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Extracellular charge adsorption influences intracellular electrochemical homeostasis in amphibian skeletal muscle.

Mehta AR, Huang CL, Skepper JN, Fraser JA - Biophys. J. (2008)

Bottom Line: The membrane potential measured by intracellular electrodes, E(m), is the sum of the transmembrane potential difference (E(1)) between inner and outer cell membrane surfaces and a smaller potential difference (E(2)) between a volume containing fixed charges on or near the outer membrane surface and the bulk extracellular space.First, analytic equations were developed to calculate the influence of charges constrained within a three-dimensional glycocalyceal matrix enveloping the cell membrane outer surface upon local electrical potentials and ion concentrations.Electron microscopy confirmed predictions of these equations that extracellular charge adsorption influences glycocalyceal volume.

View Article: PubMed Central - PubMed

Affiliation: Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom.

ABSTRACT
The membrane potential measured by intracellular electrodes, E(m), is the sum of the transmembrane potential difference (E(1)) between inner and outer cell membrane surfaces and a smaller potential difference (E(2)) between a volume containing fixed charges on or near the outer membrane surface and the bulk extracellular space. This study investigates the influence of E(2) upon transmembrane ion fluxes, and hence cellular electrochemical homeostasis, using an integrative approach that combines computational and experimental methods. First, analytic equations were developed to calculate the influence of charges constrained within a three-dimensional glycocalyceal matrix enveloping the cell membrane outer surface upon local electrical potentials and ion concentrations. Electron microscopy confirmed predictions of these equations that extracellular charge adsorption influences glycocalyceal volume. Second, the novel analytic glycocalyx formulation was incorporated into the charge-difference cellular model of Fraser and Huang to simulate the influence of extracellular fixed charges upon intracellular ionic homeostasis. Experimental measurements of E(m) supported the resulting predictions that an increased magnitude of extracellular fixed charge increases net transmembrane ionic leak currents, resulting in either a compensatory increase in Na(+)/K(+)-ATPase activity, or, in cells with reduced Na(+)/K(+)-ATPase activity, a partial dissipation of transmembrane ionic gradients and depolarization of E(m).

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The predicted influence of glycocalyceal charge upon steady-state transmembrane potentials and Na+/K+-ATPase activity. Charge-difference modeling was used to assess the influence of membrane effective Na+/K+-ATPase density (N) upon steady-state E1 (A), Em (B); Na+/K+-ATPase activity (JPump) (C), and [K+]i and [Na+]i (D) in model muscle fibers with differing glycocalyceal charges from zX(g) = −100 (A–C, upper lines, and D, outer lines) to zX(g) = +100 (A–C, lower lines, and D, inner lines) in logarithmic intervals (thus, from top to bottom, or outside to inside, zX(g) = −100, −10, −1, 0, 1, 10, and 100). η = 1.03 in each case. Note that greater values of η increase the influence of changes in zX(g) in each case (data not shown), similar to the influence shown in Fig. 4. More negative values of zX(g) produce depolarization of E1 (A) and higher steady-state Na+/K+-ATPase activity (C). However, only at lower values of N does this influence Em (B). Note that very low values of N are insufficient to maintain a stable cell in all cases, as shown (D) by the increase in the sum of [K+]i and [Na+]i, reflecting reduced [X]i, secondary to cell swelling.
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fig7: The predicted influence of glycocalyceal charge upon steady-state transmembrane potentials and Na+/K+-ATPase activity. Charge-difference modeling was used to assess the influence of membrane effective Na+/K+-ATPase density (N) upon steady-state E1 (A), Em (B); Na+/K+-ATPase activity (JPump) (C), and [K+]i and [Na+]i (D) in model muscle fibers with differing glycocalyceal charges from zX(g) = −100 (A–C, upper lines, and D, outer lines) to zX(g) = +100 (A–C, lower lines, and D, inner lines) in logarithmic intervals (thus, from top to bottom, or outside to inside, zX(g) = −100, −10, −1, 0, 1, 10, and 100). η = 1.03 in each case. Note that greater values of η increase the influence of changes in zX(g) in each case (data not shown), similar to the influence shown in Fig. 4. More negative values of zX(g) produce depolarization of E1 (A) and higher steady-state Na+/K+-ATPase activity (C). However, only at lower values of N does this influence Em (B). Note that very low values of N are insufficient to maintain a stable cell in all cases, as shown (D) by the increase in the sum of [K+]i and [Na+]i, reflecting reduced [X]i, secondary to cell swelling.

Mentions: Fig. 7, A–D, shows an extension of the theoretical analysis to explore the influence of zX(g) upon critical cellular parameters E1, Em, Na+/K+-ATPase activity (JPump), and intracellular [Na+] and [K+], respectively, for model cells with a range of values for the Na+/K+-ATPase density (N). It shows that in all cases, values of N above a certain threshold have very little influence on any modeled parameter. This is because the magnitudes of Em and the transmembrane ion gradients are then close to their respective maxima and are therefore determined by the thermodynamic constraints upon Na+/K+-ATPase activity (10) rather than by N. When N is high, more negative values of zX(g) both increase the steady-state Na+/K+-ATPase activity and depolarize E1, whereas more positive values have the opposite influence. However, despite this significant influence upon E1, zX(g) at higher values of N has very little influence on Em. Very low values of N are not compatible with cell stability. It is only when N is at an intermediate value—adequate for cellular stability while permitting little increase in pump activity—that zX(g) may significantly influence Em.


