Limits...
The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium.

Cheng X, Pinsky PM - PLoS ONE (2015)

Bottom Line: In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments.The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges.The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Stanford University, Stanford, California, United States of America.

ABSTRACT
The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is crucial for studying fluid and electrolyte homeostasis in living systems, and is an area of active research. In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments. The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges. The steady-state analysis on active membranes suggests a new pressure mechanism which balances the fluid pressure together with the osmotic pressure. The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport. We apply the proposed theory to study the transendothelial fluid pressure in the in vivo cornea, which is a crucial factor maintaining the hydration and transparency of the tissue. The results show the importance of the proposed pressure mechanism in mediating stromal fluid pressure and provide a new interpretation of the pressure modulation mechanism in the in vivo cornea.

Show MeSH

Related in: MedlinePlus

a) The cornea has three principal layers, namely the epithelium, stroma and endothelium. b) The corneal stroma is composed by collagen fibers (lamellae) packed through thickness, with keratocyte cells interspersed between adjacent lamellae. Fixed charges are associated with sulphated proteoglycans that are attached to the collagen fibrils, which form a lamella by assembling as parallel arrays following the direction of the lamella. The fixed charges give rise to a swelling tendency of the cornea [8] by the Donnan effect [17]. c) The in vivo cornea modulates the osmotic pressure by actively pumping ionic solutes (primarily bicarbonate) from stroma to aqueous humor [31]. In addition, glucose transports from aqueous humor to stroma for the metabolic activities needed by corneal cells, and lactate ion transports in the opposite direction [25]. The underlying molecular mechanisms of corneal endothelial pumping are still under investigation [31].
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4697791&req=5

pone.0145422.g003: a) The cornea has three principal layers, namely the epithelium, stroma and endothelium. b) The corneal stroma is composed by collagen fibers (lamellae) packed through thickness, with keratocyte cells interspersed between adjacent lamellae. Fixed charges are associated with sulphated proteoglycans that are attached to the collagen fibrils, which form a lamella by assembling as parallel arrays following the direction of the lamella. The fixed charges give rise to a swelling tendency of the cornea [8] by the Donnan effect [17]. c) The in vivo cornea modulates the osmotic pressure by actively pumping ionic solutes (primarily bicarbonate) from stroma to aqueous humor [31]. In addition, glucose transports from aqueous humor to stroma for the metabolic activities needed by corneal cells, and lactate ion transports in the opposite direction [25]. The underlying molecular mechanisms of corneal endothelial pumping are still under investigation [31].

Mentions: In this section an example application of Eqs (31, 40) is presented to study the endothelial transport process of the in vivo human cornea (see Fig 3). The exchange of fluid and ions across the endothelium controls the level of corneal hydration, which is a crucial factor for maintaining the transparency of the tissue [21, 24]. Fixed charges are associated with sulphated proteoglycans in the stroma (the bulk layer of the tissue), and generate osmotic pressure by Donnan effect [8, 27]. The active ionic transport processes located in the endothelium reduce the osmotic pressure by pumping ions out from the tissue. Furthermore, metabolic reactions take place in the in vivo cornea, rendering nonzero transendothelial fluxes for metabolic species (glucose, bicarbonate and lactate ions) [25, 28, 29].


The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium.

Cheng X, Pinsky PM - PLoS ONE (2015)

a) The cornea has three principal layers, namely the epithelium, stroma and endothelium. b) The corneal stroma is composed by collagen fibers (lamellae) packed through thickness, with keratocyte cells interspersed between adjacent lamellae. Fixed charges are associated with sulphated proteoglycans that are attached to the collagen fibrils, which form a lamella by assembling as parallel arrays following the direction of the lamella. The fixed charges give rise to a swelling tendency of the cornea [8] by the Donnan effect [17]. c) The in vivo cornea modulates the osmotic pressure by actively pumping ionic solutes (primarily bicarbonate) from stroma to aqueous humor [31]. In addition, glucose transports from aqueous humor to stroma for the metabolic activities needed by corneal cells, and lactate ion transports in the opposite direction [25]. The underlying molecular mechanisms of corneal endothelial pumping are still under investigation [31].
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4697791&req=5

pone.0145422.g003: a) The cornea has three principal layers, namely the epithelium, stroma and endothelium. b) The corneal stroma is composed by collagen fibers (lamellae) packed through thickness, with keratocyte cells interspersed between adjacent lamellae. Fixed charges are associated with sulphated proteoglycans that are attached to the collagen fibrils, which form a lamella by assembling as parallel arrays following the direction of the lamella. The fixed charges give rise to a swelling tendency of the cornea [8] by the Donnan effect [17]. c) The in vivo cornea modulates the osmotic pressure by actively pumping ionic solutes (primarily bicarbonate) from stroma to aqueous humor [31]. In addition, glucose transports from aqueous humor to stroma for the metabolic activities needed by corneal cells, and lactate ion transports in the opposite direction [25]. The underlying molecular mechanisms of corneal endothelial pumping are still under investigation [31].
Mentions: In this section an example application of Eqs (31, 40) is presented to study the endothelial transport process of the in vivo human cornea (see Fig 3). The exchange of fluid and ions across the endothelium controls the level of corneal hydration, which is a crucial factor for maintaining the transparency of the tissue [21, 24]. Fixed charges are associated with sulphated proteoglycans in the stroma (the bulk layer of the tissue), and generate osmotic pressure by Donnan effect [8, 27]. The active ionic transport processes located in the endothelium reduce the osmotic pressure by pumping ions out from the tissue. Furthermore, metabolic reactions take place in the in vivo cornea, rendering nonzero transendothelial fluxes for metabolic species (glucose, bicarbonate and lactate ions) [25, 28, 29].

Bottom Line: In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments.The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges.The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Stanford University, Stanford, California, United States of America.

ABSTRACT
The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is crucial for studying fluid and electrolyte homeostasis in living systems, and is an area of active research. In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments. The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges. The steady-state analysis on active membranes suggests a new pressure mechanism which balances the fluid pressure together with the osmotic pressure. The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport. We apply the proposed theory to study the transendothelial fluid pressure in the in vivo cornea, which is a crucial factor maintaining the hydration and transparency of the tissue. The results show the importance of the proposed pressure mechanism in mediating stromal fluid pressure and provide a new interpretation of the pressure modulation mechanism in the in vivo cornea.

Show MeSH
Related in: MedlinePlus