Ca2+ mobilization in nontransformed ciliary nonpigmented epithelial cells.

To investigate the calcium second messenger system in nonpigmented epithelial (NPE) cells, we studied drug-dependent cytosolic free Ca2+ concentration ([Ca2+]i) transients in cultured nontransformed human and rabbit NPE cells with a fluorescent Ca2+ indicator, fura-2, and a digital video-imaging system. The main findings of this study were: (1) The basal [Ca2+]i was 141.9 +/- 1.2 nM (mean +/- standard error of the mean, n = 401) in humans and 157.0 +/- 1.4 nM (mean +/- SEM, n = 346) in rabbits. (2) Isoproterenol (10(-4) M) had little effect on [Ca2+]i mobilization in both species. Norepinephrine (10(-4) M) and epinephrine (10(-4) M) increased [Ca2+]i in 56% and 78% of rabbit NPE cells by 1.8- and 2.1-fold of basal [Ca2+]i, respectively, but induced little [Ca2+]i change in human NPE cells. Carbachol (10(-3) M) elicited significant [Ca2+]i increase (more than 3-4-fold of basal level) in about 60-70% of NPE cells in both species. Heterogeneity was seen in the cellular response to these agonists. (3) Norepinephrine-induced response was blocked by phentolamine (10(-5) M), and the effect of carbachol was blocked by atropine (10(-4) M). (4) Time course of norepinephrine-induced [Ca2+]i change was primarily monophasic. In contrast, [Ca2+]i transients induced by carbachol were mostly biphasic. (5) The duration of carbachol- or norepinephrine-induced responses were shortened by the chelation of extracellular Ca2+ without affecting other parameters of the reaction. This study confirms the presence of the calcium signaling system in cultured nontransformed human and rabbit NPE cells.