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Sodium conductance and the activation potential in Xenopus laevis eggs.
Peres A, Mancinelli E.
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Experiments have been performed to identify the membrane permeability changes causing activation potential in Xenopus eggs. The eggs were artificially activated either by pricking or by addition of the Ca2+ ionophore A23187 to the bath. Two different ionic currents appear to control the activation potential: (i) a chloride current which develops after a delay of 30 s to 5 min from the activating stimulus and which, in low external chloride, produces a depolarization and, (ii) a voltage-dependent outward current which begins to flow when the membrane potential is more positive than about +20 mV and tends to hyperpolarize the membrane. The chloride current lasts about 3-4 min; the voltage-dependent outward current is present before activation and disappears more slowly than the Cl- current. Changes in external sodium concentration affect the reversal potential of the outward current before and after the development of the inward Cl- current. We suggest that the chloride current has the role of producing a rapid depolarization necessary to block polyspermy, while the voltage-dependent sodium outward current might prevent the depolarization from reaching excessively high values and help the repolarization phase.
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