Biophysical Journal - Cytoplasmic versus Intra-SR: the Battle of the Ca2+ Diffusion Coefficients in Cardiac Muscle

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Copyright © 2008 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 95, Issue 3, 1005-1006, 1 August 2008

doi:10.1529/biophysj.108.133926

New and Notable

Cytoplasmic versus Intra-SR: the Battle of the Ca²⁺ Diffusion Coefficients in Cardiac Muscle

Godfrey Smith^, and Niall MacQuaide

Biomedical and Life Sciences, Glasgow University, Glasgow, United Kingdom

Address reprint requests to G. L. Smith, Tel.: 00-44-14-13-30-59-63.

Steep Ca²⁺ ion concentrations gradients within the cytosol have been recorded in many cell types. These gradients allow local (<1μm) regulation of Ca²⁺ sensitive processes. In cardiac muscle, the ability to develop large cytosolic [Ca²⁺] gradients is essential to the control of the force of contraction. A factor of 10× or more differences of intracellular [Ca²⁺] can develop over a matter of ∼1μm in ∼10ms because the diffusion coefficient of Ca²⁺ is very low (∼15μm²s⁻¹, i.e., ∼50–70× less than free solution). The poor ability of Ca²⁺ to diffuse allows the force of contraction to be modulated by varying the number of sarcoplasmic reticulum (SR) Ca²⁺ release sites active during each contraction. Despite these release sites being separated by only ∼2μm, no cross talk of Ca²⁺ between adjacent SR Ca²⁺ release units is thought to occur. In theory, the same principle should apply to the luminal side of the SR since the lumen of the SR is continuous throughout the cell. Simplistically, the Ca²⁺ diffusion coefficient within the SR must be comparable to the cytosol, otherwise the adjacent regions of the SR would be depleted by active sites, and therefore the benefit of fine digital control over Ca²⁺ released would be lost. Previously, the only estimate of luminal Ca²⁺ diffusion coefficient available was ∼60μm²s⁻¹1, a value clearly higher than estimates of cytoplasmic Ca²⁺ and therefore raising a series of interesting issues related to the relatively rapid movement of Ca²⁺ within the SR. The article in this issue by Swietach et al. 2 addresses the significant technical challenges associated with measuring intra-SR Ca²⁺ mobility with elegant experimental techniques and extensive computational analysis. They arrive at a value of 8–9μm²s⁻¹ for the lumped diffusion of Ca²⁺ within the SR measured in the long-axis of the rat and guinea-pig ventricular myocytes. This value is less than current estimates of cytoplasmic diffusion coefficient and therefore supports the concept that intra-SRCa²⁺ gradients are at least as steep as cytoplasmic. This value can be used to help address a series of important quantitative questions concerning the operation of cardiac muscle under physiological and pathophysiological conditions. For example:

1. During a normal Ca²⁺ release event, how much depletion of the SR occurs? i.e., what is the minimum [Ca²⁺] achieved on the luminal side? How much depletion from adjacent junctional SR (jSR) sites occurs?
2. How fast can the Ca²⁺ be returned from the main SR Ca²⁺ uptake sites (in the network SR; i.e., nSR) to the release sites (in the jSR)?
3. How does a Ca²⁺ wave propagate along the length of a cardiac cell?

Under certain situations, Ca²⁺ release in a discrete region of the cell has been found to propagate in a nondecrementing Ca²⁺ wave along the cell length (100–120μm). A fire–diffuse–fire mechanism has been used to described the underlying mechanism 3 (the first term “fire” refers to the release of Ca²⁺ from a discrete cluster of ryanodine receptors located in the jSR; the term “diffuse” applies to the diffusion of cytoplasmic Ca²⁺ across the ∼2μm of the sarcomere; and the second term “fire” is Ca²⁺ release from the next cluster of ryanodine receptors). The triggering event is Ca²⁺-induced Ca²⁺ release, a mechanism intrinsic to the SR Ca²⁺ release channel (RyR2). For the fire–diffuse–fire mechanism to work, the cytoplasmic Ca²⁺ should be able to diffuse between adjacent RyR2 clusters more readily than intra-SR (luminal) Ca²⁺ diffuses between adjacent jSR; otherwise, when one jSR region is depleted by a fire event, this would cause net diffusion from the lumen of the (yet-to-be activated) adjacent jSR, and the Ca²⁺ wave would “fizzle-out”. As discussed by Swietach et al. 2, a low Ca²⁺ diffusion coefficient makes this scenario unlikely.

