HISTAMINE H-1 RECEPTOR-MEDIATED CA-2+ SIGNALING IN CULTURED BOVINE CORNEAL ENDOTHELIAL-CELLS

The corneal endothelium pumps ions and water from the stroma to the aqueous humor, maintaining corneal transparency. This report investigates the possibility that cultured corneal endothelial cells express neurohormonal Ca2+ signaling pathways employed by other epithelia to regulate transport or other cellular functions. Agonist-stimulated changes in intracellular calcium ([Ca2+]i) in single bovine corneal endothelial cells (BCEC) derived from confluent cultures were measured by microspectrofluorimetry using the Ca2+-sensitive probe, fura 2. Mean resting ([Ca2+]i) in BCEC was 46 +/- 2 nM (n = 124). The muscarinic cholinergic agonist, carbachol, did not mobilize Ca2+, whereas histamine induced a rapid increase in [Ca2+]i to initial peak levels of 549 +/- 22 nM (n = 46) at maximally stimulating doses. The initial rise in [Ca2+]i in response to histamine was dose dependent, with a minimum effective dose of 50 nM, EC50 = 0.84 mumol/l, and a maximum effective dose of 10 mumol/l. [Ca2+]i decreased from the initial peak, but then stabilized to form an agonist-dependent sustained elevation or abruptly fell back to baseline to begin oscillatory fluctuations. The initial peak was insensitive to removal of extracellular calcium (Ca2+o), whereas subsequent elevations in [Ca2+]i or sustained [Ca2+]i oscillations required Ca2+o. The amplitude of the oscillations in [Ca2+] increased with an increase in [histamine]. However, frequency was independent of [histamine] (mean = 0.62 spikes min-1 +/- 0.06, n = 33). Histamine-induced Ca2+ mobilization was inhibited by the H-1 receptor antagonist triprolidine, but was unaffected by ranitidine (H-2 antagonist) or thioperamide (H-3 antagonist). These results indicate that histamine H-1 receptors are expressed in BCEC cultures and that histamine elicits a dose-dependent release of intracellular calcium and promotes extracellular Ca2+ entry in these cells.