Adrenergic receptor-mediated Cl- transport in rabbit corneal endothelial cells

Effects of antiglaucoma drugs on calcium mobility in cultured corneal endothelial cells

The aim of this study was to estimate the effects of various antiglaucoma drugs including betaxolol, timolol, levobunolol, brimonidine, carteolol, dipivefrin, dorzolamide, brinzolamide, latanoprost, unoprostone, and pilocarpine on intracellular free Ca2+ ([Ca2+]i) mobility in cultured bovine corneal endothelial cells. Various antiglaucoma drugs were diluted from original concentrations to 1/ 100, 1/ 1,000, and 1/ 10,000. The [Ca2+] mobility was studied by spectrofluorophotometry after loading with the ester of fura-2 (fura-2/AM). It was found that timolol (58 microM and 5.8 microM), levobunolol (171 microM, 17.1 microM, and 1.71 microM), betaxolol (162 microM, 16.2 microM, and 1.62 microM), carteolol (680 microM and 68 microM), dipivefrin (28 microM and 2.8 microM), dorzolamide (616 microM and 61.6 microM), brinzolamide (260 microM), latanoprost (1.1 microM), unoprostone (28.2 microM, 2.82 microM, and 0.282 microM), and pilocarpine (408 micro and 40.8 microM) induced a significant increase in [Ca2+]i. Nevertheless, only brimonidine (68 microM and 6.8 microM) decreased [Ca2+]i concentration significantly. Benzalkonium chloride preservative did not affect [Ca2+]i after addition of 0.001, 0.0001 and 0.00001 mg/mL to cells. These results indicate that all antiglaucoma drugs may affect the physiologic function of corneal endothelial cells through change of [Ca2+]i mobility.

Occurrence of catecholaminergic nerve fibers in the human uveoscleral tissue in conditions of normal and raised intraocular pressure

The distribution of catecholaminergic nerve fibers (CNF) in human uveoscleral tissue was studied in six human eyes with normal intraocular pressure and in five eyes with increased pressure. The eyes with increased pressure had no visual field alterations and the patients did not have any glaucoma-related optic neuropathies. The amount of norepinephrine in these structures was also analysed. Catecholaminergic nerve fibers were detected by means of fluorescence microscopy and were counted using the quantitative analysis of images. Our results demonstrate that the occurrence of catecholaminergic nerve fibers (expressed in Conventional Units = C.U.) in human uveoscleral tissue is 15.4 +/- 1.6 C.U. in eyes with normal intraocular pressure. In eyes with increased intraocular pressure, these values were 12.2 +/- 1.2 C.U. Moreover, the amount of norepinephrine in tissue homogenates of the same eyes was evaluated and found to be 21.7 +/- 1.3 microg/gr tissue fresh weight of the human uveoscleral tissue in eyes with normal intraocular pressure. This value decreased to 18.8 +/- 1.1 microg/gr tissue fresh weight in the same tissue in conditions of raised intraocular pressure. In these experiments, the small number of eyes examined made it difficult to draw general conclusions. However, the role of human uveoscleral tissue was emphasized by the rich catecholaminergic innervation. A decrease of catecholaminergic nerve fibers and norepinephrine occurs when intraocular pressure is elevated. The modifications of these parameters, involved in the sympathetic control of aqueous humor outflow, may support the hypothesis of a possible relevant role for the human uveoscleral tissue in different pathological conditions.

Ca2+ mobilization in bovine corneal endothelial cells by P2 purinergic receptors

PURPOSE

To characterize Ca2+ mobilization by P2 receptors in the bovine corneal endothelial cells (BCEC).

METHODS

Changes in intracellular Ca2+ ([Ca2+]i) were measured by fluorescence imaging of cultured and fresh BCEC cells loaded with the Ca2+-sensitive dye Fura-PE3. Relative rates of Ca2+ influx were measured employing Mn2+ as a surrogate for Ca2+.

RESULTS

Exposure of cultured cells to uridine 5'-triphosphate (UTP), 2-methyl-thio ATP (msATP) and ATP caused biphasic changes in [Ca2+]i consisting of a peak followed by a plateau phase. Based on the peak responses to 100 microM agonist, the magnitude of UTP responses were similar to that of ATP but greater than that of msATP or ADP. UTP and msATP stimulated Mn2+ influx following [Ca2+]i peak similar to that observed in response to cyclopiazonic acid (CPA), an inhibitor of ER Ca2+-ATPase. Under Ca2+-free conditions, peak responses were similar to those in the presence of external Ca2+, but reduced when the cells were pre-exposed to CPA. Reactive Blue-2 (RB2), inhibited msATP responses by 60.4 +/- 18.8% but UTP responses by only 10.6 +/- 9.5%. Repeated exposures to UTP or msATP reduced [Ca2+]i mobilization indicating homologous desensitization. Response to UTP was not affected by a prior exposure to msATP. However, response to msATP was reduced by a prior exposure to UTP indicating mixed heterologous desensitization. Fresh cells responded to UTP (50 microM) with temporal characteristics of [Ca2+]i mobilization similar to that of cultured cells.

CONCLUSION

BCEC express P2 receptors belonging to the P2Y subfamily. The emptying of the IP3-sensitive stores, leading to the initial peak in [Ca2+]i response, subsequently caused capacitative Ca2+ influx leading to the onset of the plateau phase. A significant homologous desensitization to UTP and msATP, selective heterologous desensitization between UTP and msATP, and selective inhibition by RB2 indicate the coexistence of multiple P2Y receptors.

Cyclic AMP activates anion channels in cultured bovine corneal endothelial cells

Ion coupled fluid transport by the corneal endothelium is stimulated by adenosine through a cAMP dependent mechanism. This study examines if anion conductance is enhanced by cAMP and, hence by adenosine. Cl- fluxes, measured by changes in fluorescence of the Cl- sensitive dye SPQ, following removal or re-addition of Cl- Ringer, could be accelerated by 20 microM forskolin or 10 microM adenosine. The cAMP cocktail (20 microM forskolin + 100 microM IBMX + 100 microM cpt-cAMP) had no effect on resting [Cl-]i. However, when Cl- influx was inhibited by 100 microM furosemide, net Cl- efflux was observed in response to the cAMP cocktail. Exposure to the cAMP cocktail alone depolarized the resting membrane potential. Conversely, the cAMP cocktail caused a relative hyperpolarization in cells which had been previously depolarized beyond the equilibrium potential for Cl- (ECl-), by application of 1 microM Gramicidin D. cAMP dependent changes in membrane potential could be inhibited by 50 microM NPPB, but not by 200 microM DPC, 100 microM H2DIDS or 50 microM glibenclamide. Taken together, these results are consistent with NPPB-sensitive, cAMP activated Cl- channels. To examine if these channels are permeable to HCO3-, changes in pHi in response to the cAMP cocktail were measured in acidified and depolarized cells in the absence of Na+. The cAMP cocktail caused an increase in pHi only when HCO3- was present, consistent with HCO3- influx. In control HCO3- Ringer, the cAMP cocktail caused a transient decrease in pHi, which could not be accounted for by inhibition of Na+:nHCO3- cotransport or stimulation of Cl-/HCO3- exchange. These results are consistent with conductive HCO3- efflux through cAMP activated channels. We conclude that cultured bovine corneal endothelial cells possess cAMP activated anion channels.