Rapid activation of the cardiac ryanodine receptor by submillisecond calcium stimuli

The local control concept of excitation-contraction coupling in the heart p ostulates that the activity of the sarcoplasmic reticulum ryanodine recepto r channels (RyR) is controlled by Ca2+ entry through adjoining sarcolemmal single dihydropyridine receptor channels (DHPRs). One unverified premise of this hypothesis is that the RyR must be fast enough to track the brief (<0 .5 ms) Ca2+ elevations accompanying single DHPR channel openings. To define the kinetic limits of effective trigger Ca2+ signals, we recorded activity of single cardiac RyRs in lipid bilayers during rapid and transient increa ses in Ca2+ generated by flash photolysis of DM-nitrophen. Application of s uch Ca2+ spikes (amplitude similar to 10-30 mu M, duration similar to 0.1-0 .4 ms) resulted in activation of the RyRs with a probability that increased steeply (apparent Hill slope similar to 2.5) with spike amplitude. The tim e constants of RyR activation were 0.07-0.27 ms, decreasing with spike ampl itude. To fit the rising portion of the open probability, a single exponent ial function had to be raised to a power n similar to 3. We show that these data could be adequately described with a gating scheme incorporating four sequential Ca2+-sensitive closed states between the resting and the first open states. These results provide evidence that brief Ca2+ triggers are ad equate to activate the RyR, and support the possibility that RyR channels a re governed by single DHPR openings. They also provide evidence for the ass umption that RyR activation requires binding of multiple Ca2+ ions in accor dance with the tetrameric organization of the channel protein.