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Numerical analysis of Ca2+ signaling in rat ventricular myocytes with realistic transverse-axial tubular geometry and inhibited sarcoplasmic reticulum.

Cheng Y, Yu Z, Hoshijima M, Holst MJ, McCulloch AD, McCammon JA, Michailova AP - PLoS Comput. Biol. (2010)

Bottom Line: In agreement with experiment, in the presence of fluorescence dye and inhibited sarcoplasmic reticulum, the lack of detectible differences in the depolarization-evoked Ca(2+) transients was found when the Ca(2+) flux was heterogeneously distributed along the sarcolemma.Even at modest elevation of Ca(2+), reached during Ca(2+) influx, large and steep Ca(2+) gradients are found in the narrow sub-sarcolemmal space.The model predicts that the branched t-tubule structure and changes in the normal Ca(2+) flux density along the cell membrane support initiation and propagation of Ca(2+) waves in rat myocytes.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of California San Diego, La Jolla, California, United States of America.

ABSTRACT
The t-tubules of mammalian ventricular myocytes are invaginations of the cell membrane that occur at each Z-line. These invaginations branch within the cell to form a complex network that allows rapid propagation of the electrical signal, and hence synchronous rise of intracellular calcium (Ca(2+)). To investigate how the t-tubule microanatomy and the distribution of membrane Ca(2+) flux affect cardiac excitation-contraction coupling we developed a 3-D continuum model of Ca(2+) signaling, buffering and diffusion in rat ventricular myocytes. The transverse-axial t-tubule geometry was derived from light microscopy structural data. To solve the nonlinear reaction-diffusion system we extended SMOL software tool (http://mccammon.ucsd.edu/smol/). The analysis suggests that the quantitative understanding of the Ca(2+) signaling requires more accurate knowledge of the t-tubule ultra-structure and Ca(2+) flux distribution along the sarcolemma. The results reveal the important role for mobile and stationary Ca(2+) buffers, including the Ca(2+) indicator dye. In agreement with experiment, in the presence of fluorescence dye and inhibited sarcoplasmic reticulum, the lack of detectible differences in the depolarization-evoked Ca(2+) transients was found when the Ca(2+) flux was heterogeneously distributed along the sarcolemma. In the absence of fluorescence dye, strongly non-uniform Ca(2+) signals are predicted. Even at modest elevation of Ca(2+), reached during Ca(2+) influx, large and steep Ca(2+) gradients are found in the narrow sub-sarcolemmal space. The model predicts that the branched t-tubule structure and changes in the normal Ca(2+) flux density along the cell membrane support initiation and propagation of Ca(2+) waves in rat myocytes.

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Effects of reduced extracellular [Na+] on subcellular [Ca2+]i signals.(A–B) The voltage-clamp protocol and whole-cell L-type Ca2+ current. (C) Quantitative comparison of the effects of changes in [Na+]e on the global Na+/Ca2+ flux (solid lines - [Na+]e 140 mM, dashed lines – zero [Na+]e). (D) Predicted Ca2+ leak with zero [Na+]e. (E) Quantitative comparison of the effects of changes in [Na+]e on the global Ca2+ transient. (F) Calcium concentrations visualized as line-scan images in transverse cell direction with zero [Na+]e. (G) Quantitative comparison of the effects of changes in [Na+]e on local Ca2+ transients. In this numerical experiment Fluo-3 was zero, Ca2+ fluxes heterogeneously distributed via the sarcolemma, the line-scan positioned at 200nm away from the t-tubule membrane at the angle 120°. Along the scanning line the featured spots were chosen to be the same as in Fig. 6G.
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pcbi-1000972-g008: Effects of reduced extracellular [Na+] on subcellular [Ca2+]i signals.(A–B) The voltage-clamp protocol and whole-cell L-type Ca2+ current. (C) Quantitative comparison of the effects of changes in [Na+]e on the global Na+/Ca2+ flux (solid lines - [Na+]e 140 mM, dashed lines – zero [Na+]e). (D) Predicted Ca2+ leak with zero [Na+]e. (E) Quantitative comparison of the effects of changes in [Na+]e on the global Ca2+ transient. (F) Calcium concentrations visualized as line-scan images in transverse cell direction with zero [Na+]e. (G) Quantitative comparison of the effects of changes in [Na+]e on local Ca2+ transients. In this numerical experiment Fluo-3 was zero, Ca2+ fluxes heterogeneously distributed via the sarcolemma, the line-scan positioned at 200nm away from the t-tubule membrane at the angle 120°. Along the scanning line the featured spots were chosen to be the same as in Fig. 6G.

