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Effects of Magnesium on Cardiac Excitation-Contraction Coupling

Department of Bioengineering, University of California San Diego, La Jolla CA, USA

Address reprint requests to: Anushka Michailova, Ph.D., Department of Bioengineering, PFBH 241, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0412. E-mail: amihaylo{at}bioeng.ucsd.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION AND CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

 
Objective: Magnesium regulates a large number of cellular processes. Small changes in intracellular free Mg²⁺ ([Mg²⁺]_i) may have important effects on cardiac excitability and contractility. We investigated the effects of [Mg²⁺]_i on cardiac excitation-contraction coupling.

Methods: We used our ionic-metabolic model that incorporates equations for Ca²⁺ and Mg²⁺ buffering and transport by ATP and ADP and equations for MgATP regulation of ion transporters (Na⁺-K⁺ pump, sarcolemmal and sarcoplasmic Ca²⁺ pumps).

Results: Model results indicate that variations in cytosolic Mg²⁺ level might sensitively affect diastolic and systolic Ca²⁺, sarcoplasmic Ca²⁺ content, Ca²⁺ influx through L-type channels, efficiency of the Na⁺/Ca²⁺ exchanger and action potential shape. The analysis suggests that the most important reason for the observed effects is a modified normal function of sarcoplasmic Ca²⁺-ATPase pump by altered diastolic MgATP levels.

Conclusion: The model is able to reproduce qualitatively a sequence of events that correspond well with experimental observations during cardiac excitation-contraction coupling in mammalian ventricular myocytes.

Key words: Mg²⁺, Ca²⁺, cardiac excitation-contraction coupling, computer modelling

	INTRODUCTION

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Magnesium ions (Mg²⁺) play a fundamental role in cellular function, but the effects of alterations in the concentration of intracellular free magnesium ([Mg²⁺]_i) on cell excitability and cardiac excitation-contraction coupling (E-C coupling) are incompletely understood.

In 2001, we [1] extended the model of the ventricular myocyte by Winslow et al [2] incorporating equations for Ca²⁺ and Mg²⁺ buffering and transport by ATP and ADP and equations for MgATP regulation of ion transporters (Na⁺-K⁺ pump, sarcolemmal and sarcoplasmic Ca²⁺ pumps). This new ionic/metabolic model provided an opportunity for the first time to examine and predict theoretically how the variations in [Mg²⁺]_i might affect ionic currents, Ca²⁺ transient and action potential shape.

The model analyses suggest that the changes in [Mg²⁺]_i may significantly affect normal cell function. Our model predictions are in general agreement with experimental data measured under normal and pathological conditions during cardiac excitation-contraction coupling in mammalian ventricular myocytes [3–5]. Preliminary results of this work have been presented at the Gordon Research Conferences "Magnesium in Biochemical Processes and Medicine" in abstract form [6].

	MATERIALS AND METHODS

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Taking into consideration experimental and theoretical observations [2,7] that Ca²⁺ near the plasma membrane can reach concentrations much higher that those in the myoplasm, we [1] allowed free and bound Mg²⁺, ATP and ADP concentrations near the membrane to differ from free and bound Mg²⁺, ATP and ADP concentrations in the myoplasm. Experimental and theoretical studies also indicate that ATP and ADP are mobile buffers [7,8]. To simulate diffusion of Mg²⁺, ATP and ADP from the subspace to the myoplasm we included fluxes for Mg²⁺, CaATP, MgATP, CaADP, and MgADP. In the myocytes, ATP and ADP not only bind and transport Ca²⁺ and Mg²⁺ but also regulate intracellular enzymes, ATP-dependent transporters and channels [7,9]. ATP (as MgATP) is the preferred substrate for various ATPases.

From these experimental observations, we [1] assumed that changes in diastolic and systolic myoplasmic MgATP concentration ([MgATP]_i) regulate SR Ca²⁺-ATPase, Na⁺-K⁺ ATPase and sarcolemmal Ca²⁺ ATPase. In the model, pump currents and fluxes are proportional to [MgATP]_i and changes in ATP and ADP concentrations indirectly regulate L-type Ca²⁺ current (I_Ca) via subspace Ca²⁺ concentration changes.

	RESULTS

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Cytosolic Free Magnesium
Simulations with the model showed that a fall in free Mg²⁺ (respectively in total cytosolic Mg²⁺): (a) significantly prolonged action potential duration (Fig. 1F); (b) increased Ca²⁺ current through L-type channels (Fig. 1D); (c) increased diastolic Ca²⁺, slightly decreased systolic Ca²⁺ peak and Ca²⁺ signal decayed more slowly than the normal Ca²⁺ signal (Fig. 1A); (d) modified normal Ca²⁺ transient in the sarcoplasmic reticulum (Fig. 1B); (e) sensitively affected the efficiency of the Na⁺/Ca²⁺ (I_NaCa) exchanger in extruding Ca²⁺ (Fig. 1E).

View larger version (25K): [in this window] [in a new window]   Fig. 1. Model outputs in response to 1-Hz pulse train (9–10 s). Panels A–B, D–F: Free Mg2+ 1 mM (solid line), 0.5 mM (dash-dot line), 0.3 mM (dotted line), 0.2 mM (dash line), 0.1 mM (dash-dot-dot line). Bottom trace in (D) shows an expanded view of the peak of L-type Ca2+ current at different [Mg2+]i. Panel C: Free Mg2+ 1 mM (solid line), 3 mM (dotted line), 5 mM (dash line). [ATP]tot 7 mM, [ADP]tot 5 µM.

