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

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.

AKT activation promotes PTEN hamartoma tumor syndrome–associated cataract development

“…Na + transport is coupled to fluid circulation, creating a microcirculatory system that carries nutrients to the fiber cells and allows removal of metabolic waste. The intracellular passage of fluid through gap junction channels is driven by a standing hydrostatic pressure gradient within the lens (19)(20)(21). The ensemble activity of the various membrane channels and transporters that drive this microcirculatory system overcomes the lack of a lens vasculature and supports clarity, consistent with the finding that mutations in many of these channel genes have been linked to congenital cataract (22).…”

“…The lens depends on the coordinated activity of different channels and transporters to create a circulating current, with Na + being the primary current carrier (16,17). The transport of Na + is coupled to circulation of fluid, and the intracellular passage of fluid through gap junction channels is driven by a hydrostatic pressure gradient (19)(20)(21). If the normal lens circulation was altered in PTEN KO animals, a buildup of osmotically active solutes within the lens could increase intracellular pressure at the lens surface, leading to the observed rupture phenotype.…”

“…If the normal lens circulation was altered in PTEN KO animals, a buildup of osmotically active solutes within the lens could increase intracellular pressure at the lens surface, leading to the observed rupture phenotype. We previously developed a technique for directly measuring hydrostatic pressure in the lens (19,26) and used this method here to determine whether progression of the PTEN KO phenotype correlated with changes in lens intracellular pressure ( Figure 3). To derive quantitative values for the central and surface pressures, pooled data were graphed as a function of normalized distance from the lens center, then fit with an equation based on lens structure and experimentally derived transport parameters of the lens (19).…”

Connexin 46 (Cx46) Gap Junctions Provide a Pathway for the Delivery of Glutathione to the Lens Nucleus

“…Previous studies showed that inhibition of the Na ϩ -K ϩ -ATPase by ouabain or by replacement of extracellular Na ϩ with K ϩ significantly reduced the lens hydrostatic pressure and disrupted the lens circulation (16). WT whole lenses were incubated for 3 h in artificial aqueous humor (AAH control) and in AAH containing 1 mM ouabain or in sodium-free AAH, and GSH content was measured in cortical and nuclear fractions of lenses.…”

“…The circulating ionic current generates fluid flow through gap junctions. Passive gap junction-mediated intracellular fluid flow is driven by hydrostatic pressure, which is highest in lens central cells and lowest in the peripheral cells (16). Solutes such as glucose are carried by extracellular water flow and are taken up into inner fiber cells by membrane glucose transporters (17).…”