RESPONSE OF RYANODINE RECEPTOR CHANNELS TO CA2+ STEPS PRODUCED BY RAPID SOLUTION

We used a flow method for Ca2+ activation of sheep cardiac and rabbit skeletal ryanodine receptor (RyR) channels in lipid bilayers, which ac tivated RyRs in < 20 ms and maintained a steady [Ca2+] for 5 s, [Ca2+] was rapidly altered by flowing Ca2+-buffered solutions containing 100 or 200 mu M Ca2+ from a perfusion tube inserted in the cis, myoplasmi c chamber above the bilayer. During steps from 0.1 to 100 mu M, [Ca2+] reached 0.3 mu M (activation threshold) and 10 mu M (maximum P-o) in times consistent with predictions of a solution exchange model, Immedi ately following rapid RyR activation, P-o was 0.67 (cardiac) and 0.45 (skeletal) at a holding voltage of +40 mV (cis/trans), P-o then declin ed (at constant [Ca2+]) in 70% of channels (n = 25) with time constant s ranging from .5 to 15 s. The mechanism for P-o decline, whether it b e adaptation or inactivation, was not determined in this study, cis, 2 mM Mg2+ reduced the initial P-o for skeletal RyRs to 0.21 and margina lly slowed the declining phase. During very rapid falls in [Ca2+] from mM (inhibited) to sub-mu M (sub-activating) levels, skeletal RyR did not open. We conclude the RyR gates responsible for Ca2+-dependent act ivation and inhibition of skeletal RyRs can gate independently.