Correspondence to: Sandor Györke, Department of Physiology, Texas Tech University Health Science Center, 3601 4th Street Lubbock, Texas 79430., physg{at}ttuhsc.edu (E-mail), Fax: 806-743-1512; (fax)
Several perspectives on Ca2+ sparks were recently published. A common theme was the importance of defining the mechanisms that terminate local SR Ca2+ release. We propose that the time- and Ca2+-dependent modal gating behavior of single ryanodine receptor (RyR) channels is the negative control mechanism that terminates local Ca2+ release. Specifically, the observed "inactivation" and "adaptation" phenomena are two manifestations of the same general mechanism (i.e., modal RyR gating). We hope this new unified view of RyR negative control mechanisms may lead to new insights into local intracellular Ca2+ signaling in heart.
The local Ca2+ spark is thought to represent the elementary intracellular Ca2+ release unit in adult mammalian cardiac muscle (
In 1993, we argued that conventional wisdom was insufficient to explain why repeated fast Ca2+ stimuli triggered transiently and repeatedly activated single RyR channels in planar bilayers (
Adaptation: Phenomenon Not Mechanism
6 yr and volumes of new data have provided further insight into RyR adaptation. We believe RyR adaptation should be viewed as a physiologically important phenomenon and not as a molecular mechanism. There is now substantial evidence that the adaptation phenomenon is due to a transient, Ca2+-dependent shift in the modal gating behavior of the RyR channel (
The RyR adaptation phenomenon is observed when the channel is activated by a free Ca2+ waveform generated by laser flash photolysis of DM-nitrophen (
When true step Ca2+ stimuli (without fast Ca2+ spikes) are composed of single RyR channels in planar bilayers, these step-like Ca2+ stimuli trigger bursts of RyR channel activity that spontaneously decay over time (decay, ~12 s range;
RyR Negative Control: The Cellular Level
Defining the mechanisms that terminate elementary SR Ca2+ release events (i.e., Ca2+ sparks) is an important step towards understanding how release is regulated. The candidate negative control mechanisms include: (a) Ca2+-dependent inactivation, (b) adaptation, and (c) use-dependent inactivation (
The hallmark of the adaptation phenomenon is thought to be the ability of apparently "refractory" RyR channels to reactivate in response to a larger Ca2+ stimulus. This, however, is not likely to be relevant to regulation of CICR in situ, as even small trigger Ca2+ stimuli in situ may elevate the local free Ca2+ concentration in the diadic cleft to very high levels representing maximal activating stimuli for the local RyRs. These high Ca2+ levels should result in maximal occupation of Ca2+ binding sites that govern the equilibrium between the high and low Po modes, and thus the reactivation by even larger Ca2+ stimuli would not occur. Perhaps the more physiologically relevant feature of adaptation is the underlying modal gating shift. In the presence of a sustained trigger Ca2+ signal, a time- and Ca2+-dependent shift to the low- and zero-Po mode would cause a decline in channel activity. The implication is that the decreasing RyR channel activity would always appear as a consequence of earlier channel activation. Thus (provided it is sufficiently fast) the shift in gating modes could account for apparent use-dependent properties of Ca2+ release inactivation in situ (
Many vesicle Ca2+ flux studies (
Thus, we propose that the modal RyR gating behavior may represent a common factor that underlies the apparently different mechanisms of Ca2+-dependent inactivation, use-dependent inactivation, and adaptation. The implication is that the negative control mechanisms that counter the inherent positive feedback of CICR may be a time- and Ca2+-dependent shift in the modal gating behavior of the RyR channel. The intent of our proposition is to simply stimulate discussion. It is clear that additional experimentation and a far more detailed theoretical framework is required to understand termination of the SR Ca2+ release process in heart. Nevertheless, even a relatively speculative exchange of scientific ideas can generate new and interesting ideas and future directions.
Acknowledgements
The author thanks Michael Fill for stimulating discussions and his input in writing this comment.
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