Limits...
Lens intracellular hydrostatic pressure is generated by the circulation of sodium and modulated by gap junction coupling.

Gao J, Sun X, Moore LC, White TW, Brink PR, Mathias RT - J. Gen. Physiol. (2011)

Bottom Line: Intracellular hydrostatic pressure in lenses from these mouse models varied inversely with the number of channels.When the lens' circulation of Na(+) was either blocked or reduced, intracellular hydrostatic pressure in central fiber cells was either eliminated or reduced proportionally.These data are consistent with our hypotheses: fluid circulates through the lens; the intracellular leg of fluid circulation is through gap junction channels and is driven by hydrostatic pressure; and the fluid flow is generated by membrane transport of sodium.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physiology and Biophysics, SUNY at Stony Brook, NY 11794, USA.

ABSTRACT
We recently modeled fluid flow through gap junction channels coupling the pigmented and nonpigmented layers of the ciliary body. The model suggested the channels could transport the secretion of aqueous humor, but flow would be driven by hydrostatic pressure rather than osmosis. The pressure required to drive fluid through a single layer of gap junctions might be just a few mmHg and difficult to measure. In the lens, however, there is a circulation of Na(+) that may be coupled to intracellular fluid flow. Based on this hypothesis, the fluid would cross hundreds of layers of gap junctions, and this might require a large hydrostatic gradient. Therefore, we measured hydrostatic pressure as a function of distance from the center of the lens using an intracellular microelectrode-based pressure-sensing system. In wild-type mouse lenses, intracellular pressure varied from ∼330 mmHg at the center to zero at the surface. We have several knockout/knock-in mouse models with differing levels of expression of gap junction channels coupling lens fiber cells. Intracellular hydrostatic pressure in lenses from these mouse models varied inversely with the number of channels. When the lens' circulation of Na(+) was either blocked or reduced, intracellular hydrostatic pressure in central fiber cells was either eliminated or reduced proportionally. These data are consistent with our hypotheses: fluid circulates through the lens; the intracellular leg of fluid circulation is through gap junction channels and is driven by hydrostatic pressure; and the fluid flow is generated by membrane transport of sodium.

Show MeSH

Related in: MedlinePlus

The effect on intracellular hydrostatic pressure of approximately halving the number of gap junction channels coupling the MFs. (A) The standing hydrostatic pressure gradient in lenses from Cx46+/− KO mice, which were ∼2 mo old. The hydrostatic pressure (pi mmHg) is graphed as a function of normalized distance (r/a) from the lens center, where a (cm) is the lens radius, and r (cm) is the distance from the lens center. The data are from 12 lenses from six mice. The pressures at two to six radial locations were recorded from each lens. The smooth curve is the best fit of Eq. 7 to the data. Because the manometer can only measure ∼400 mmHg, the pressures at locations closer to the lens center than ∼0.55a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in Cx46+/− KO lenses was ∼50% of that in WT lenses (Mathias et al., 2010). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or approximately two times greater in the KO than WT lenses. The best fits of the model to the data give the ratio of pi(0) in Cx46+/− KO/WT lenses as 1.92. (B) An over-plot of the Cx46+/− and WT data.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC3105514&req=5

fig6: The effect on intracellular hydrostatic pressure of approximately halving the number of gap junction channels coupling the MFs. (A) The standing hydrostatic pressure gradient in lenses from Cx46+/− KO mice, which were ∼2 mo old. The hydrostatic pressure (pi mmHg) is graphed as a function of normalized distance (r/a) from the lens center, where a (cm) is the lens radius, and r (cm) is the distance from the lens center. The data are from 12 lenses from six mice. The pressures at two to six radial locations were recorded from each lens. The smooth curve is the best fit of Eq. 7 to the data. Because the manometer can only measure ∼400 mmHg, the pressures at locations closer to the lens center than ∼0.55a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in Cx46+/− KO lenses was ∼50% of that in WT lenses (Mathias et al., 2010). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or approximately two times greater in the KO than WT lenses. The best fits of the model to the data give the ratio of pi(0) in Cx46+/− KO/WT lenses as 1.92. (B) An over-plot of the Cx46+/− and WT data.

