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The ion selectivity of a membrane conductance inactivated by extracellular calcium in Xenopus oocytes
Author(s) -
Zhang Yong,
McBride Don W.,
Hamill Owen P.
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.763bp.x
Subject(s) - chemistry , reversal potential , xenopus , niflumic acid , voltage clamp , membrane potential , conductance , biophysics , amiloride , extracellular , calcium , toad , selectivity , analytical chemistry (journal) , ion , patch clamp , sodium , chromatography , biochemistry , endocrinology , organic chemistry , biology , catalysis , gene , medicine , receptor , mathematics , combinatorics
1 The ion selectivity of a membrane ion conductance that is inactivated by extracellular calcium (Ca 2+ o ) in Xenopus oocytes has been studied using the voltage‐clamp technique. 2 The reversal potential of the Ca 2+ o ‐sensitive current (I c ) was measured using voltage ramps (‐80 to +40 mV) as a function of the external concentration (12‐240 mM) of NaCl or KCl. The direction and amplitude of the shifts in reversal potentials are consistent with permeability ratios of 1:0.99:0.24 for K + :Na + :Cl − . 3 Current‐voltage ( I‐V ) relations of I c , determined during either voltage ramps of 0.5 s duration or at steady state, displayed pronounced rectification at both hyperpolarized and depolarized potentials. However, instantaneous I‐V relations showed less rectification and could be fitted by the constant field equation assuming the above K + :Na + :Cl − permeability ratios. 4 Ion substitution experiments indicated that relatively large organic monovalent cations and anions are permeant through I c channels with the permeability ratios K + :NMDG + :TEA + :TPA + :TBA + :Gluc − = 1:0.45:0.35:0.2:0.2:0.2. 5 External amiloride (200 μM), gentamicin (220 μM), flufenamic acid (40 μM), niflumic acid (100 μM), Gd 3+ (0.3 μM) or Ca 2+ (200 μM) caused reversible block of I c without changing its reversal potential. 6 Preinjection of oocytes with antisense oligonucleotide against connexin 38, the Xenopus hemi‐gap‐junctional protein, inhibited I c by 80 % without affecting its ion selectivity, thus confirming and extending the recent suggestion of Ebihara that I c represents current carried through hemi‐gap‐junctional channels. 7 In vitro and in vivo maturation of oocytes resulted in a significant decrease in I c conductance to 7 % and 2 % of control values, respectively. This developmental downregulation of I c minimizes any toxic effect I c activation would have when the mature egg is released into Ca 2+ o ‐free pond water. 8 The results of this study are discussed in relation to other Ca 2+ o ‐inactivated conductances seen in a wide variety of cell types and which have previously been interpreted as arising either from Ca 2+ o ‐masked channels or from changes in the ion selectivity of voltage‐gated Ca 2+ or K + channels.