Extracellular charge adsorption influences intracellular electrochemical homeostasis in amphibian skeletal muscle.

Mehta AR, Huang CL, Skepper JN, Fraser JA - Biophys. J. (2008)

The predicted influence of glycocalyceal charge upon steady-state transmembrane potentials and Na+/K+-ATPase activity. Charge-difference modeling was used to assess the influence of membrane effective Na+/K+-ATPase density (N) upon steady-state E1 (A), Em (B); Na+/K+-ATPase activity (JPump) (C), and [K+]i and [Na+]i (D) in model muscle fibers with differing glycocalyceal charges from zX(g) = −100 (A–C, upper lines, and D, outer lines) to zX(g) = +100 (A–C, lower lines, and D, inner lines) in logarithmic intervals (thus, from top to bottom, or outside to inside, zX(g) = −100, −10, −1, 0, 1, 10, and 100). η = 1.03 in each case. Note that greater values of η increase the influence of changes in zX(g) in each case (data not shown), similar to the influence shown in Fig. 4. More negative values of zX(g) produce depolarization of E1 (A) and higher steady-state Na+/K+-ATPase activity (C). However, only at lower values of N does this influence Em (B). Note that very low values of N are insufficient to maintain a stable cell in all cases, as shown (D) by the increase in the sum of [K+]i and [Na+]i, reflecting reduced [X]i, secondary to cell swelling.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2480687&req=5

fig7: The predicted influence of glycocalyceal charge upon steady-state transmembrane potentials and Na+/K+-ATPase activity. Charge-difference modeling was used to assess the influence of membrane effective Na+/K+-ATPase density (N) upon steady-state E1 (A), Em (B); Na+/K+-ATPase activity (JPump) (C), and [K+]i and [Na+]i (D) in model muscle fibers with differing glycocalyceal charges from zX(g) = −100 (A–C, upper lines, and D, outer lines) to zX(g) = +100 (A–C, lower lines, and D, inner lines) in logarithmic intervals (thus, from top to bottom, or outside to inside, zX(g) = −100, −10, −1, 0, 1, 10, and 100). η = 1.03 in each case. Note that greater values of η increase the influence of changes in zX(g) in each case (data not shown), similar to the influence shown in Fig. 4. More negative values of zX(g) produce depolarization of E1 (A) and higher steady-state Na+/K+-ATPase activity (C). However, only at lower values of N does this influence Em (B). Note that very low values of N are insufficient to maintain a stable cell in all cases, as shown (D) by the increase in the sum of [K+]i and [Na+]i, reflecting reduced [X]i, secondary to cell swelling.
Mentions: Fig. 7, A–D, shows an extension of the theoretical analysis to explore the influence of zX(g) upon critical cellular parameters E1, Em, Na+/K+-ATPase activity (JPump), and intracellular [Na+] and [K+], respectively, for model cells with a range of values for the Na+/K+-ATPase density (N). It shows that in all cases, values of N above a certain threshold have very little influence on any modeled parameter. This is because the magnitudes of Em and the transmembrane ion gradients are then close to their respective maxima and are therefore determined by the thermodynamic constraints upon Na+/K+-ATPase activity (10) rather than by N. When N is high, more negative values of zX(g) both increase the steady-state Na+/K+-ATPase activity and depolarize E1, whereas more positive values have the opposite influence. However, despite this significant influence upon E1, zX(g) at higher values of N has very little influence on Em. Very low values of N are not compatible with cell stability. It is only when N is at an intermediate value—adequate for cellular stability while permitting little increase in pump activity—that zX(g) may significantly influence Em.

Bottom Line: The membrane potential measured by intracellular electrodes, E(m), is the sum of the transmembrane potential difference (E(1)) between inner and outer cell membrane surfaces and a smaller potential difference (E(2)) between a volume containing fixed charges on or near the outer membrane surface and the bulk extracellular space.First, analytic equations were developed to calculate the influence of charges constrained within a three-dimensional glycocalyceal matrix enveloping the cell membrane outer surface upon local electrical potentials and ion concentrations.Electron microscopy confirmed predictions of these equations that extracellular charge adsorption influences glycocalyceal volume.

View Article: PubMed Central - PubMed

Affiliation: Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom.

ABSTRACT
The membrane potential measured by intracellular electrodes, E(m), is the sum of the transmembrane potential difference (E(1)) between inner and outer cell membrane surfaces and a smaller potential difference (E(2)) between a volume containing fixed charges on or near the outer membrane surface and the bulk extracellular space. This study investigates the influence of E(2) upon transmembrane ion fluxes, and hence cellular electrochemical homeostasis, using an integrative approach that combines computational and experimental methods. First, analytic equations were developed to calculate the influence of charges constrained within a three-dimensional glycocalyceal matrix enveloping the cell membrane outer surface upon local electrical potentials and ion concentrations. Electron microscopy confirmed predictions of these equations that extracellular charge adsorption influences glycocalyceal volume. Second, the novel analytic glycocalyx formulation was incorporated into the charge-difference cellular model of Fraser and Huang to simulate the influence of extracellular fixed charges upon intracellular ionic homeostasis. Experimental measurements of E(m) supported the resulting predictions that an increased magnitude of extracellular fixed charge increases net transmembrane ionic leak currents, resulting in either a compensatory increase in Na(+)/K(+)-ATPase activity, or, in cells with reduced Na(+)/K(+)-ATPase activity, a partial dissipation of transmembrane ionic gradients and depolarization of E(m).

Show MeSH
Related in: MedlinePlus