Mentions: Homozygous KO of Cx46 (Cx46−/−) caused complete loss of gap junction coupling between MFs, leading to loss of calcium homeostasis in MFs and a dense central cataract (Gao et al., 2004). However, heterozygous KO of Cx46 (Cx46+/−) yielded healthy lenses that were transparent, but GDF went from ∼1 S/cm2 in WT to 0.75 S/cm2 in Cx46+/− lenses, and GMF went from 0.5 S/cm2 to 0.25 S/cm2, because of about a 50% reduction in the amount of Cx46 protein (Mathias et al., 2010). Pooled pressure measurements from 12 Cx46+/− lenses are graphed as a function of normalized radial location in Fig. 6 A. At ∼45% of the distance into the lenses (r/a = 0.55), the pressures consistently exceeded 400 mmHg and could not be determined using the manometer. So again, as in Fig. 5, the average central pressure was estimated from curve fitting Eq. 7 to the peripheral pressure data. Based on the curve fit, the average value of pi(0) was 632 mmHg, or ∼1.9 times that in WT lenses, so halving the number of MF gap junction channels almost doubled the pressure gradient, as expected for water flow through fiber cell gap junction channels. Moreover, in these lenses, the change in slope of the pressure versus radial location at r = b is quite noticeable, even in the raw data. This is also consistent with the water flowing through fiber cell gap junction channels, because gap junction coupling conductance abruptly decreases by about a factor of 3 at the DF to MF transition in Cx46+/− lenses. However, as described below, parallels between DF/MF coupling conductance and hydraulic conductivity are not perfect. In particular, the value of GDF in Cx46+/− lenses was ∼25% lower than in WT lenses, whereas there is no noticeable difference in the best-fit values ΛDF between these two types of lenses. This may simply be because of variability in the pressure data, as discussed in the next paragraph. Fig. 6 B provides an over-plot of pressure data from Cx46(+/−) and WT lenses. In the range of radial locations where Cx46+/− pressures in the MF could be measured, the pressure data are consistently higher than those measured in WT lenses, and the slope of the MF pressure gradient in Cx46+/− lenses is clearly much steeper than that in WT lenses. These data are consistent with water flow through lens fiber cell gap junctions.


Lens intracellular hydrostatic pressure is generated by the circulation of sodium and modulated by gap junction coupling.

Gao J, Sun X, Moore LC, White TW, Brink PR, Mathias RT - J. Gen. Physiol. (2011)

The effect on intracellular hydrostatic pressure of approximately halving the number of gap junction channels coupling the MFs. (A) The standing hydrostatic pressure gradient in lenses from Cx46+/− KO mice, which were ∼2 mo old. The hydrostatic pressure (pi mmHg) is graphed as a function of normalized distance (r/a) from the lens center, where a (cm) is the lens radius, and r (cm) is the distance from the lens center. The data are from 12 lenses from six mice. The pressures at two to six radial locations were recorded from each lens. The smooth curve is the best fit of Eq. 7 to the data. Because the manometer can only measure ∼400 mmHg, the pressures at locations closer to the lens center than ∼0.55a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in Cx46+/− KO lenses was ∼50% of that in WT lenses (Mathias et al., 2010). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or approximately two times greater in the KO than WT lenses. The best fits of the model to the data give the ratio of pi(0) in Cx46+/− KO/WT lenses as 1.92. (B) An over-plot of the Cx46+/− and WT data.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3105514&req=5

