Single channel recordings of Ca²⁺-activated K⁺ currents in rat muscle cell culture

Abstract

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^1–7. Using the extracellular patch clamp technique^8–12 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 <10⁻⁸ M, but opens with increasing frequency as the [Ca²⁺]_i exceeds 5 × 10⁻⁷ M. 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.