Stationary Outward and Transient Ca<SUP>2&#8290;</SUP>-Dependent Currents in Hamster Oocytes

Stationary Outward and Transient Ca<SUP>2&#8290;</SUP>-Dependent Currents in Hamster Oocytes

<P> The outward currents elicited in hamster eggs by depolarizing pulses were studied. The currents appeared to comprise at least two components, a transient outward component (<I>I</I><SUB>to</SUB>) and a steady-state outward component (<I>I</I><SUB>∞</SUB>). <I>I</I><SUB>to </SUB>was transiently followed by the cessation of inward Ca<SUP>2&#8290;</SUP> current (<I>I</I><SUB>Ca</SUB>), and its current-voltage (I-V) relation was a mirror image of that of <I>I</I><SUB>Ca</SUB>. Either blockade of <I>I</I><SUB>Ca</SUB> by Co<SUP>2&#8290;</SUP> or replacement of Ca<SUP>2&#8290;</SUP> with Sr<SUP>2&#8290;</SUP> abolished <I>I</I><SUB>to </SUB>without change in <I>I</I><SUB>∞</SUB>. Intracellular EGTA (10 mM) inhibited <I>I</I><SUB>to</SUB> but not <I>I</I><SUB>∞</SUB>. suggesting strongly that generation of <I>I</I><SUB>to</SUB> requires intracellular Ca<SUP>2&#8290;</SUP>. Apamin (1 nM) abolished selectively <I>I</I><SUB>to, </SUB>indicating<SUB> </SUB>that <I>I</I><SUB>to </SUB>is Ca<SUP>2&#8290;</SUP>-dependent K<SUP>&#8290;</SUP> current. On the other hand, <I>I</I><SUB>∞</SUB> was Ca<SUP>2&#8290;</SUP>- independent. Both <I>I</I><SUB>to</SUB> and <I>I</I><SUB>∞ </SUB>were completely inhibited by internal Cs<SUP>&#8290;</SUP> and external TEA. The estimated reversal potential of <I>I</I><SUB>to </SUB>was close to the theoretical <I>E</I><SUB>K</SUB>. Taken together, both outward currents were carried by K<SUP>&#8290;</SUP> channels. From these results, <I>I</I><SUB>to</SUB> is likely to be a current responsible for the hyperpolarizing responses seen in hamster eggs at fertilization.