Single Channel Measurements of the Calcium Release Channel from Skeletal Muscle Sarcoplasmic Reticulum Activation by Ca " and ATP and Modulation by Mg "

A high-conductance (100 pS in 53 mM trans Ca Ca channel was incorporated from heavy-density skeletal muscle sarcoplasmic reticulum (SR) fractions into planar lipid bilayers of the Mueller-Rudin type . cis Ca21 in the range of 2-950,uM increased open probability (P.) in single channel records without affecting open event lifetimes . Millimolar ATP was found to be as good as or better than Ca in activation ; however, both Ca2' and ATP were required to fully activate the channel, i .e ., to bring Po = l . Exponential fits to open and closed single channel lifetimes suggested that the channel may exist in many distinct states . Two open and two closed states were identified when the channel was activated by either Ca or ATP alone or by Ca plus nucleotide . Mgt was found to permeate the SR Ca channel in a trans-to-cis direction such that img2+/iCa2+ = 0.40 . cis Mgt+ was inhibitory and in single channel recordings produced an unresolvable flickering of Caand nucleotide-activated channels . At nanomolar cis Cat+ , 4 uM Mgt+ completely inhibited nucleotide-activated channels . In the presence of 2 'UM cis Ca the nucleotide-activated macroscopic Ba conductance was inhibited by cis Mgt+ with an IC50 equal to 1 .5 mM .