We investigated the possibility that cultured corneal endothelial cells express receptors that are coupled to the phosphoinositide cycle/intracellular Ca2+signaling pathway. Agonist-stimulated changes in intracellular calcium ([Ca2+]i) in single bovine and human corneal endothelial cells (BCEC and HCEC, respectively) derived from confluent cultures were measured by microspectrofluorimetry using the Ca2+-sensitive probe, fura-2. Total inositol phosphates accumulated during a 30 min incubation in the presence or absence of agonists was determined in Li+containing medium with cells pre-labelled for 48 hrs with 10μCi/ml3H-myoinositol. Histamine (HA), ADP and ATP induced a rapid increase in [Ca2+]i. Subsequently, [Ca2+]idecreased to either a stable, agonist-dependent sustained elevation, or fell back to baseline to begin oscillatory fluctuations. The initial rise in [Ca2+]iwas insensitive to removal of extracellular calcium (Ca2+○), whereas the stable elevations in [Ca2+]iand the [Ca2+]ioscillations required Ca2+0. In contrast, bradykinin (BK) and endothelin-1 (ET-1) elicited an initial rise in [Ca2+]ithat returned to prestimulatory levels within 2 min despite the continued presence of agonist. The Ca2+-mobilizing agonists carbachol, phenylephrine, adenosine and substance P were all ineffective in elevating [Ca2+]i. Histamine-induced Ca2+mobilization was inhibited by the Hi-receptor antagonist triprolidine, but triprolidine had no effect on either BK or ATP stimulation of Ca2+mobilization. In BCEC, 100μM HA significantly increased total inositol phosphate accumulation (18.8-fold over unstimulated controls) and was 90% inhibited by 0.5μM triprolidine. BK and ATP also significantly increased formation of inositol phosphates in BCEC. These results indicate that the signal transduction pathway involving PI turnover and Ca2+mobilization is expressed in BCEC and HCEC cultures and is activated by HA, BK, ATP, ADP and ET-1.