Abstract
In porcine aortic smooth muscle cells, neuropeptide Y (NPY) stimulates mobilization of Ca2+ from intracellular store sites via Y1 receptors. However, it has been debated whether or not Ca2+ mobilization by Y1 receptors depends on the generation of inositol 1, 4, 5-trisphosphate [Ins(1, 4, 5)P3] following activation of phospholipase C. To examine this question, we studied the effect of U73122, an inhibitor of phospholipase C-mediated inositol phosphate accumulation, on the NPY-induced rise in cytosolic free Ca2+ concentration ( [Ca2+]i) in comparison with that on angiotensin II (All)-induced [Ca2+]i, increase, which is dependent on Ins(1, 4, 5)P3 generation. Digital-imaging microscopy study using the Ca2+ sensitive dye fura-2 revealed that application of All induced a rapid but transient [Ca2+]i increase in a single cell, arising from intracellular calcium stores. Application of NPY to the same cell induced a [Ca2+]i rise with a pattern similar to that induced by All. All increased the formation of Ins(1, 4, 5)P3 by about 3.0-fold, while the NPY-induced formation was very small. U73122 completely inhibited not only Ins(1, 4, 5)P3 synthesis, but also Ca2+ mobilization induced by either agonist. The effect of U73122 on the NPY-induced [Ca2+]i increase was about 10-fold more potent than that on the All-induced one. U73343, an inactive analog of U73122, had no influence on any of the All- and NPY-mediated effects. Herbimycin A completely inhibited the platelet-derived growth factor-induced [Ca2+]i increase, but had no effect on the NPY-induced [Ca2+]i increase, indicating that phospholipase C-γ is not involved in the NPY effect. These results suggest that NPY-induced Ca2+ mobilization from intracellular stores in porcine smooth muscle cells is secondary to the very small generation of Ins(1, 4, 5)P3 following stimulation of phospholipase C-β, which may account for the hypersensitivity of the NPY effect to U73122.