Mentions: In this study, we also examined the effects of NCX inhibition on the voltage-clamp induced Ca2+ signals in the absence of fluorescent indicator. The inhibition of NCX forward mode was achieved by removing extracellular sodium (i.e. 0 mM [Na+]e). To adjust Ca2+ flux via Ca2+ leak to match Ca2+ influx via NCX, so that at rest no net movement across the cell membrane to occur, we estimated Ca2+ leak constant () assuming [Na+]e zero (see Table 3). Under these conditions NCX operated only in Ca2+ entry mode while membrane leak pumped Ca2+ out of the cell (see Figs. 8C–D dashed lines). Figures 8F–G (see dashed lines) show the predicted local [Ca2+]i transients and the corresponding line-scan image. The line scan-image demonstrates that [Ca2+]i distribution was again non-uniform but rather different compared to that predicted with 140 mM [Na+]e (compare Fig. 8F with Fig. 6F). The results in Fig. 8G demonstrate that: (1) local [Ca2+]i peaks in the featured spots (0.17 µm, 3.09 µm and 5.45 µm) increased and that this increase was more pronounced near t-tubule mouth; (2) upon repolarization [Ca2+]i suddenly dropped because rate decreased while rate remained unchanged; (3) the decay of Ca2+ transient near the outer cell edge was slow; (4) [Ca2+]i in the featured spots, 3.09 µm and 5.45 µm, begun slowly to increase when t>70 ms because rate remained unchanged but rate slightly decreased. With [Na+]e zero versus 140 mM [Na+]e SCHs increased: SCH(tIca-peak) by 1.29 folds; SCH(t70 ms) by 1.4 folds; SCH(t[Ca]i-peak) by 1.43 folds; SCH(t100ms) by 1.65 folds; and SCH(t200ms) by 3.04 folds (see Fig. 8G). Furthermore, the NCX inhibition had visible effect on the global [Ca2+]i transient, i.e. global Ca2+ peak increased and during membrane repolarization initially [Ca2+]i suddenly decreased and after that rapidly equilibrated. In agreement with experimental data reported previously [65], the model also predicts increase in global [Ca2+]i peak and no changes in the time to peak when [Na+]e is completely substituted with 140 mM Li+, (see dashed line in Fig. 8E). New finding is that, with zero extracellular Na+, the Ca2+ signal spreads from the external membrane to the cell center as continuum Ca2+ wave initiated after 56 ms but very soon this wave faltered (see Video S3, right panel).


Numerical analysis of Ca2+ signaling in rat ventricular myocytes with realistic transverse-axial tubular geometry and inhibited sarcoplasmic reticulum.

Cheng Y, Yu Z, Hoshijima M, Holst MJ, McCulloch AD, McCammon JA, Michailova AP - PLoS Comput. Biol. (2010)

Effects of reduced extracellular [Na+] on subcellular [Ca2+]i signals.(A–B) The voltage-clamp protocol and whole-cell L-type Ca2+ current. (C) Quantitative comparison of the effects of changes in [Na+]e on the global Na+/Ca2+ flux (solid lines - [Na+]e 140 mM, dashed lines – zero [Na+]e). (D) Predicted Ca2+ leak with zero [Na+]e. (E) Quantitative comparison of the effects of changes in [Na+]e on the global Ca2+ transient. (F) Calcium concentrations visualized as line-scan images in transverse cell direction with zero [Na+]e. (G) Quantitative comparison of the effects of changes in [Na+]e on local Ca2+ transients. In this numerical experiment Fluo-3 was zero, Ca2+ fluxes heterogeneously distributed via the sarcolemma, the line-scan positioned at 200nm away from the t-tubule membrane at the angle 120°. Along the scanning line the featured spots were chosen to be the same as in Fig. 6G.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2965743&req=5

