Limits...
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).

Show MeSH

Related in: MedlinePlus

The influence of zX(g) on the relationship between extracellular [K+] and Em compared with the effects of corresponding alterations in [Ca2+]e. (A) Charge-difference modeling of the influence of five different concentrations of [K+]e (diamonds, 0.75 mM; triangles, 1 mM; circles, 1.5 mM; squares, 2.5 mM; and crosses, 5.0 mM) upon steady-state Em. Note that zX(g) has little influence on Em at 5.0 or 2.5 mM [K+]e, provokes a maximum of ∼10 mV difference at 0.75 mM [K+]e, and can produce 20 mV shifts in Em at intermediate [K+]e values of 1 to 1.5 mM. (B) Comparison of model predictions (open bars) to experimental data (solid bars) for normal (1.8 mM) versus low (nominally zero) [Ca2+]e solutions over a range of [K+]e. The experimental data demonstrate a significant (p < 0.01) depolarization of Em in low compared with normal [Ca2+]e solutions at 1 mM [K+]e, but no significant difference in Em (p > 0.01) in the different [Ca2+]e conditions at normal or very low values of [K+]e, confirming model predictions.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2480687&req=5

fig8: The influence of zX(g) on the relationship between extracellular [K+] and Em compared with the effects of corresponding alterations in [Ca2+]e. (A) Charge-difference modeling of the influence of five different concentrations of [K+]e (diamonds, 0.75 mM; triangles, 1 mM; circles, 1.5 mM; squares, 2.5 mM; and crosses, 5.0 mM) upon steady-state Em. Note that zX(g) has little influence on Em at 5.0 or 2.5 mM [K+]e, provokes a maximum of ∼10 mV difference at 0.75 mM [K+]e, and can produce 20 mV shifts in Em at intermediate [K+]e values of 1 to 1.5 mM. (B) Comparison of model predictions (open bars) to experimental data (solid bars) for normal (1.8 mM) versus low (nominally zero) [Ca2+]e solutions over a range of [K+]e. The experimental data demonstrate a significant (p < 0.01) depolarization of Em in low compared with normal [Ca2+]e solutions at 1 mM [K+]e, but no significant difference in Em (p > 0.01) in the different [Ca2+]e conditions at normal or very low values of [K+]e, confirming model predictions.

Mentions: Fig. 8 demonstrates one such manipulation of extracellular conditions. It shows the influence of changes in extracellular [K+] upon Em. This is a maneuver expected to decrease the energetic favorability of Na+/K+-ATPase activity, while also increasing passive transmembrane leak currents. Fig. 8 A demonstrates the results of charge-difference modeling of the influence of zX(g) upon Em at different values of [K+]e from 5 mM to 0.75 mM for a model cell with a constant value of N within the physiological range for skeletal muscle. At higher values of [K+]e of 5 mM and 2.5 mM (Fig. 8 A, crosses and squares, respectively), zX(g) has little influence upon Em, although, as expected, the lower value of [K+]e produces a more polarized model cell. This lack of influence of zX(g) upon Em reflects the fact that Na+/K+-ATPase activity is able to increase at more negative values of zX(g), thus largely offsetting the resultant increase in leak currents. However, further reduction in [K+]e to 1.5 mM or 1 mM permitted two further observations. First, relative depolarization of Em was seen with reductions in [K+]e at certain (with 1.5 mM [K+]e) or all (with 1 mM [K+]e) values of zX(g). This results primarily from a reduction in Na+/K+-ATPase activity at low values of [K+]e. Second, this reduction in Na+/K+-ATPase activity had the effect of pushing N into the window of values within which zX(g) was able to have a significant influence on Em, such that more negative values of zX(g) produced depolarization. In contrast, this effect was not seen in solutions of higher [K+]e. Finally, when [K+]e was reduced further, the influence of zX(g) upon Em was also reduced, reflecting severe limitation of Na+/K+-ATPase activity at all values of zX(g).


