ATP Release Via Connexin Hemichannels Controls Intercellular Propagation of Ca Waves in Corneal Endothelial Cells - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 180

IC. In addition, it is plausible that hemichannel block can have a beneficial effect
by preventing disturbances of intracellular homeostasis caused by solute entry/loss
through the channels.
The influence of the organization and/or contractility of the cortical actin
cytoskeleton on hemichannels suggests altered activity of hemichannels in response
to hypoxia and aging, since under these conditions, the corneal endothelial cells
show significant alterations in the actin cytoskeleton, as suggested by polymegath-
ism and pleomorphism [48, 127].
Our experiments also demonstrated that exposure of BCEC to adenosine, which
increases intracellular cAMP through activation of A2B receptors resulting in
inhibition of RhoA activation [105], opposes the thrombin effect on IC. Our obser-
vations on the effect of adenosine add yet another mechanism to the growing
repertoire of defense mechanisms by which adenosine may protect and enhance
the physiological functions of corneal endothelium. While adenosine is known to
enhance fluid transport [86], to activate cAMP-activated Cl - channels [12, 112]
and to rescue the loss of barrier integrity by reducing MLC phosphorylation [105,
93], our experiments demonstrate that adenosine can influence the coordination
of cellular responses to stimuli. Despite the focus of this study on the thrombin-
induced inhibition of the IC, our observations have more general implications in
that our experiments demonstrated that the action of adenosine in preventing the
inhibition of IC by thrombine was mediated via a pathway involving MLC. Since
pro-inflammatory molecules can cause MLC phosphorylation, it is plausible that
by inducing MLC dephosphorylation, adenosine could be beneficial in overcom-
ing the potential threat to loss of IC concomitant with its ability to rescue barrier
integrity. Furthermore, our finding that GJIC is enhanced in response to increase
levels of cAMP is supported by a previous finding that HCO 3 - , which activates sol-
uble adenylate cyclase [111], enhances dye coupling in the corneal endothelium
[124]. Our finding that adenosine not only affects GJIC, but also overcomes the
thrombin-induced inhibition of hemichannels was surprising. However, it is in line
with our findings that the major effect of thrombin on IC is through inhibition of
PIC. Improving PIC could be helpful especially for cells that have lost direct contact
with neighboring cells.
10.4.3 IC Via ATP in Different Cell Types
Together with the major progress in the field of purinergic receptors and charac-
terization of ectonucleotidases, IC via ATP release and its potential physiological
relevance, have been examined in a large number of cell types/tissues. It has become
clear that, in addition to other mechanisms, hemichannels are an important pathway
for ATP release, playing an important role in PIC in a large number of phys-
iological and pathophysiological processes. A brief summary is provided in the
Table 10.2.