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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.

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The effect on intracellular hydrostatic pressure of reducing the number of gap junction channels coupling the differentiating and MFs. (A) The standing hydrostatic pressure gradient in lenses from GPX-1 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 10 lenses from five 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 ∼0.4a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in the GPX-1 KO lenses was ∼60% of that in WT lenses (Wang et al., 2009). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or ∼1.67 times greater than in WT. The best fits of the model to the data give the ratio of pi(0) in GPX-1 KO/WT lenses as 1.52. (B) An over-plot of the GPX-1 KO and WT data.
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fig5: The effect on intracellular hydrostatic pressure of reducing the number of gap junction channels coupling the differentiating and MFs. (A) The standing hydrostatic pressure gradient in lenses from GPX-1 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 10 lenses from five 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 ∼0.4a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in the GPX-1 KO lenses was ∼60% of that in WT lenses (Wang et al., 2009). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or ∼1.67 times greater than in WT. The best fits of the model to the data give the ratio of pi(0) in GPX-1 KO/WT lenses as 1.52. (B) An over-plot of the GPX-1 KO and WT data.

Mentions: GPX is a cytoplasmic enzyme that protects the lens against oxidative damage by H2O2 (Reddy, 1990). KO of GPX results in age-onset nuclear cataracts (Reddy et al., 2001); however, in KO mice at 2 mo of age, the lenses are transparent and have normal transport properties except for reductions in gap junction coupling conductance. Western blots suggest that these reductions are a result of loss of both Cx46 and Cx50 (Wang et al., 2009). Based on curve fits of series resistance data from 2-mo-old WT and GPX KO lenses, GMF was reduced to ∼59% normal (Wang et al., 2009). Pooled pressure measurements from 10 lenses from GPX KO mice are graphed as a function of radial location in Fig. 5 A. At ∼60% of the radial distance into the lenses (r/a = 0.4), the pressures significantly exceeded 400 mmHg, which is the maximum pressure the manometer can measure. Hence, we do not have data on the central pressures, other than that they exceeded 400 mmHg. However, by curve fitting Eq. 7 to the peripheral data, we can project the average central pressure. Based on the curve fit, the average value of pi(0) was ∼496 mmHg, or ∼1.5 times that in WT lenses, consistent with water flow through a reduced number of gap junction channels. Fig. 5 B is an over-plot of the GPX KO and WT pressure data. In the range of radial locations where pressure in both types of lenses could be measured, the pressures in the KO lenses are consistently higher than those in WT lenses.


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 reducing the number of gap junction channels coupling the differentiating and MFs. (A) The standing hydrostatic pressure gradient in lenses from GPX-1 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 10 lenses from five 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 ∼0.4a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in the GPX-1 KO lenses was ∼60% of that in WT lenses (Wang et al., 2009). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or ∼1.67 times greater than in WT. The best fits of the model to the data give the ratio of pi(0) in GPX-1 KO/WT lenses as 1.52. (B) An over-plot of the GPX-1 KO and WT data.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig5: The effect on intracellular hydrostatic pressure of reducing the number of gap junction channels coupling the differentiating and MFs. (A) The standing hydrostatic pressure gradient in lenses from GPX-1 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 10 lenses from five 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 ∼0.4a could not be determined, other than that they exceeded 400 mmHg. The MF coupling conductance in the GPX-1 KO lenses was ∼60% of that in WT lenses (Wang et al., 2009). Based on the derivation of Eq. 7, the pressure gradient should be approximately inversely proportional to the MF coupling conductance, or ∼1.67 times greater than in WT. The best fits of the model to the data give the ratio of pi(0) in GPX-1 KO/WT lenses as 1.52. (B) An over-plot of the GPX-1 KO and WT data.
Mentions: GPX is a cytoplasmic enzyme that protects the lens against oxidative damage by H2O2 (Reddy, 1990). KO of GPX results in age-onset nuclear cataracts (Reddy et al., 2001); however, in KO mice at 2 mo of age, the lenses are transparent and have normal transport properties except for reductions in gap junction coupling conductance. Western blots suggest that these reductions are a result of loss of both Cx46 and Cx50 (Wang et al., 2009). Based on curve fits of series resistance data from 2-mo-old WT and GPX KO lenses, GMF was reduced to ∼59% normal (Wang et al., 2009). Pooled pressure measurements from 10 lenses from GPX KO mice are graphed as a function of radial location in Fig. 5 A. At ∼60% of the radial distance into the lenses (r/a = 0.4), the pressures significantly exceeded 400 mmHg, which is the maximum pressure the manometer can measure. Hence, we do not have data on the central pressures, other than that they exceeded 400 mmHg. However, by curve fitting Eq. 7 to the peripheral data, we can project the average central pressure. Based on the curve fit, the average value of pi(0) was ∼496 mmHg, or ∼1.5 times that in WT lenses, consistent with water flow through a reduced number of gap junction channels. Fig. 5 B is an over-plot of the GPX KO and WT pressure data. In the range of radial locations where pressure in both types of lenses could be measured, the pressures in the KO lenses are consistently higher than those in WT lenses.

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