Limits...
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.

Show MeSH
Simulated peak open probabilities of Model 4Ca with four Ca2+ binding sites (Table ) in response to rectangular Ca2+ pulses, plotted as a function of pulse amplitude and duration. The x axis represents the amplitude of free Ca2+ concentration during the pulse, the y axis represents the duration of the pulse, and the z axis represents the peak open probability of the channel estimated from the ensemble average of 4,096 episodes.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2230654&req=5

Figure 7: Simulated peak open probabilities of Model 4Ca with four Ca2+ binding sites (Table ) in response to rectangular Ca2+ pulses, plotted as a function of pulse amplitude and duration. The x axis represents the amplitude of free Ca2+ concentration during the pulse, the y axis represents the duration of the pulse, and the z axis represents the peak open probability of the channel estimated from the ensemble average of 4,096 episodes.

Mentions: The chemistry of DMN limits flash-photolysis experiments to a rather narrow range of amplitude-duration characteristics of Ca2+ spikes. In contrast, the parameters of local Ca2+ signals associated with the activity of DHPRs vary widely. Therefore, to gain further insight into the dependence of the channel activation on the characteristics of the trigger signal, we performed simulations in response to a broad range of rectangular Ca2+ pulses using Model 4Ca with four Ca2+ binding sites described above. The properties of the Ca2+ pulse in the physiological range of durations and amplitudes had a profound effect on peak open probability of the RyR, as illustrated in Fig. 7. Calcium elevations lasting <10 μs had negligible probability to open the RyR in the whole amplitude range. To increase the peak open probability from 5 to 95%, the amplitude of the calcium pulse has to be increased by ∼10-fold for any pulse duration. Prolongation of the Ca2+ pulses above 1 ms was not effective in increasing peak Po of the RyR. In the high Ca2+ pulse amplitude range (>10 μM), the dependence of peak Po on pulse duration was very steep for short pulse durations (0.1–0.5 ms).


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

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

Simulated peak open probabilities of Model 4Ca with four Ca2+ binding sites (Table ) in response to rectangular Ca2+ pulses, plotted as a function of pulse amplitude and duration. The x axis represents the amplitude of free Ca2+ concentration during the pulse, the y axis represents the duration of the pulse, and the z axis represents the peak open probability of the channel estimated from the ensemble average of 4,096 episodes.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 7: Simulated peak open probabilities of Model 4Ca with four Ca2+ binding sites (Table ) in response to rectangular Ca2+ pulses, plotted as a function of pulse amplitude and duration. The x axis represents the amplitude of free Ca2+ concentration during the pulse, the y axis represents the duration of the pulse, and the z axis represents the peak open probability of the channel estimated from the ensemble average of 4,096 episodes.
Mentions: The chemistry of DMN limits flash-photolysis experiments to a rather narrow range of amplitude-duration characteristics of Ca2+ spikes. In contrast, the parameters of local Ca2+ signals associated with the activity of DHPRs vary widely. Therefore, to gain further insight into the dependence of the channel activation on the characteristics of the trigger signal, we performed simulations in response to a broad range of rectangular Ca2+ pulses using Model 4Ca with four Ca2+ binding sites described above. The properties of the Ca2+ pulse in the physiological range of durations and amplitudes had a profound effect on peak open probability of the RyR, as illustrated in Fig. 7. Calcium elevations lasting <10 μs had negligible probability to open the RyR in the whole amplitude range. To increase the peak open probability from 5 to 95%, the amplitude of the calcium pulse has to be increased by ∼10-fold for any pulse duration. Prolongation of the Ca2+ pulses above 1 ms was not effective in increasing peak Po of the RyR. In the high Ca2+ pulse amplitude range (>10 μM), the dependence of peak Po on pulse duration was very steep for short pulse durations (0.1–0.5 ms).

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