details
    ISSN: 1432-1424
    Keywords: Key words:Xenopus oocytes — Cl− channel — Calcium — Selectivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Removal of extracellular divalent cations activated a Cl− channel in the plasma membrane of Xenopus laevis oocytes. This so-called Ca2+-inactivated Cl− channel (CaIC) was present in every oocyte and was investigated using two-electrode whole-cell voltage clamp and single-channel patch-clamp techniques. Beside other Cl− channel inhibitors, anthracene-9-carboxylic acid (9-AC) and 3′azido-3′deoxythymidine (AZT), a nucleoside analogue commonly used as an antiviral drug, blocked at least partly the CalC-mediated currents. Using the Cl−-sensitive dye 6-methoxy-N-(sulfopropyl)quinolinium (SPQ) we could visualize the transport of Cl− from the oocyte cytoplasm to the surrounding medium after activation of the CaIC by Ca2+ removal. In the absence of external Cl− and Ca2+, the emission intensity of SPQ declined continuously, indicating a quenching of fluorescence by the efflux of Cl− in the millimolar range. In the presence of external Ca2+, no emission changes could be observed during the same time period. Chelating external Ca2+ in absence of Cl− immediately activated Ca2+-inactivated Cl− channels leading to subsequent emission decrease of SPQ. Investigations on the selectivity of the CaIC revealed only poor discrimination between different anions. With single-channel measurements, we found an anion selectivity sequence I− 〉 Br− 〉 Cl−≫ gluconate as it is also typical for maxi Cl− channels. Contrary to the majority of all other transport systems of the Xenopus oocyte, which show reduced activity due to membrane depolarization or endocytotic removal of the transport protein from the plasma membrane during oocyte maturation, the CaIC remained active in maturated oocytes. Single-channel measurements on maturated oocytes, also known as eggs, showed the presence of Ca2+-inactivated Cl− channels. However, this egg CaIC revealed an altered sensitivity to external Ca2+ concentrations. All these data confirm and extend our previous observations on the CaIC and give clear evidence that this channel is peculiar among all Cl− channels described up to now.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002329900161
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  • 4
    Electronic Resource
    Electronic Resource
    Further Characteristics of the Ca2+-inactivated Cl− Channel in Xenopus laevis Oocytes (1999)
    Springer
    The journal of membrane biology 172 (1999), S. 169-179 
    Details
    ISSN: 1432-1424
    Keywords: Key words: Ca2+-inactivated Cl− channel — Inhibition profile — Capacitance measurements
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. Removal of extracellular Ca2+ activates ion channels in the plasma membrane of defolliculated oocytes of the South Africa clawed toad Xenopus laevis. At present, there is controversy about the nature of the Ca2+-inactivated ion channels. Recently, we identified one of these channels as a Ca2+-inactivated Cl− channel (CaIC) using single channel analysis. In this work we confirm and extend previous observations on the CaIC by presenting a decisive extension of the regulation and inhibition profile. CaIC current is reversibly blocked by the divalent and trivalent cations Zn2+ (half-maximal blocker concentration, K1/2= 8 μm), Cu2+ (K1/2= 120 μm) and Gd3+ (K1/2= 20 μm). Furthermore, CaIC is inhibited by the specific Cl− channel blocker NPPB (K1/2≈ 3 μm). Interestingly, CaIC-mediated currents are further sensitive to the cation channel inhibitor amiloride (500 μm) but insensitive to its high affinity analogue benzamil (100 μm). An investigation of the pH-dependence of the CaIC revealed a reduction of currents in the acidic range. Using simultaneous measurements of membrane current (I m ), conductance (G m ) and capacitance (C m ) we demonstrate that Ca2+ removal leads to instant activation of CaIC already present in the plasma membrane. Since C m remains constant upon Ca2+ depletion while I m and G m increase drastically, no exocytotic transport of CaIC from intracellular pools and functional insertion into the plasma membrane is involved in the large CaIC currents. A detailed overview of applicable blockers is given. These blockers are useful when oocytes are utilized as an expression system for foreign proteins whose investigations require Ca2+-free solutions and disturbances by CaIC currents are unwanted. We further compare and discuss our results with data of Ca2+-inactivated cation channels reported by other groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002329900594
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  • 5
    Electronic Resource
    Electronic Resource
    Endogenous Ion Channels in Oocytes of Xenopus laevis: Recent Developments (1999)
    Springer
    The journal of membrane biology 170 (1999), S. 1-12 
    Details
    ISSN: 1432-1424
    Keywords: Key words:Xenopus laevis oocyte — Endogenous ion channels — Ion transport — Membrane transport
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002329900532
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  • 6
    Electronic Resource
    Electronic Resource
    Endogenousd-glucose transport in oocytes ofXenopus laevis (1989)
    Springer
    The journal of membrane biology 111 (1989), S. 93-102 
    Details
    ISSN: 1432-1424
