Human conjunctival epithelial cells express histamine-1 receptors coupled to phosphoinositide turnover and intracellular calcium mobilization: role in ocular allergic and inflammatory diseases.

Dispase-dissociated primary cultures of human conjunctival epithelial (HCE) cells were stimulated with histamine and the generation of inositol phosphates ([3H]IPs) from [3H]phosphoinositide (PI) hydrolysis and the mobilization of intracellular calcium ([Ca2+]i) were studied using ion exchange chromatography and Fura-2 fluorescence techniques, respectively. Histamine (100 microM) maximally stimulated PI turnover in HCE cells by 210 +/- 10% (n = 21) above basal levels and with a potency (EC50) of 3.3 microM (n = 4). Histamine (EC50 = 5.8 microM, n = 3) rapidly mobilized [Ca2+]i which peaked within 10 sec but which was still significantly elevated 20 min after stimulation. The histamine-induced [Ca2+]i responses did not desensitize upon repeated applications of histamine. The effects of histamine (100 microM) on PI turnover and [Ca2+]i were potently antagonized by the H1-antagonists, emedastine (IC50 = 1.6-2.9 nM), triprolidine (IC50 = 3.1 nM) and levocabastine (IC50 = 8 nM), but weakly by the H2-(ranitidine/cimetidine) and H3-(thioperamide) antagonists (IC50s = 10-100 microM). In conclusion, HCE cells have been shown to possess functional H1-histamine receptors that couple to inositol phosphates generation which then mobilize intracellular calcium. These intracellular signaling mechanisms may be intimately linked with the process of inflammatory cytokine secretion from the HCE cells after stimulation by histamine released from the conjunctival mast cells. The current results strongly suggest that the HCE cells are active participants in mediating, and perhaps amplifying, the pro-inflammatory and allergic effects of histamine which is released from conjunctival mast cells during ocular allergic and inflammatory reactions.