Reconstitution in planar lipid bilayers of a Ca2+-dependent K+ channel from transverse tubule membranes isolated from rabbit skeletal muscle.
Addition of membrane vesicles prepared from transverse tubule (T-tubule) membranes of rabbit skeletal muscle to the aqueous phase of a planar lipid bilayer induces a stepwise increase in conductance. This conductance is both voltage and Ca2+ dependent. At 1 mM Ca2+, the steady-state conductance is maximal at voltages higher than +20 mV and decreases for more negative voltages. (Voltages refer to the side to which the vesicles are added, cis) Decreasing the Ca2+ concentration reversibly shifts the conductance-voltage curve toward the right along the voltage axis. Furthermore, Ca2+ can activate the conductance only if added to the cis compartment. Neither Mg2+, Ba2+, nor Cd2+ can activate the conductance induced by T-tubule vesicles. Addition of 5 mM tetraethylammonium ion to the trans, but not the cis, side abolishes the T-tubule-induced conductance. The Ca2+-dependent conductance appears as a consequence of ionic channel formation. Single-channel activity appears in bursts followed by periods of time in which the channel remains "silent". The conductance of the open channel averages 226 pS in 0.1 M KC1 and is voltage and Ca2+ independent. However, the fraction of time that the channel remains in the open state is voltage and Ca2+ dependent in a manner that parallels the voltage and Ca2+ dependence of the multichannel membrane. The channel is 6.6 times more permeable to K+ than to Na+ and is impermeable to C1-.
D000818:Animals; D002118:Calcium / Q000502:physiology; D002413:Cations, Divalent; D002462:Cell Membrane / Q000502:physiology; D002474:Cell-Free System; D004553:Electric Conductivity; D007473:Ion Channels / Q000502:physiology*; D008051:Lipid Bilayers; D008564:Membrane Potentials; D011188:Potassium / Q000502:physiology*; D011817:Rabbits; D012508:Sarcolemma / Q000502:physiology* / Q000648:ultrastructure
D002413:Cations, Divalent; D007473:Ion Channels; D008051:Lipid Bilayers; D011188:Potassium; D002118:Calcium