Surprisingly, the model predicted that increasing free Mg²⁺ from 1 mM to 5 mM slightly affected action potential shape (Fig. 1C), systolic Ca²⁺ transient, I_Ca and I_NaCa currents as well diastolic MgATP levels (i.e. the activity of SR Ca²⁺-ATPase pump), (not shown).

Cytosolic ATP, ADP and Mg²⁺
An application of this model analysis was our ability to predict how the simultaneous changes in [ATP]_tot, [ADP]_tot and [Mg²⁺]_i might affect cardiac E-C coupling.

Adding 1 mM Mg²⁺, 7 mM ATP and 5 µM ADP (normal conditions) in the Winslow et al. model [2] slightly affected the amplitude and time course of intracellular Ca²⁺ signals [1] and decreased action potential duration, insignificantly (Fig. 2), solid line. However, the block of oxidative metabolism (i.e. fall [ATP]_tot/[ADP]_tot) [1] significantly reduced the sarcoplasmic Ca²⁺ content, increased diastolic Ca²⁺, lowered systolic Ca²⁺, increased Ca²⁺ influx through L-type channels and decreased the efficiency of the Na⁺/Ca²⁺ exchanger in extruding Ca²⁺. These simulations also resulted in an increase in action potential duration with 4 mM [K⁺]_o (Fig. 2), dash-dot-dot, but the model did not include the ATP-sensitive K⁺ channels, which contribute to action potential shortening observed during ischemia.

View larger version (12K): [in this window] [in a new window]   Fig. 2. Adding ATP and ADP as mobile Ca2+ and Mg2+ buffers slightly shorten the action potential (9–10 s). ATP, ADP and Mg2+ not included (dash-dot line). Normal conditions: [ATP]tot 7 mM, [ADP]tot 5 µM, [Mg2+]i 1 mM (solid line). Metabolic inhibition: [ATP]tot 3 mM, [ADP]tot 3 mM, [Mg2+]i 2.25 mM (dash-dot-dot line).

	DISCUSSION AND CONCLUSIONS

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The main goal of the present study was to examine how the changes in free intracellular Mg²⁺ might modulate the integrated process of E-C coupling in heart cells.

The most important result was the observation that changes in the diastolic [MgATP]_i, as a consequence of the changes in free cytosolic Mg²⁺ and/or [ATP]_tot/[ADP]_tot ratio (respectively [Mg²⁺]_i), might significantly affect sarcoplasmic Ca²⁺-ATPase pump activities, in turn altering normal ion and buffer concentrations, ion currents and action potential shape.

Our theoretical predictions have resulted in a close qualitative agreement with the experimental data [3–5,9], but this model was not able to explain the experimental observations that: (a) the variations in [Mg²⁺]_i (1–0.1 mM) have a pronounced effect on L-type Ca²⁺ current; (b) the elevations in [Mg²⁺]_i (1–9.4 mM) shorten the action potential and suppress the L-type Ca²⁺ current; and (c) the fall in [ATP]_tot/[ADP]_tot decreases action potential duration. Therefore, we will continue to refine and extend the model since many of the important processes occurring in normal conditions and during metabolic inhibition are not yet included.

	ACKNOWLEDGMENTS

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This work was supported by National Biomedical Computational Resource (2 P41 RR08605) and the National Space Biomedical Research Institute (IHF00207).

Received August 5, 2004.

	REFERENCES

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1. Michailova A, McCulloch A: Model study of ATP and ADP buffering, transport of Ca²⁺ and Mg²⁺, and regulation of ion pumps in ventricular myocyte.Biophys J81 :614 –629,2001 .[Abstract/Free Full Text]
2. Winslow RL, Rice J, Jafri S, Marban E, O’Rourke B: Mechanisms of altered excitation-contraction coupling in canine tachycardia-induced heart failure. II. Model studies.Circ Res84 :571 –586,1999 .[Abstract/Free Full Text]
3. Agus ZA, Kelepouris E, Dukes I, Morad M: Cytosolic magnesium modulates calcium channel activity in mammalian ventricular cells.Am J Physiol256 :C425 –C455,1989 .
4. Tashiro M, Spencer CI, Berlin JR: Modulation of cardiac excitation-contraction coupling by cytosolic [Mg²⁺].Biophys J82 :352 ,2002 .
5. Wei S, Quigley JF, Hanlon SU, O’Rourke B, Haigney MCP: Cytosolic free magnesium modulates Na/Ca exchange currents in pig myocytes.Cardiovascular Res53 :334 –340,2002 .[Abstract/Free Full Text]
6. Michailova A, Thomas ME, McCulloch A:Coupling ionic currents and calcium and magnesium cycling with ATP and ADP in a ventricular myocyte model. Gordon Research Conferences: Magnesium in biochemical processes and medicine, Ventura, USA,2002 .
7. Bers DM: "Excitation-Contraction Coupling and Cardiac Contractile Force ." Dortrecht, Boston, London: Kluwer Academic Press,2001 .
8. Michailova A, DelPrincipe F, Egger M, Niggli E: Spatiotemporal features of Ca²⁺ signaling, buffering and diffusion in atrial myocytes with inhibited sarcoplasmic reticulum.Biophys J.83 :3134 –3151,2002 .[Abstract/Free Full Text]
9. Carmeliet E: Cardiac ionic currents and acute ischemia: from channels to arrhythmias.Physiol Rev79 :917 –1017,1999 .[Abstract/Free Full Text]
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