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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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Charge-difference modeling of the influence of glycocalyceal charge upon steady-state transmembrane potentials. CD modeling was used to assess the influence of glycocalyceal fixed charge valency (zX(g)) upon transmembrane potentials. Negative fixed charges cause negative glycocalyceal potentials (E2) and relative depolarization of E1 such that Em is unchanged, whereas positive fixed charges produce positive surface potentials and hyperpolarization of E1, again leaving Em unchanged. Note that large magnitudes of E2 are prevented because osmotic swelling of the glycocalyx reduces fixed charge density as zX(g) increases. As shown in Fig. 4, higher magnitudes of η permit higher magnitudes of fixed charge density and hence increase the influence of zX(g) upon E2.
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fig6: Charge-difference modeling of the influence of glycocalyceal charge upon steady-state transmembrane potentials. CD modeling was used to assess the influence of glycocalyceal fixed charge valency (zX(g)) upon transmembrane potentials. Negative fixed charges cause negative glycocalyceal potentials (E2) and relative depolarization of E1 such that Em is unchanged, whereas positive fixed charges produce positive surface potentials and hyperpolarization of E1, again leaving Em unchanged. Note that large magnitudes of E2 are prevented because osmotic swelling of the glycocalyx reduces fixed charge density as zX(g) increases. As shown in Fig. 4, higher magnitudes of η permit higher magnitudes of fixed charge density and hence increase the influence of zX(g) upon E2.

Mentions: The CD model of Fraser and Huang (5) was modified as described in Methods to include the glycocalyx GD formulation, thereby allowing simulation of the influence of glycocalyceal charge upon intracellular ion concentrations. Thus, transmembrane ion fluxes were calculated in the CD model from the electrochemical gradients between the bulk intracellular space and the glycocalyx, while the potential and ion concentrations within the glycocalyx were calculated using the GD analysis. Thus, Fig. 6 demonstrates CD modeling of the influence of glycocalyceal charge upon a model resting cell with ion channel and pump density parameters within the physiological range for skeletal muscle. It shows that under physiological conditions, the model predicts that zX(g) influences E1 and E2, but not Em (Fig. 1).


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

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

Charge-difference modeling of the influence of glycocalyceal charge upon steady-state transmembrane potentials. CD modeling was used to assess the influence of glycocalyceal fixed charge valency (zX(g)) upon transmembrane potentials. Negative fixed charges cause negative glycocalyceal potentials (E2) and relative depolarization of E1 such that Em is unchanged, whereas positive fixed charges produce positive surface potentials and hyperpolarization of E1, again leaving Em unchanged. Note that large magnitudes of E2 are prevented because osmotic swelling of the glycocalyx reduces fixed charge density as zX(g) increases. As shown in Fig. 4, higher magnitudes of η permit higher magnitudes of fixed charge density and hence increase the influence of zX(g) upon E2.
© Copyright Policy
Related In: Results  -  Collection

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

fig6: Charge-difference modeling of the influence of glycocalyceal charge upon steady-state transmembrane potentials. CD modeling was used to assess the influence of glycocalyceal fixed charge valency (zX(g)) upon transmembrane potentials. Negative fixed charges cause negative glycocalyceal potentials (E2) and relative depolarization of E1 such that Em is unchanged, whereas positive fixed charges produce positive surface potentials and hyperpolarization of E1, again leaving Em unchanged. Note that large magnitudes of E2 are prevented because osmotic swelling of the glycocalyx reduces fixed charge density as zX(g) increases. As shown in Fig. 4, higher magnitudes of η permit higher magnitudes of fixed charge density and hence increase the influence of zX(g) upon E2.
Mentions: The CD model of Fraser and Huang (5) was modified as described in Methods to include the glycocalyx GD formulation, thereby allowing simulation of the influence of glycocalyceal charge upon intracellular ion concentrations. Thus, transmembrane ion fluxes were calculated in the CD model from the electrochemical gradients between the bulk intracellular space and the glycocalyx, while the potential and ion concentrations within the glycocalyx were calculated using the GD analysis. Thus, Fig. 6 demonstrates CD modeling of the influence of glycocalyceal charge upon a model resting cell with ion channel and pump density parameters within the physiological range for skeletal muscle. It shows that under physiological conditions, the model predicts that zX(g) influences E1 and E2, but not Em (Fig. 1).

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