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Rapid activation of the cardiac ryanodine receptor by submillisecond calcium stimuli.

Zahradníková A, Zahradník I, Györke I, Györke S - J. Gen. Physiol. (1999)

Bottom Line: To define the kinetic limits of effective trigger Ca(2+) signals, we recorded activity of single cardiac RyRs in lipid bilayers during rapid and transient increases in Ca(2+) generated by flash photolysis of DM-nitrophen.These results provide evidence that brief Ca(2+) 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 Ca(2+) ions in accordance with the tetrameric organization of the channel protein.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Physiology, Slovak Academy of Sciences, Bratislava, Slovak Republic 83334, USA.

ABSTRACT
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 Ca(2+) 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) Ca(2+) elevations accompanying single DHPR channel openings. To define the kinetic limits of effective trigger Ca(2+) signals, we recorded activity of single cardiac RyRs in lipid bilayers during rapid and transient increases in Ca(2+) generated by flash photolysis of DM-nitrophen. Application of such Ca(2+) spikes (amplitude approximately 10-30 microM, duration approximately 0.1-0.4 ms) resulted in activation of the RyRs with a probability that increased steeply (apparent Hill slope approximately 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 approximately 3. We show that these data could be adequately described with a gating scheme incorporating four sequential Ca(2+)-sensitive closed states between the resting and the first open states. These results provide evidence that brief Ca(2+) 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 Ca(2+) ions in accordance with the tetrameric organization of the channel protein.

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Description of the calcium dependence of the kinetics and amplitude of the ensemble open probability by different models of RyR gating. (A–E) Superposition of the experimental channel responses (thin noisy lines; data from the experiment shown in Fig. 4) and theoretical responses of the models (thick lines): Model 1Ca (A), Model 2Ca (B), Model 3Ca (C), Model 4Ca (D), and Model 5Ca (E). (F) The relationship between the peak open probability and the Ca2+ spike amplitude. The symbols with their standard deviations represent measured peak open probability at different levels of peak calcium during the spike. The lines represent the theoretical dose–response curves () for models with one to five Ca2+ binding sites. Labels correspond to the number of Ca2+ binding steps in the model.
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Figure 5: Description of the calcium dependence of the kinetics and amplitude of the ensemble open probability by different models of RyR gating. (A–E) Superposition of the experimental channel responses (thin noisy lines; data from the experiment shown in Fig. 4) and theoretical responses of the models (thick lines): Model 1Ca (A), Model 2Ca (B), Model 3Ca (C), Model 4Ca (D), and Model 5Ca (E). (F) The relationship between the peak open probability and the Ca2+ spike amplitude. The symbols with their standard deviations represent measured peak open probability at different levels of peak calcium during the spike. The lines represent the theoretical dose–response curves () for models with one to five Ca2+ binding sites. Labels correspond to the number of Ca2+ binding steps in the model.

Mentions: To simulate the RyR response to Ca2+ spikes, we used our previously published minimal gating model of RyR with one Ca2+ binding step (Zahradníková and Zahradník 1996; see Fig. 5 and Model 1Ca in Table ). As alternative models, we used extensions of Model 1Ca, incorporating consecutive binding of two to five Ca2+ ions. It was assumed that Ca2+ binding sites are identical and behave independently. Subsequent gating steps are possible only if all calcium binding sites are occupied (Table , Model 2Ca–Model 5Ca). The rate constants of transitions not involving Ca2+ binding were unchanged. In models with multiple Ca2+ binding steps, the ratios of the on and off rates for calcium binding were calculated from the apparent peak and steady state calcium sensitivities of the channel Po (see Zahradníková and Zahradník 1996) to provide a mean value identical to that of Model 1Ca. The on rates were optimized for best description of the rate of RyR activation.


Rapid activation of the cardiac ryanodine receptor by submillisecond calcium stimuli.

Zahradníková A, Zahradník I, Györke I, Györke S - J. Gen. Physiol. (1999)

Description of the calcium dependence of the kinetics and amplitude of the ensemble open probability by different models of RyR gating. (A–E) Superposition of the experimental channel responses (thin noisy lines; data from the experiment shown in Fig. 4) and theoretical responses of the models (thick lines): Model 1Ca (A), Model 2Ca (B), Model 3Ca (C), Model 4Ca (D), and Model 5Ca (E). (F) The relationship between the peak open probability and the Ca2+ spike amplitude. The symbols with their standard deviations represent measured peak open probability at different levels of peak calcium during the spike. The lines represent the theoretical dose–response curves () for models with one to five Ca2+ binding sites. Labels correspond to the number of Ca2+ binding steps in the model.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230654&req=5

Figure 5: Description of the calcium dependence of the kinetics and amplitude of the ensemble open probability by different models of RyR gating. (A–E) Superposition of the experimental channel responses (thin noisy lines; data from the experiment shown in Fig. 4) and theoretical responses of the models (thick lines): Model 1Ca (A), Model 2Ca (B), Model 3Ca (C), Model 4Ca (D), and Model 5Ca (E). (F) The relationship between the peak open probability and the Ca2+ spike amplitude. The symbols with their standard deviations represent measured peak open probability at different levels of peak calcium during the spike. The lines represent the theoretical dose–response curves () for models with one to five Ca2+ binding sites. Labels correspond to the number of Ca2+ binding steps in the model.
Mentions: To simulate the RyR response to Ca2+ spikes, we used our previously published minimal gating model of RyR with one Ca2+ binding step (Zahradníková and Zahradník 1996; see Fig. 5 and Model 1Ca in Table ). As alternative models, we used extensions of Model 1Ca, incorporating consecutive binding of two to five Ca2+ ions. It was assumed that Ca2+ binding sites are identical and behave independently. Subsequent gating steps are possible only if all calcium binding sites are occupied (Table , Model 2Ca–Model 5Ca). The rate constants of transitions not involving Ca2+ binding were unchanged. In models with multiple Ca2+ binding steps, the ratios of the on and off rates for calcium binding were calculated from the apparent peak and steady state calcium sensitivities of the channel Po (see Zahradníková and Zahradník 1996) to provide a mean value identical to that of Model 1Ca. The on rates were optimized for best description of the rate of RyR activation.

Bottom Line: To define the kinetic limits of effective trigger Ca(2+) signals, we recorded activity of single cardiac RyRs in lipid bilayers during rapid and transient increases in Ca(2+) generated by flash photolysis of DM-nitrophen.These results provide evidence that brief Ca(2+) 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 Ca(2+) ions in accordance with the tetrameric organization of the channel protein.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Physiology, Slovak Academy of Sciences, Bratislava, Slovak Republic 83334, USA.

ABSTRACT
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 Ca(2+) 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) Ca(2+) elevations accompanying single DHPR channel openings. To define the kinetic limits of effective trigger Ca(2+) signals, we recorded activity of single cardiac RyRs in lipid bilayers during rapid and transient increases in Ca(2+) generated by flash photolysis of DM-nitrophen. Application of such Ca(2+) spikes (amplitude approximately 10-30 microM, duration approximately 0.1-0.4 ms) resulted in activation of the RyRs with a probability that increased steeply (apparent Hill slope approximately 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 approximately 3. We show that these data could be adequately described with a gating scheme incorporating four sequential Ca(2+)-sensitive closed states between the resting and the first open states. These results provide evidence that brief Ca(2+) 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 Ca(2+) ions in accordance with the tetrameric organization of the channel protein.

Show MeSH
Related in: MedlinePlus