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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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Related in: MedlinePlus

A sketch of the hypotheses being tested in this study. (A) The net flux of Na+, followed by fluid, enters the lens at both poles and exits at the equator. The interesting pattern of circulation ensures maximum stirring of the fluid, which is hypothesized to act as a micro circulatory system for the avascular lens (Mathias et al., 2007). (B) A more detailed view of Na+ and K+ fluxes. Na+ flows into the lens along the extracellular spaces between cells, moves down its electrochemical gradient to enter fiber cells, reverses its direction, and flows back to the lens surface, where the Na/K ATPase transports it out of the lens to complete the circulation. The circulating pattern of flow shown in A occurs because gap junctions coupling DF cells direct the intracellular leg of the circulation to the equator. (C) Our first hypothesis is that water circulates through the lens as shown in this panel. Our second hypothesis is that the water circulation follows and is driven by the Na+ flux. Water enters each fiber cell through AQP0 resulting from local osmotic gradients created by the transmembrane Na+ flux, and leaves the lens through AQP1 resulting from local osmotic gradients generated by the Na/K ATPase. For the intracellular water to flow back to the surface of the lens, we hypothesize that a hydrostatic pressure (pi mmHg) develops that drives the water from cell to cell through gap junctions. (D) The predicted hydrostatic pressure gradient. The intracellular hydrostatic pressure 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 equation relates intracellular water flow (ui cm/s) to the hydrostatic pressure gradient (dpi/dr mmHg/cm), the hydraulic conductivity of a single gap junction channel (Lj (cm3/s)/(mmHg)), the number of gap junction channels per area of cell-to-cell contact (Nj cm−2), and the fiber cell width (w cm).
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fig1: A sketch of the hypotheses being tested in this study. (A) The net flux of Na+, followed by fluid, enters the lens at both poles and exits at the equator. The interesting pattern of circulation ensures maximum stirring of the fluid, which is hypothesized to act as a micro circulatory system for the avascular lens (Mathias et al., 2007). (B) A more detailed view of Na+ and K+ fluxes. Na+ flows into the lens along the extracellular spaces between cells, moves down its electrochemical gradient to enter fiber cells, reverses its direction, and flows back to the lens surface, where the Na/K ATPase transports it out of the lens to complete the circulation. The circulating pattern of flow shown in A occurs because gap junctions coupling DF cells direct the intracellular leg of the circulation to the equator. (C) Our first hypothesis is that water circulates through the lens as shown in this panel. Our second hypothesis is that the water circulation follows and is driven by the Na+ flux. Water enters each fiber cell through AQP0 resulting from local osmotic gradients created by the transmembrane Na+ flux, and leaves the lens through AQP1 resulting from local osmotic gradients generated by the Na/K ATPase. For the intracellular water to flow back to the surface of the lens, we hypothesize that a hydrostatic pressure (pi mmHg) develops that drives the water from cell to cell through gap junctions. (D) The predicted hydrostatic pressure gradient. The intracellular hydrostatic pressure 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 equation relates intracellular water flow (ui cm/s) to the hydrostatic pressure gradient (dpi/dr mmHg/cm), the hydraulic conductivity of a single gap junction channel (Lj (cm3/s)/(mmHg)), the number of gap junction channels per area of cell-to-cell contact (Nj cm−2), and the fiber cell width (w cm).

Mentions: The lens has an internal circulation of Na+ that enters at both poles and exits at the equator (see Fig. 1 A). This circulation was recently reviewed (Mathias et al., 2007), so only a brief overview will be presented. Fig. 1 B shows a more detailed view of the entry of Na+ into the extracellular spaces between lens cells where it flows toward the lens center. There is a large fiber cell transmembrane electrochemical gradient for sodium, causing it to move from the extracellular spaces into the intracellular compartment, where it reverses direction and is driven by an intracellular electrochemical gradient to flow through gap junction channels back to the surface. It is directed to flow in the interesting circulating pattern shown in Fig. 1 A because gap junction coupling conductance in the equatorial region of differentiating fibers (DFs; the outer shell of fiber cells that retain their organelles) is very high (Baldo and Mathias, 1992). Hence, once a sodium ion enters the intracellular compartment, DF gap junctions facilitate flow to the equator. Moreover, equatorial epithelial cells have a relatively high expression of Na/K ATPase protein (Gao et al., 2000), and almost all of the Na/K ATPase activity in the lens occurs at the equatorial surface where sodium is transported out of the lens (Candia and Zamudio, 2002). Lens K+ channels colocalize with the Na/K ATPase in lens epithelial cells (Mathias et al., 1997). Thus, K+ efflux colocalizes with K+ influx, and there is little net circulation of K+, as shown in Fig. 1 B. Fiber cells have Cl− channels as well as Na+ leak channels (Webb and Donaldson, 2008), but Cl− is close to electrochemical equilibrium, so the flux is small relative to the Na+ flux and has only small modulatory effects on the overall circulation. To summarize, the circulation of Na+ ultimately exists because of energy supplied by the Na/K ATPase, but it is directly generated by the fiber cell transmembrane electrochemical gradient for Na+. The circulating pattern of Na+ current is a result of the concentration of gap junction coupling conductance in the equatorial DF and the relatively high Na/K ATPase activity in the equatorial epithelium.