pcbi-1000972-g008: Effects of reduced extracellular [Na+] on subcellular [Ca2+]i signals.(A–B) The voltage-clamp protocol and whole-cell L-type Ca2+ current. (C) Quantitative comparison of the effects of changes in [Na+]e on the global Na+/Ca2+ flux (solid lines - [Na+]e 140 mM, dashed lines – zero [Na+]e). (D) Predicted Ca2+ leak with zero [Na+]e. (E) Quantitative comparison of the effects of changes in [Na+]e on the global Ca2+ transient. (F) Calcium concentrations visualized as line-scan images in transverse cell direction with zero [Na+]e. (G) Quantitative comparison of the effects of changes in [Na+]e on local Ca2+ transients. In this numerical experiment Fluo-3 was zero, Ca2+ fluxes heterogeneously distributed via the sarcolemma, the line-scan positioned at 200nm away from the t-tubule membrane at the angle 120°. Along the scanning line the featured spots were chosen to be the same as in Fig. 6G.
Mentions: In this study, we also examined the effects of NCX inhibition on the voltage-clamp induced Ca2+ signals in the absence of fluorescent indicator. The inhibition of NCX forward mode was achieved by removing extracellular sodium (i.e. 0 mM [Na+]e). To adjust Ca2+ flux via Ca2+ leak to match Ca2+ influx via NCX, so that at rest no net movement across the cell membrane to occur, we estimated Ca2+ leak constant () assuming [Na+]e zero (see Table 3). Under these conditions NCX operated only in Ca2+ entry mode while membrane leak pumped Ca2+ out of the cell (see Figs. 8C–D dashed lines). Figures 8F–G (see dashed lines) show the predicted local [Ca2+]i transients and the corresponding line-scan image. The line scan-image demonstrates that [Ca2+]i distribution was again non-uniform but rather different compared to that predicted with 140 mM [Na+]e (compare Fig. 8F with Fig. 6F). The results in Fig. 8G demonstrate that: (1) local [Ca2+]i peaks in the featured spots (0.17 µm, 3.09 µm and 5.45 µm) increased and that this increase was more pronounced near t-tubule mouth; (2) upon repolarization [Ca2+]i suddenly dropped because rate decreased while rate remained unchanged; (3) the decay of Ca2+ transient near the outer cell edge was slow; (4) [Ca2+]i in the featured spots, 3.09 µm and 5.45 µm, begun slowly to increase when t>70 ms because rate remained unchanged but rate slightly decreased. With [Na+]e zero versus 140 mM [Na+]e SCHs increased: SCH(tIca-peak) by 1.29 folds; SCH(t70 ms) by 1.4 folds; SCH(t[Ca]i-peak) by 1.43 folds; SCH(t100ms) by 1.65 folds; and SCH(t200ms) by 3.04 folds (see Fig. 8G). Furthermore, the NCX inhibition had visible effect on the global [Ca2+]i transient, i.e. global Ca2+ peak increased and during membrane repolarization initially [Ca2+]i suddenly decreased and after that rapidly equilibrated. In agreement with experimental data reported previously [65], the model also predicts increase in global [Ca2+]i peak and no changes in the time to peak when [Na+]e is completely substituted with 140 mM Li+, (see dashed line in Fig. 8E). New finding is that, with zero extracellular Na+, the Ca2+ signal spreads from the external membrane to the cell center as continuum Ca2+ wave initiated after 56 ms but very soon this wave faltered (see Video S3, right panel).

Bottom Line: In agreement with experiment, in the presence of fluorescence dye and inhibited sarcoplasmic reticulum, the lack of detectible differences in the depolarization-evoked Ca(2+) transients was found when the Ca(2+) flux was heterogeneously distributed along the sarcolemma.Even at modest elevation of Ca(2+), reached during Ca(2+) influx, large and steep Ca(2+) gradients are found in the narrow sub-sarcolemmal space.The model predicts that the branched t-tubule structure and changes in the normal Ca(2+) flux density along the cell membrane support initiation and propagation of Ca(2+) waves in rat myocytes.

View Article: PubMed Central - PubMed

Affiliation: Department of Bioengineering, University of California San Diego, La Jolla, California, United States of America.

ABSTRACT
The t-tubules of mammalian ventricular myocytes are invaginations of the cell membrane that occur at each Z-line. These invaginations branch within the cell to form a complex network that allows rapid propagation of the electrical signal, and hence synchronous rise of intracellular calcium (Ca(2+)). To investigate how the t-tubule microanatomy and the distribution of membrane Ca(2+) flux affect cardiac excitation-contraction coupling we developed a 3-D continuum model of Ca(2+) signaling, buffering and diffusion in rat ventricular myocytes. The transverse-axial t-tubule geometry was derived from light microscopy structural data. To solve the nonlinear reaction-diffusion system we extended SMOL software tool (http://mccammon.ucsd.edu/smol/). The analysis suggests that the quantitative understanding of the Ca(2+) signaling requires more accurate knowledge of the t-tubule ultra-structure and Ca(2+) flux distribution along the sarcolemma. The results reveal the important role for mobile and stationary Ca(2+) buffers, including the Ca(2+) indicator dye. In agreement with experiment, in the presence of fluorescence dye and inhibited sarcoplasmic reticulum, the lack of detectible differences in the depolarization-evoked Ca(2+) transients was found when the Ca(2+) flux was heterogeneously distributed along the sarcolemma. In the absence of fluorescence dye, strongly non-uniform Ca(2+) signals are predicted. Even at modest elevation of Ca(2+), reached during Ca(2+) influx, large and steep Ca(2+) gradients are found in the narrow sub-sarcolemmal space. The model predicts that the branched t-tubule structure and changes in the normal Ca(2+) flux density along the cell membrane support initiation and propagation of Ca(2+) waves in rat myocytes.

Show MeSH
Related in: MedlinePlus