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Abstract
Various kinds of neuropeptides have been identified to be immunoreactive in the drainage angle of mammalian eyes. However, little is known about second messenger system involvement with these peptides. To determine whether some of these peptides are linked to a calcium signalling system in the trabecular meshwork (TM) cells, their effects on [Ca2+]i transients in fura-2 loaded cultured bovine TM cells were studied with a digital video-imaging system. The main findings of this study were: (1) The basal [Ca2+]i was 164.0 +/- 1.0 nM (mean +/- standard error of the mean, n = 668). (2) Of the neuropeptides examined, neuropeptide Y (NPY) (10(-6)M) is the most potent because it increased [Ca2+]i by about four-fold from the basal level. Other peptides--substance P, bombesin, calcitonin gene-related peptide, and vasoactive intestinal peptide induced smaller increases in [Ca2+]i. (3) We defined a response as positive if [Ca2+]i increased to a value that was 1.2-fold over the basal level. The majority of the TM cells reacted to NPY, whereas only 20-30% of the cells reacted to any of the other peptides. (4) The chelation of extracellular Ca2+ shortened the half-life of a NPY-induced response without affecting its latency. (5) NPY (10(-6)M) significantly increased the formation of inositol triphosphate following a 15 sec exposure. The same was the case for inositol monophosphate and inositol diphosphate. The results of this study suggest that in bovine TM cells, NPY stimulation is coupled to Ca2+ signalling through an increase in polyphosphoinositide turnover.