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transport systems for lactate/H + , which serve to help counteract any
osmotic effects of metabolism-related excess, lactate production (Leung
et al. 2011).
The Corneal Endothelial Barrier and the Intraocular Pressure
A lesser fl uid barrier is present at the level of the corneal endothelium where
the cell-cell junctions result in a 'leaky' membrane; it is likely that there
is routinely some leakage of aqueous humor from the anterior chamber
into the relatively dehydrated corneal stroma. If the endothelial barrier
is severely damaged, then rapid fl uid fl ow into the stroma will occur,
with a very substantial increase in corneal thickness (Maurice and
Giardini 1951).
The normal leakage (posterior-to-anterior) is likely dependent upon
intraocular pressure (IOP), with increased posterior-to-anterior flow
occurring under some conditions of clinically-relevant ocular hypertension
(e.g. acute-onset elevated IOP). Overall, the function of the endothelium
can be considered as a means to regulate the fl uid balance between the
corneal stroma and the aqueous humour. It is generally accepted that the
endothelium accomplishes this by acting both as a 'leaky' fl uid barrier and
as a fl uid pump (Edelhauser 2000). In the laboratory and using isolated
corneal stroma-endothelial preparations, is possible to measure a net fl uid
pump activity, in the stroma-to-aqueous (i.e. anterior-to-posterior) direction
(Doughty and Maurice 1988); the 'leak' is in the opposite direction and
dominates when the endothelial function is severely compromised.
This 'pump' is often considered to be a means by which fl uid could
be removed from the corneal stroma in the living eye (Leung et al. 2011),
a feature that has been attributed to an active metabolism and ability to
actively transport ions such as bicarbonate.
However, fl uid can apparently be 'pumped' (transported) in the nominal
absence of ions such as bicarbonate (Doughty and Maurice 1988, Doughty
et al. 1993, Diecke et al. 2007), so is not obviously exclusively dependent
on a 'bicarbonate pump'. A set of water transport proteins, the aquaporins
may be responsible for such fl uid movement (Levin and Verkman 2006). A
case can also be made that some of the bicarbonate-pH effects could be on
the corneal stroma per se, and that those cellular metabolic activities that
appear to regulate corneal endothelial function can be equally applied to
the keratocytes. Since these cells are attached to the collagen fi brils, they
could serve to reorganize the collagen matrix and lamellae of the corneal
stroma and so indirectly change the fl uid distribution (hydration) within
the stroma (Doughty 1989b, 2001, 2003b).
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