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

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

The influence of zX(g) on the relationship between extracellular [K+] and Em compared with the effects of corresponding alterations in [Ca2+]e. (A) Charge-difference modeling of the influence of five different concentrations of [K+]e (diamonds, 0.75 mM; triangles, 1 mM; circles, 1.5 mM; squares, 2.5 mM; and crosses, 5.0 mM) upon steady-state Em. Note that zX(g) has little influence on Em at 5.0 or 2.5 mM [K+]e, provokes a maximum of ∼10 mV difference at 0.75 mM [K+]e, and can produce 20 mV shifts in Em at intermediate [K+]e values of 1 to 1.5 mM. (B) Comparison of model predictions (open bars) to experimental data (solid bars) for normal (1.8 mM) versus low (nominally zero) [Ca2+]e solutions over a range of [K+]e. The experimental data demonstrate a significant (p < 0.01) depolarization of Em in low compared with normal [Ca2+]e solutions at 1 mM [K+]e, but no significant difference in Em (p > 0.01) in the different [Ca2+]e conditions at normal or very low values of [K+]e, confirming model predictions.
© Copyright Policy
Related In: Results  -  Collection

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

fig8: The influence of zX(g) on the relationship between extracellular [K+] and Em compared with the effects of corresponding alterations in [Ca2+]e. (A) Charge-difference modeling of the influence of five different concentrations of [K+]e (diamonds, 0.75 mM; triangles, 1 mM; circles, 1.5 mM; squares, 2.5 mM; and crosses, 5.0 mM) upon steady-state Em. Note that zX(g) has little influence on Em at 5.0 or 2.5 mM [K+]e, provokes a maximum of ∼10 mV difference at 0.75 mM [K+]e, and can produce 20 mV shifts in Em at intermediate [K+]e values of 1 to 1.5 mM. (B) Comparison of model predictions (open bars) to experimental data (solid bars) for normal (1.8 mM) versus low (nominally zero) [Ca2+]e solutions over a range of [K+]e. The experimental data demonstrate a significant (p < 0.01) depolarization of Em in low compared with normal [Ca2+]e solutions at 1 mM [K+]e, but no significant difference in Em (p > 0.01) in the different [Ca2+]e conditions at normal or very low values of [K+]e, confirming model predictions.
Mentions: Fig. 8 demonstrates one such manipulation of extracellular conditions. It shows the influence of changes in extracellular [K+] upon Em. This is a maneuver expected to decrease the energetic favorability of Na+/K+-ATPase activity, while also increasing passive transmembrane leak currents. Fig. 8 A demonstrates the results of charge-difference modeling of the influence of zX(g) upon Em at different values of [K+]e from 5 mM to 0.75 mM for a model cell with a constant value of N within the physiological range for skeletal muscle. At higher values of [K+]e of 5 mM and 2.5 mM (Fig. 8 A, crosses and squares, respectively), zX(g) has little influence upon Em, although, as expected, the lower value of [K+]e produces a more polarized model cell. This lack of influence of zX(g) upon Em reflects the fact that Na+/K+-ATPase activity is able to increase at more negative values of zX(g), thus largely offsetting the resultant increase in leak currents. However, further reduction in [K+]e to 1.5 mM or 1 mM permitted two further observations. First, relative depolarization of Em was seen with reductions in [K+]e at certain (with 1.5 mM [K+]e) or all (with 1 mM [K+]e) values of zX(g). This results primarily from a reduction in Na+/K+-ATPase activity at low values of [K+]e. Second, this reduction in Na+/K+-ATPase activity had the effect of pushing N into the window of values within which zX(g) was able to have a significant influence on Em, such that more negative values of zX(g) produced depolarization. In contrast, this effect was not seen in solutions of higher [K+]e. Finally, when [K+]e was reduced further, the influence of zX(g) upon Em was also reduced, reflecting severe limitation of Na+/K+-ATPase activity at all values of zX(g).

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