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Thrombin Inhibits Intercellular Calcium Wave Propagation in Corneal Endothelial Cells by Modulation of Hemichannels and Gap Junctions

¹From the Laboratory of Physiology, KULeuven, Leuven, Belgium; and the ²School of Optometry, Indiana University, Bloomington, Indiana.

PURPOSE. Thrombin, a serine protease, breaks down the barrier integrity of corneal endothelial cells by phosphorylation of the regulatory light chain of myosin II (myosin light chain; MLC), which induces contractility of the actin cytoskeleton. This study was undertaken to investigate the effect of thrombin on gap junctional (GJIC) and paracrine (PIC) intercellular communication in cultured bovine corneal endothelial cells (BCECs).

METHODS. An intercellular Ca²⁺ wave, a form of cell–cell communication, was elicited by applying a mechanical stimulus to a single cell in a confluent monolayer. Changes in [Ca²⁺]_i were imaged by fluorescence microscopy with a fluorescent calcium indicator, and the images were used to calculate the area reached by the Ca²⁺ wave (active area). GJIC was assessed by fluorescence recovery after photobleaching (FRAP). Activity of hemichannels was assayed by lucifer yellow (LY) uptake and also by adenosine triphosphate (ATP) release by using the luciferin–luciferase technique.

RESULTS. RT-PCR showed transcripts for PAR-1 and -2 receptors, but not for PAR-4 receptors. Immunocytochemistry showed thrombin-sensitive PAR receptors as well as trypsin-sensitive PAR-2 receptors. Both thrombin and the selective PAR-1 agonist TRAP-6 reduced the active area of the Ca²⁺ wave. These agents also reduced the fluorescence recovery in FRAP experiments. The effect of thrombin on the Ca²⁺ wave was inhibited by a peptide antagonist of PAR-1, but not by a PAR-4 antagonist. Pretreatment with ML-7 (an MLCK inhibitor), Y-27632 (a Rho kinase inhibitor) or chelerythrine (a PKC inhibitor) prevented the effect of thrombin on the Ca²⁺ wave. Activation of PAR-1 did not affect the Ca²⁺ wave propagation in cells pretreated with Gap26, which blocks hemichannels. However, PAR-1 activation decreased the active area in cells pretreated with Gap27, which inhibits gap junctions. Thrombin abolished enhancement of the Ca²⁺ wave propagation by ARL-67156 (inhibitor of ecto-ATPases). The effect of the PAR-1 agonists on the Ca²⁺ wave was not detectable in cells pretreated with exogenous apyrases.

CONCLUSIONS. Thrombin inhibits intercellular Ca²⁺ wave propagation in BCECs. This effect is due to activation of PAR-1 receptors and involves MLC phosphorylation by MLCK-, PKC- and Rho kinase–sensitive pathways. Thrombin mainly inhibits the ATP-mediated PIC pathway, and also reduces GJIC to a lesser extent.

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