Rapid Activation of the Cardiac Ryanodine Receptor by Submillisecond Calcium Stimuli

The local control concept of excitation–contraction coupling in the heart postulates that the activity of the sarcoplasmic reticulum ryanodine receptor 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 increases in Ca2+ generated by flash photolysis of DM-nitrophen. Application of such Ca2+ spikes (amplitude ∼10–30 μM, duration ∼0.1–0.4 ms) resulted in activation of the RyRs with a probability that increased steeply (apparent Hill slope ∼2.5) with spike amplitude. The time constants of RyR activation were 0.07–0.27 ms, decreasing with spike amplitude. To fit the rising portion of the open probability, a single exponential function had to be raised to a power n ∼ 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 adequate to activate the RyR, and support the possibility that RyR channels are governed by single DHPR openings. They also provide evidence for the assumption that RyR activation requires binding of multiple Ca2+ ions in accordance with the tetrameric organization of the channel protein.