    Keywords: Xenopus oocyte ; glucose ; cotransport ; flux ; voltage clamp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Endogenous glucose uptake by the oocytes ofXenopus laevis consists of two distinct components: one that is independent of extracellular Na+, and the other one that represents Na+-glucose cotransport. The latter shows similar characteristics as 2 Na+-1 glucose cotransport of epithelial cells: The similarities include the dependencies on external concentrations of Na+, glucose, and phlorizin, and on pH. As in epithelial cells, the glucose uptake in oocytes can also be stimulated by lanthanides. Both the electrogenic cotransport and the inhibition by phlorizin are voltage-dependent; the data are compatible with the assumption that the membrane potential acts as a driving force for the reaction cycle of the transport process. In particular, hyperpolarization seems to stimulat transport by recruitment of substrate binding sites to the outer membrane surface. The results described pertain to oocytes arrested in the prophase of the first meiotic division; maturation of the oocytes leads to a downregulation of both the Na+-independent and the Na+-dependent transport systems. The effect on the Na+-dependent cotransport is the consequence of a change of driving force due to membrane depolarization associated with the maturation process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869212
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  • 7
    Electronic Resource
    Electronic Resource
    The Ca2+-induced leak current in Xenopus oocytes is indeed mediated through a Cl− channel (1995)
    Springer
    The journal of membrane biology 148 (1995), S. 263-275 
    Details
    ISSN: 1432-1424
    Keywords: Xenopus oocytes ; Cl− channel ; Divalent cations ; Leak current
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Defolliculated oocytes of Xenopus laevis responded to removal of external divalent cations with large depolarizations and, when voltage clamped, with huge currents. Single channel analysis revealed a Cl− channel with a slope conductance of about 90 pS at positive membrane potentials with at least four substates. Single channel amplitudes and mean channel currents had a reversal potential of approximately −15 mV as predicted by the Nernst equation for a channel perfectly selective for Cl−. Readdition of Ca2+ immediately inactivated the channel and restored the former membrane potential or clamp current. The inward currents were mediated by a Ca2+ inactivated Cl− channel (CaIC). The inhibitory potency of Ca2+ was a function of the external Ca2+ concentration with a half maximal blocker concentration of about 20 μm. These channels were inhibited by the Cl− channel blockers flufenamic acid, niflumic acid and diphenylamine-2-carboxylate (DPC). In contrast, 4,4′-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonicacid (SITS), another Cl− channel blocker, led to activation of this Cl− channel. Like other Cl− channels, the CaIC was activated by cytosolic cAMP. Extracellular ATP inhibited the channel while ADP was without any effect. Injection of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activating phorbol ester, stimulated the Cl− current. Cytochalasin D, an actin filament disrupting compound, reversibly decreased the clamp current demonstrating an influence of the cytoskeleton. The results indicate that removal of divalent cations activates Cl− channels in Xenopus oocytes which share several features with Cl− channels of the CLC family. The former so-called leak current of oocytes under divalent cation-free conditions is nothing else than an activation of Cl− channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00235044
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  • 8
    Electronic Resource
    Electronic Resource
    Transport of Charged Dipeptides by the Intestinal H+/Peptide Symporter PepT1 Expressed in Xenopus laevis Oocytes (1997)
    Springer
    The journal of membrane biology 155 (1997), S. 247 -256 
    Details
    ISSN: 1432-1424
    Keywords: Key words: Intestinal peptide transporter — Expression — Substrate specificity — Two-electrode voltage-clamp technique
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. The cloned intestinal peptide transporter is capable of electrogenic H+-coupled cotransport of neutral di- and tripeptides and selected peptide mimetics. Since the mechanism by which PepT1 transports substrates that carry a net negative or positive charge at neutral pH is poorly understood, we determined in Xenopus oocytes expressing PepT1 the characteristics of transport of differently charged glycylpeptides. Transport function of PepT1 was assessed by flux studies employing a radiolabeled dipeptide and by the two-electrode voltage-clamp-technique. Our studies show, that the transporter is capable of translocating all substrates by an electrogenic process that follows Michaelis Menten kinetics. Whereas the apparent K0.5 value of a zwitterionic substrate is only moderately affected by alterations in pH or membrane potential, K0.5 values of charged substrates are strongly dependent on both, pH and membrane potential. Whereas the affinity of the anionic dipeptide increased dramatically by lowering the pH, a cationic substrate shows only a weak affinity for PepT1 at all pH values (5.5–8.0). The driving force for uptake is provided mainly by the inside negative transmembrane electrical potential. In addition, affinity for proton interaction with PepT1 was found to depend on membrane potential and proton binding subsequently affects the substrate affinity. Furthermore, our studies suggest, that uptake of the zwitterionic form of a charged substrate contributes to overall transport and that consequently the stoichiometry of the flux-coupling ratios for peptide: H+/H3O+ cotransport may vary depending on pH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002329900177