fig6: The effect on intracellular hydrostatic pressure of approximately halving the number of gap junction channels coupling the MFs. (A) The standing hydrostatic pressure gradient in lenses from Cx46+/− KO mice, which were ∼2 mo old. The hydrostatic pressure (pi mmHg) is graphed as a function of normalized distance (r/a) from the lens center, where a (cm) is the lens radius, and r (cm) is the distance from the lens center. The data are from 12 lenses from six mice. The pressures at two to six radial locations were recorded from each lens. The smooth curve is the best fit of Eq. 7 to the data. Because the manometer can only measure ∼400 mmHg, the pressures at locations closer to the lens center than ∼0.55a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in Cx46+/− KO lenses was ∼50% of that in WT lenses (Mathias et al., 2010). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or approximately two times greater in the KO than WT lenses. The best fits of the model to the data give the ratio of pi(0) in Cx46+/− KO/WT lenses as 1.92. (B) An over-plot of the Cx46+/− and WT data.
Mentions: Homozygous KO of Cx46 (Cx46−/−) caused complete loss of gap junction coupling between MFs, leading to loss of calcium homeostasis in MFs and a dense central cataract (Gao et al., 2004). However, heterozygous KO of Cx46 (Cx46+/−) yielded healthy lenses that were transparent, but GDF went from ∼1 S/cm2 in WT to 0.75 S/cm2 in Cx46+/− lenses, and GMF went from 0.5 S/cm2 to 0.25 S/cm2, because of about a 50% reduction in the amount of Cx46 protein (Mathias et al., 2010). Pooled pressure measurements from 12 Cx46+/− lenses are graphed as a function of normalized radial location in Fig. 6 A. At ∼45% of the distance into the lenses (r/a = 0.55), the pressures consistently exceeded 400 mmHg and could not be determined using the manometer. So again, as in Fig. 5, the average central pressure was estimated from curve fitting Eq. 7 to the peripheral pressure data. Based on the curve fit, the average value of pi(0) was 632 mmHg, or ∼1.9 times that in WT lenses, so halving the number of MF gap junction channels almost doubled the pressure gradient, as expected for water flow through fiber cell gap junction channels. Moreover, in these lenses, the change in slope of the pressure versus radial location at r = b is quite noticeable, even in the raw data. This is also consistent with the water flowing through fiber cell gap junction channels, because gap junction coupling conductance abruptly decreases by about a factor of 3 at the DF to MF transition in Cx46+/− lenses. However, as described below, parallels between DF/MF coupling conductance and hydraulic conductivity are not perfect. In particular, the value of GDF in Cx46+/− lenses was ∼25% lower than in WT lenses, whereas there is no noticeable difference in the best-fit values ΛDF between these two types of lenses. This may simply be because of variability in the pressure data, as discussed in the next paragraph. Fig. 6 B provides an over-plot of pressure data from Cx46(+/−) and WT lenses. In the range of radial locations where Cx46+/− pressures in the MF could be measured, the pressure data are consistently higher than those measured in WT lenses, and the slope of the MF pressure gradient in Cx46+/− lenses is clearly much steeper than that in WT lenses. These data are consistent with water flow through lens fiber cell gap junctions.

Bottom Line: Intracellular hydrostatic pressure in lenses from these mouse models varied inversely with the number of channels.When the lens' circulation of Na(+) was either blocked or reduced, intracellular hydrostatic pressure in central fiber cells was either eliminated or reduced proportionally.These data are consistent with our hypotheses: fluid circulates through the lens; the intracellular leg of fluid circulation is through gap junction channels and is driven by hydrostatic pressure; and the fluid flow is generated by membrane transport of sodium.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physiology and Biophysics, SUNY at Stony Brook, NY 11794, USA.

ABSTRACT
We recently modeled fluid flow through gap junction channels coupling the pigmented and nonpigmented layers of the ciliary body. The model suggested the channels could transport the secretion of aqueous humor, but flow would be driven by hydrostatic pressure rather than osmosis. The pressure required to drive fluid through a single layer of gap junctions might be just a few mmHg and difficult to measure. In the lens, however, there is a circulation of Na(+) that may be coupled to intracellular fluid flow. Based on this hypothesis, the fluid would cross hundreds of layers of gap junctions, and this might require a large hydrostatic gradient. Therefore, we measured hydrostatic pressure as a function of distance from the center of the lens using an intracellular microelectrode-based pressure-sensing system. In wild-type mouse lenses, intracellular pressure varied from ∼330 mmHg at the center to zero at the surface. We have several knockout/knock-in mouse models with differing levels of expression of gap junction channels coupling lens fiber cells. Intracellular hydrostatic pressure in lenses from these mouse models varied inversely with the number of channels. When the lens' circulation of Na(+) was either blocked or reduced, intracellular hydrostatic pressure in central fiber cells was either eliminated or reduced proportionally. These data are consistent with our hypotheses: fluid circulates through the lens; the intracellular leg of fluid circulation is through gap junction channels and is driven by hydrostatic pressure; and the fluid flow is generated by membrane transport of sodium.

Show MeSH
Related in: MedlinePlus