Many nerve and muscle cells possess a Ca2+-activated K+ conductance which gives rise to a long-lasting hyperpolarization following Ca2+ entry during an action potential and which appears different from the classical Hodgkin and Huxley voltage-activated K+ conductance. Using the extracellular patch clamp techique to record the currents from patches of intact and isolated plasma cell membrane from rat myotubes, we have observed single channel currents that would give rise to this Ca2+-activated K+ conductance. The channels are highly selective to K+ and have a conductance near 100 pS at physiological [K+], a value 10-20 times greater than that of the classical voltage-activated K+ channels. The frequency with which these channels open and effective open times are controlled in a reversible manner by the free [Ca2+] on the intracellular side of the membrane; the channel seldom opens when the [Ca2+](i) is <10-8M, but opens with increasing frequency as the [Ca2+](i) exceeds 5x10-7M. With 0.5 μM Ca2+(i), both opening frequency and effective open times depend on membrane potential, increasing as the intracellular side of the membrane is made more positive. At higher levels of Ca2+(i) (50 μM), the channel is almost continuously active, even at hyperpolarized membrane potentials.
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