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  • 9
    Electronic Resource
    Electronic Resource
    Ion transport across leech integument (1993)
    Springer
    Journal of comparative physiology 163 (1993), S. 153-159 
    Details
    ISSN: 1432-136X
    Keywords: Na+ channels ; Amiloride ; Benzamil ; Noise analysis ; Leech, Hirudo medicinalis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The dorsal skin of the leech Hirudo medicinalis was used for electrophysiological measurements performed in Ussing chambers. The leech skin is a tight epithelium (transepithelial resistance = 10.5±0.5 kΩ· cm-2) with an initial short-circuit current of 29.0±2.9 μA·cm-2. Removal of Na+ from the apical bath medium reduced short-circuit current about 55%. Ouabain (50μmol·l-1) added to the basolateral solution, depressed the short-circuit current completely. The Na+ current saturated at a concentration of 90 mmol Na+·l-1 in the apical solution (K M=11.2±1.8 mmol·l-1). Amiloride (100 μmol·l-1) on the apical side inhibited ca. 40% of the Na+ current and indicated the presence of Na+ channels. The dependence of Na+ current on the amiloride concentration followed Michaclis-Menten kinetics (K i=2.9±0.4 μmol·l-1). The amiloride analogue benzamil had a higher affinity to the Na+ channel (K i=0.7±0.2 μmol·l-1). Thus, Na+ channels in leech integument are less sensitive to amiloride than channels known from vertebrate epithelia. With 20 mmol Na+·l-1 in the mucosal solution the tissue showed an optimum amiloride-inhibitable current, and the amiloride-sensitive current under this condition was 86.8±2.3% of total short-circuit current. Higher Na+ concentrations lead to a decrease in amiloride-blockade short-circuit current. Sitmulation of the tissue with cyclic adenosine monophosphate (100 μmol·l-1) and isobutylmethylxanthine (1 mmol·l-1) nearly doubled short-circuit current and increased amiloride-sensitive Na+ currents by 50%. By current fluctuation analysis we estimated single Na+ channel current (2.7±0.9 pA) and Na+ channel density (3.6±0.6 channels·μm-2) under control conditions. After cyclic adenosine monophosphate stimulation Na+ channel density increased to 5.4±1.1 channels·μm-2, whereas single Na+ channel current showed no significant change (1.9±0.2 pA). These data present a detailed investigation of an invertebrate epithelial Na+ channel, and show the similarities and differences to vertebrate Na+ channels. Whereas the channel properties are different from the classical vertebrate Na+ channel, the regulation by cyclic adenosine monophosphate seems similar. Stimulation of Na+ uptake by cyclic adenosine monophosphate is mediated by an increasing number of Na+ channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00263601
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  • 10
    Electronic Resource
    Electronic Resource
    Electrogenic cation transport across leech caecal epithelium (1996)
    Springer
    Journal of comparative physiology 166 (1996), S. 435-442 
    Details
    ISSN: 1432-136X
    Keywords: non-selective cation channel ; leech caecum ; leaky epithelium ; leech,Hirudo medicinalis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract Electrogenic cation transport across the caecal epithelium of the leechHirudo medicinalis was investigated using modified Ussing chambers. Transepithelial resistance (R T ) and potential difference (V T ) were 61.0±3.5 Ω·cm2 and-1.1±0.2 mV (n=149), respectively, indicating that leech caecal epithelium is a “leaky” epithelium. Under control conditions short circuit current (I SC ) and transepithelial Na+ transport rate (I Na ) averaged at 22.1±1.5 μA·cm-2 and 49.7±2.6 μA·cm-2, respectively. Mucosal application of amiloride (100 μmol·l-1) or benzamil (50 μmol·l-1) influenced neitherI SC norI Na . The transport system in the apical membrane showed no pronounced cation selectively and a linear dependence on mucosal Na+ concentration. Removal of mucosal Ca2+ increasedI SC by about 50% due to an increase of transepithelial Na+ transport. Trivalent cations (La3+ and Tb3+, 1 mmol·l-1 both) added to the mucosal Ringer solution reducedI Na by more than 40%. Serosal ouabain (1 mmol·l-1) almost halvedI SC andI Na while 0.1% (=54 mmol·l-1) DNP decreasedI Na to 11.8±5.1% of initial values. Serosal addition of cAMP increased bothI SC andI Na whereas the neurotransmitters FMRFamide, acetylcholine, GABA,l-dopa, serotonin and dopamine failed to show any effects; octopamine, glycine andl-glutamate reducedI Na markedly. On the basis of these results we conclude that in leech caecal epithelium apical uptake of monovalent cations is mediated by non-selective cation conductances which are sensitive to extracellular Ca2+ but insensitive to amiloride. Basolaterally Na+ is extruded via ouabain-sensitive and-insensitive ATPases. cAMP activates Na+ transport across leech caecal epithelium, although the physiological stimulus for CAMP-production remains unknown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02337888
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