Cytoplasmic Ca2+ does not inhibit the cardiac muscle sarcoplasmic reticulum ryanodine receptor Ca2+ channel, although Ca(2+)-induced Ca2+ inactivation of Ca2+ release is observed in native vesicles.

Single channel properties of cardiac and fast-twitch skeletal muscle sarcoplasmic reticulum (SR) release channels were compared in a planar bilayer by fusing SR membranes in a Cs(+)-conducting medium. We found that the pharmacology, Cs+ conductance and selectivity to monovalent and divalent cations of the two channels were similar. The cardiac SR channel exhibited multiple kinetic states. The open and closed lifetimes were not altered from a range of 10(-7) to 10(-3) M Ca2+, but the proportion of closed and open states shifted to shorter closings and openings, respectively. However, while the single channel activity of the skeletal SR channel was activated and inactivated by micromolar and millimolar Ca2+, respectively, the cardiac SR channel remained activated in the presence of high [Ca2+]. In correlation to these studies, [3H]ryanodine binding by the receptors of the two channel receptors was inhibited by high [Ca2+] in skeletal but not in cardiac membranes in the presence of adenine nucleotides. There is, however, a minor inhibition of [3H]ryanodine binding of cardiac SR at millimolar Ca2+ in the absence of adenine nucleotides. When Ca(2+)-induced Ca2+ release was examined from preloaded native SR vesicles, the release rates followed a normal biphasic curve, with Ca(2+)-induced inactivation at high [Ca2+] for both cardiac and skeletal SR. Our data suggest that the molecular basis of regulation of the SR Ca2+ release channel in cardiac and skeletal muscle is different, and that the cardiac SR channel isoform lacks a Ca(2+)-inactivated site.