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1. The action potential and the membrane current of the mouse oocyte were analysed by current-clamp and voltage-clamp techniques and they were compared with those of other animal oocytes. 2. The matured and unfertilized oocyte of the mouse in standard medium with 6 mM-K showed the resting potential of -23-1+/-2-9 mV. The resting potential was relatively large in the medium with 20 mM-Ca or 10 mM-Mn, being -35-7+/-2-6 mV and further increased to -46-9+/-4-8 mV with replacement of Na in the medium by choline. 3. At the cessation of large hyperpolarization below -90 mV in standard medium, a regenerative potential was often elicited in the form of an off-response. The off-response depended upon the external concentration of Ca. In 20 mM-Ca medium it was constantly observed with hyperpolarization below -60 mV. Its critical level was -40 mV and its overshoot was +15 mV. 4. The time and potential-dependent inward current was observed both in standard and 20 mM-Ca media under voltage-clamp condition. In 20 mM-Ca medium the inward current was observed by depolarization beyond -40 mV and showed its maximum at -15 mV. It was greatly reduced by replacing the external Ca with Mn but retained by substituting Sr or Ba for Ca. The selectivity ratios among these alkali earth cations were Ca:Sr:Ba=1-0:1-4:0-7. 5. The current-voltage relation in Ca and Na-deficient and 10 mM-Mn medium was linear from -200 to +25 mV. The hyperpolarization below -200 mV revealed an inward-going rectification. The depolarization above +50 mV under voltage-clamp condition induced the outward surge current with activation and inactivation processes. 6. In contrast to the mouse oocyte, the matured and unfertilized oocyte of the sea urchin showed a large resting potential of -70 mV in 30 Ca ASW and the depolarization beyond -40 mV elicited an action potential with an overshoot of 20 mV. The action potential showed a notch in the rising phase and lasted about 1 to 2 sec. 7. Under the voltage-clamp condition both Ca inward current and the outward surge current were observed in the sea urchin oocyte membrane just as in the mouse oocyte membrane. 8. The selectivity ratios among alkali earth cations, Ca:Sr:Ba, for 'Ca channels' of the oocyte membranes were 1-0:1-4:0-7 in the mouse, 1-0:1-7:1-1 in the tunicate and 1-0:0-7:0-5 in the sea urchin. When the current density through Ca channels are revised in terms of the respective critical levels for Ca channels, the revised selectivity sequences become Ca greater than Sr greater than Ba, being common to all three species.
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