The ion selectivity of a membrane conductance inactivated by extracellular calcium in Xenopus oocytes

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Abstract
The ion selectivity of a membrane ion conductance that is inactivated by extracellular calcium (Ca2+o) in Xenopus oocytes has been studied using the voltage‐clamp technique. The reversal potential of the Ca2+o‐sensitive current (Ic) 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−. Current‐voltage (I‐V) relations of Ic, 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. Ion substitution experiments indicated that relatively large organic monovalent cations and anions are permeant through Ic channels with the permeability ratios K+:NMDG+:TEA+:TPA+:TBA+:Gluc−= 1:0.45:0.35:0.2:0.2:0.2. External amiloride (200 μM), gentamicin (220 μM), flufenamic acid (40 μM), niflumic acid (100 μM), Gd3+ (0.3 μM) or Ca2+ (200 μM) caused reversible block of Ic without changing its reversal potential. Preinjection of oocytes with antisense oligonucleotide against connexin 38, the Xenopus hemi‐gap‐junctional protein, inhibited Ic by 80 % without affecting its ion selectivity, thus confirming and extending the recent suggestion of Ebihara that Ic represents current carried through hemi‐gap‐junctional channels. In vitro and in vivo maturation of oocytes resulted in a significant decrease in Ic conductance to 7 % and 2 % of control values, respectively. This developmental downregulation of Ic minimizes any toxic effect Ic activation would have when the mature egg is released into Ca2+o‐free pond water. The results of this study are discussed in relation to other Ca2+o‐inactivated conductances seen in a wide variety of cell types and which have previously been interpreted as arising either from Ca2+o‐masked channels or from changes in the ion selectivity of voltage‐gated Ca2+ or K+ channels.