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1. 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. 2. 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-. 3. 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. 4. 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. 5. External amiloride (200 microM), gentamicin (220 microM), flufenamic acid (40 microM), niflumic acid (100 microM), Gd3+ (0.3 microM) or Ca2+ (200 microM) caused reversible block of Ic without changing its reversal potential. 6. 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. 7. 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. 8. 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.
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