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

1. The ion selectivity of a membrane ion conductance that is inactivated by extracellular calcium (Ca(o)²⁺) in Xenopus oocytes has been studied using the voltage-clamp technique. 2. The reversal potential of the Ca(o)²⁺-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), Gd³⁺ (0.3 μM) or Ca²⁺ (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(o)²⁺-free pond water. 8. The results of this study are discussed in relation to other Ca(o)²⁺-inactivated conductances seen in a wide variety of cell types and which have previously been interpreted as arising either from Ca(o)²⁺-masked channels or from changes in the ion selectivity of voltage-gated Ca²⁺ or K⁺ channels.