Maintenance of corneal hydration is provided by the ion and fluid
secretion properties of the corneal endothelium. The endothelium is a
thin monolayer of cells that are well coupled to each other through gap
junctions but show discontinuous bands of tight junctions leading to
very low transendothelial electrical resistance (20–40Ω
/cm
2). Being so leaky, the endothelium is not
readily amenable to the study of transport by the usual measurements of
short circuit current or net fluxes of radioactive tracers in the
Ussing-type chamber. Nevertheless, early attempts have shown that short
circuit current is dependent on the presence of bicarbonate and reduced
by carbonic anhydrase inhibitors,
1 2 which is consistent
with the reduction in fluid transport by these same
manipulations.
3 4 More recently, it has been shown that
uptake of HCO
3 − by the
endothelium is provided by an electrogenic
Na
+-nHCO
3 − cotransporter
(
n ≤ 2), which is most likely positioned at the
basolateral membrane.
5 Furthermore, it has been shown that
Cl
− is necessary for fluid transport.
6 Intracellular [Cl
−] is above electrochemical equilibrium
in cultured endothelial cells,
7 indicating that a
transporter uptake mechanism is likely to be present. One possibility
for Cl
− uptake is coupling of
Na
+/H
+ exchange with
Cl
−/HCO
3 − exchange. However,
recently we have shown that the anion exchanger is not expressed in
cultured endothelium, and expression in freshly isolated tissue is very
low.
5 Another possible uptake mechanism, used by many
secretory epithelia, is through the loop diuretic-sensitive
Na
+-K
+-2Cl
− cotransporter. In
fact, bumetanide-sensitive Rb
+ uptake, further activated by
cell shrinkage, has been demonstrated in cultured bovine corneal
endothelial cells (BCECs),
8 indicating the presence of
Na
+-K
+-2Cl
− cotransport, a role in
regulatory volume increase and possible role in fluid transport. On the
other hand, Rb
+ uptake studies using fresh rabbit corneas
have failed to show bumetanide-sensitive uptake in endothelium bathed
in isotonic or hypertonic media.
9 Further, fluid transport
studies using freshly isolated rabbit corneas have so far failed to
show bumetanide sensitivity of fluid transport.
4 9 Taken
together, these studies indicate that the
Na
+-K
+-2Cl
− cotransporter may not
be present in fresh endothelium, and its expression could be
upregulated by culturing.