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)

A sketch of the hypotheses being tested in this study. (A) The net flux of Na+, followed by fluid, enters the lens at both poles and exits at the equator. The interesting pattern of circulation ensures maximum stirring of the fluid, which is hypothesized to act as a micro circulatory system for the avascular lens (Mathias et al., 2007). (B) A more detailed view of Na+ and K+ fluxes. Na+ flows into the lens along the extracellular spaces between cells, moves down its electrochemical gradient to enter fiber cells, reverses its direction, and flows back to the lens surface, where the Na/K ATPase transports it out of the lens to complete the circulation. The circulating pattern of flow shown in A occurs because gap junctions coupling DF cells direct the intracellular leg of the circulation to the equator. (C) Our first hypothesis is that water circulates through the lens as shown in this panel. Our second hypothesis is that the water circulation follows and is driven by the Na+ flux. Water enters each fiber cell through AQP0 resulting from local osmotic gradients created by the transmembrane Na+ flux, and leaves the lens through AQP1 resulting from local osmotic gradients generated by the Na/K ATPase. For the intracellular water to flow back to the surface of the lens, we hypothesize that a hydrostatic pressure (pi mmHg) develops that drives the water from cell to cell through gap junctions. (D) The predicted hydrostatic pressure gradient. The intracellular hydrostatic pressure 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 equation relates intracellular water flow (ui cm/s) to the hydrostatic pressure gradient (dpi/dr mmHg/cm), the hydraulic conductivity of a single gap junction channel (Lj (cm3/s)/(mmHg)), the number of gap junction channels per area of cell-to-cell contact (Nj cm−2), and the fiber cell width (w cm).
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig1: A sketch of the hypotheses being tested in this study. (A) The net flux of Na+, followed by fluid, enters the lens at both poles and exits at the equator. The interesting pattern of circulation ensures maximum stirring of the fluid, which is hypothesized to act as a micro circulatory system for the avascular lens (Mathias et al., 2007). (B) A more detailed view of Na+ and K+ fluxes. Na+ flows into the lens along the extracellular spaces between cells, moves down its electrochemical gradient to enter fiber cells, reverses its direction, and flows back to the lens surface, where the Na/K ATPase transports it out of the lens to complete the circulation. The circulating pattern of flow shown in A occurs because gap junctions coupling DF cells direct the intracellular leg of the circulation to the equator. (C) Our first hypothesis is that water circulates through the lens as shown in this panel. Our second hypothesis is that the water circulation follows and is driven by the Na+ flux. Water enters each fiber cell through AQP0 resulting from local osmotic gradients created by the transmembrane Na+ flux, and leaves the lens through AQP1 resulting from local osmotic gradients generated by the Na/K ATPase. For the intracellular water to flow back to the surface of the lens, we hypothesize that a hydrostatic pressure (pi mmHg) develops that drives the water from cell to cell through gap junctions. (D) The predicted hydrostatic pressure gradient. The intracellular hydrostatic pressure 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 equation relates intracellular water flow (ui cm/s) to the hydrostatic pressure gradient (dpi/dr mmHg/cm), the hydraulic conductivity of a single gap junction channel (Lj (cm3/s)/(mmHg)), the number of gap junction channels per area of cell-to-cell contact (Nj cm−2), and the fiber cell width (w cm).
Mentions: The lens has an internal circulation of Na+ that enters at both poles and exits at the equator (see Fig. 1 A). This circulation was recently reviewed (Mathias et al., 2007), so only a brief overview will be presented. Fig. 1 B shows a more detailed view of the entry of Na+ into the extracellular spaces between lens cells where it flows toward the lens center. There is a large fiber cell transmembrane electrochemical gradient for sodium, causing it to move from the extracellular spaces into the intracellular compartment, where it reverses direction and is driven by an intracellular electrochemical gradient to flow through gap junction channels back to the surface. It is directed to flow in the interesting circulating pattern shown in Fig. 1 A because gap junction coupling conductance in the equatorial region of differentiating fibers (DFs; the outer shell of fiber cells that retain their organelles) is very high (Baldo and Mathias, 1992). Hence, once a sodium ion enters the intracellular compartment, DF gap junctions facilitate flow to the equator. Moreover, equatorial epithelial cells have a relatively high expression of Na/K ATPase protein (Gao et al., 2000), and almost all of the Na/K ATPase activity in the lens occurs at the equatorial surface where sodium is transported out of the lens (Candia and Zamudio, 2002). Lens K+ channels colocalize with the Na/K ATPase in lens epithelial cells (Mathias et al., 1997). Thus, K+ efflux colocalizes with K+ influx, and there is little net circulation of K+, as shown in Fig. 1 B. Fiber cells have Cl− channels as well as Na+ leak channels (Webb and Donaldson, 2008), but Cl− is close to electrochemical equilibrium, so the flux is small relative to the Na+ flux and has only small modulatory effects on the overall circulation. To summarize, the circulation of Na+ ultimately exists because of energy supplied by the Na/K ATPase, but it is directly generated by the fiber cell transmembrane electrochemical gradient for Na+. The circulating pattern of Na+ current is a result of the concentration of gap junction coupling conductance in the equatorial DF and the relatively high Na/K ATPase activity in the equatorial epithelium.

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