But there is still a lot to debate. For example:

1. Previous measurements of Ca²⁺ diffusion using a luminal Ca²⁺ indicator in rabbit cardiomyocytes arrived at a considerably higher value 1. Why the discrepancy? Swietach et al. suggest that SR Ca²⁺ leak during the period of SR depletion can explain the difference. In their study, SR Ca²⁺ leak was blocked by 0.3 mM or 2 mM tetracaine. But Wu and Bers 1 were aware of this possibility, and indicated that SR Ca²⁺ leak was not significant over the timescale of their measurements.
2. Keller et al. 4 recently published data to suggest that rapid inhibition of the SR Ca²⁺ pump slowed the propagation of the Ca²⁺ wave. Why should inhibition of a Ca²⁺ pump immediately in front of the Ca²⁺ wave slow propagation in a fire–diffuse–fire situation? The explanation suggested by Keller et al. was that Ca²⁺ wave propagation required a component of SR Ca²⁺ uptake and diffusion within the SR, i.e., a fire–SR Ca²⁺ uptake–diffusion (intra-SR)–fire mechanism. But as pointed out by Swietach et al., the low intra-SR diffusion coefficient makes this mechanism unlikely. But the experimental observation remains and requires an explanation. One resolution is simply that SR Ca²⁺ uptake is required to prime the SR Ca²⁺ release channel for the fire event, but the uptake occurs at the release site on the jSR (no intra-SR diffusion required). A significant amount of SERCA is thought to exist at jSR, thus the propagation process could be fire–diffuse–SR Ca²⁺ uptake–fire. As described in an abstract presented to the most recent Biophysical Society meeting by Ramay et al. 5, this latter mechanism can generate a propagated Ca²⁺ wave using very low intra-SR Ca²⁺ diffusion characteristics. Experimental evidence for this mechanism will come from high resolution measurements of intra-SR Ca²⁺.
3. The cytoplasmic Ca²⁺ diffusion coefficient will be dependent on a number of factors including the range of [Ca²⁺] involved and physiological status of the cell. This was nicely illustrated by another recent Biophysical Society abstract, stating that factors that will contribute to the Ca²⁺ diffusion coefficient, i.e., the Ca²⁺ buffering associated with the SR Ca²⁺ pump and the myofilaments, will change during β-adrenergic stimulation 6. As discussed above, whether or not local Ca²⁺ release propagates throughout the cell may depend on the relative values of cytoplasmic and luminal Ca²⁺ diffusion coefficients. In theory, Ca²⁺ waves may be suppressed by increasing the intra-SR diffusion coefficient. Under these circumstances, the jSR region depleted by a fire event would deplete the lumen of the jSR ahead of the Ca²⁺ wavefront.
4. The intra-SR Ca²⁺ diffusion studied by Swietach et al. was along the longitudinal axis of the cardiac cell. The intra-SR Ca²⁺ diffusion coefficient may not be the same in the other two planes of the cell.

In summary, the article by Swietach et al. in this issue provides new data to inform computational models and to encourage discussion about the topic of intra-SR Ca²⁺ diffusion that hither-to had received scant attention. The implications of this data for quantitative models of Ca²⁺ fluxes in cardiac muscle are significant.

References

1. Wu, X., and Bers, D.M. (2006). Sarcoplasmic reticulum and nuclear envelope are one highly interconnected Ca²⁺ store throughout cardiac myocyte. Circ. Res. 99, 283–291. CrossRef | PubMed

2. Swietach, P., Spitzer, K.W., and Vaughan-Jones, R.D. (2008). Ca²⁺-mobility in the sarcoplasmic reticulum of ventricular myocytes is low. Biophys. J. 95, 1412–1427. Abstract | Full Text | PDF (970 kb) | CrossRef | PubMed

3. Keizer, J., and Smith, G.D. (1998). Spark-to-wave transition: saltatory transmission of calcium waves in cardiac myocytes. Biophys. Chem. 72, 87–100. CrossRef | PubMed

4. Keller, M., Kao, J.P., Egger, M., and Niggli, E. (2007). Calcium waves driven by “sensitization” wave-fronts. Cardiovasc. Res. 74, 39–45. CrossRef | PubMed

5. Ramay, H., Jafri, M.S., Lederer, W.J., and Sobie, E.A. (2008). Propagation of Ca²⁺ waves in ventricular myocytes.. 2008 Biophysical Society Meeting Abstracts. Biophys. J. , Supplement, Abstract, 488-Pos. PubMed

6. Briston, S.J., Trafford, A.W., Eisner, D.A., and Dibb, K.M. (2008). Buffering and SR Ca content in phospholamban knockout mouse dura β-adrenergic stimulation. 2008 Biophysical Society Meeting Abstracts. Biophys. J. , Supplement, Abstract, 1534-Pos. PubMed

Publication Information

Received: April 7, 2008
Accepted: April 16, 2008

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