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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 of approximately eliminating the transmembrane electrochemical gradient for Na+ on central hydrostatic pressure in WT mouse lenses. The high K+/low Na+ external solution contained 140 mM K+ and 5 mM Na+. (A) Typical data from one lens showing the time course of the reduction in central hydrostatic pressure associated with elimination of the electrochemical gradient for Na+. After ∼120 min in 5 mM of extracellular Na+, the pressure near the center of the lens dropped to near zero. Upon restoration of normal extracellular Na+, the pressure began to recover, but we did not wait for full recovery, which was unlikely because the long exposure to low extracellular Na+ affected many transport systems and was probably not completely reversible. (B) The average hydrostatic pressure in lenses immersed in high K+/low Na+ solution (seven lenses from seven mice) compared with the pressure in normal Tyrode’s solution (five lenses from five mice). The central pressure in lenses immersed in high K+/low Na+ solution consistently fell to near zero in a time period of ∼2 h. In control lenses, the hydrostatic pressure remained relatively constant over a period of more than 2 h. After a period of 140 min, the average pressure was 85% of its initial value.
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fig7: The effect of approximately eliminating the transmembrane electrochemical gradient for Na+ on central hydrostatic pressure in WT mouse lenses. The high K+/low Na+ external solution contained 140 mM K+ and 5 mM Na+. (A) Typical data from one lens showing the time course of the reduction in central hydrostatic pressure associated with elimination of the electrochemical gradient for Na+. After ∼120 min in 5 mM of extracellular Na+, the pressure near the center of the lens dropped to near zero. Upon restoration of normal extracellular Na+, the pressure began to recover, but we did not wait for full recovery, which was unlikely because the long exposure to low extracellular Na+ affected many transport systems and was probably not completely reversible. (B) The average hydrostatic pressure in lenses immersed in high K+/low Na+ solution (seven lenses from seven mice) compared with the pressure in normal Tyrode’s solution (five lenses from five mice). The central pressure in lenses immersed in high K+/low Na+ solution consistently fell to near zero in a time period of ∼2 h. In control lenses, the hydrostatic pressure remained relatively constant over a period of more than 2 h. After a period of 140 min, the average pressure was 85% of its initial value.

Mentions: Fig. 7 A shows the effect of the low Na+/high K+ external solution on pressure in a central fiber cell in a typical lens. The pressure declined to near zero in a period of ∼2 h, after which restoring normal external Tyrode’s solution caused the beginning of recovery of pressure. We never waited for total recovery, which we considered unlikely to occur given the severity of this treatment and its effects on Na+-dependent transport systems. Fig. 7 B shows the averaged normalized pressure in central fiber cells from seven lenses graphed as a function of time in low Na+/high K+ solution (filled circles). The pressure consistently fell to near zero in a time period of ∼2 h. The average normalized pressure in five control lenses in normal Tyrode’s solution for the same period of time (Fig. 7 B, open circles) remained relatively constant; the pressure at the end of 2 h was ∼85% of the initial value.


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 of approximately eliminating the transmembrane electrochemical gradient for Na+ on central hydrostatic pressure in WT mouse lenses. The high K+/low Na+ external solution contained 140 mM K+ and 5 mM Na+. (A) Typical data from one lens showing the time course of the reduction in central hydrostatic pressure associated with elimination of the electrochemical gradient for Na+. After ∼120 min in 5 mM of extracellular Na+, the pressure near the center of the lens dropped to near zero. Upon restoration of normal extracellular Na+, the pressure began to recover, but we did not wait for full recovery, which was unlikely because the long exposure to low extracellular Na+ affected many transport systems and was probably not completely reversible. (B) The average hydrostatic pressure in lenses immersed in high K+/low Na+ solution (seven lenses from seven mice) compared with the pressure in normal Tyrode’s solution (five lenses from five mice). The central pressure in lenses immersed in high K+/low Na+ solution consistently fell to near zero in a time period of ∼2 h. In control lenses, the hydrostatic pressure remained relatively constant over a period of more than 2 h. After a period of 140 min, the average pressure was 85% of its initial value.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig7: The effect of approximately eliminating the transmembrane electrochemical gradient for Na+ on central hydrostatic pressure in WT mouse lenses. The high K+/low Na+ external solution contained 140 mM K+ and 5 mM Na+. (A) Typical data from one lens showing the time course of the reduction in central hydrostatic pressure associated with elimination of the electrochemical gradient for Na+. After ∼120 min in 5 mM of extracellular Na+, the pressure near the center of the lens dropped to near zero. Upon restoration of normal extracellular Na+, the pressure began to recover, but we did not wait for full recovery, which was unlikely because the long exposure to low extracellular Na+ affected many transport systems and was probably not completely reversible. (B) The average hydrostatic pressure in lenses immersed in high K+/low Na+ solution (seven lenses from seven mice) compared with the pressure in normal Tyrode’s solution (five lenses from five mice). The central pressure in lenses immersed in high K+/low Na+ solution consistently fell to near zero in a time period of ∼2 h. In control lenses, the hydrostatic pressure remained relatively constant over a period of more than 2 h. After a period of 140 min, the average pressure was 85% of its initial value.
Mentions: Fig. 7 A shows the effect of the low Na+/high K+ external solution on pressure in a central fiber cell in a typical lens. The pressure declined to near zero in a period of ∼2 h, after which restoring normal external Tyrode’s solution caused the beginning of recovery of pressure. We never waited for total recovery, which we considered unlikely to occur given the severity of this treatment and its effects on Na+-dependent transport systems. Fig. 7 B shows the averaged normalized pressure in central fiber cells from seven lenses graphed as a function of time in low Na+/high K+ solution (filled circles). The pressure consistently fell to near zero in a time period of ∼2 h. The average normalized pressure in five control lenses in normal Tyrode’s solution for the same period of time (Fig. 7 B, open circles) remained relatively constant; the pressure at the end of 2 h was ∼85% of the initial value.

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