Single apamin-blocked Ca-activated K⁺ channels of small conductance in cultured rat skeletal muscle

Abstract

Action potentials in many excitable cells are followed by a prolonged afterhyperpolarization^1–3 that modulates repetitive firing^2,4,5. Although it is established that the afterhyperpolarization is produced by Ca-activated K⁺ currents^1,2,4, the basis of these currents is not known. The large conductance (250 pS) Ca-activated K⁺ channel (BK channel^6,7) is not a major contributor to the afterhyperpolarization in non-innervated skeletal muscle and some nerve cells, because apamin, a neurotoxic component of bee venom, abolishes the afterhyperpolarization^8–11 but does not block BK channels⁹, and 5 mM extracellular tetraethylammoniumion (TEA) blocks BK channels^9,12 but does not reduce the afterhyperpolarization^2,4,8–11. We now report single-channel currents from small conductance (10–14 pS) Ca-activated K⁺ channels (SK channels) with the necessary properties to account for the afterhyperpolarization. SK channels are blocked by apamin but not by 5 mM external TEA (TEA_o). They are also highly Ca-sensitive at the negative membrane potentials associated with the afterhyperpolarization.