In porcine aortic smooth muscle cells, neuropeptide Y (NPY) stimulates mobilization of CA(2+) from intracellular store sites via Y1 receptors. However, it has been debated whether or not Ca(2+) 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 Ca(2+) concentration ([Ca(2+)]i) in comparison with that on angiotensin II (AII)-induced [Ca(2+)]i increase, which is dependent on Ins(1,4,5)P3 generation. Digital-imaging microscopy study using the Ca(2+)-sensitive dye fura-2 revealed that application of AII induced a rapid but transient [Ca(2+)]i increase in a single cell, arising from intracellular calcium stores. Application of NPY to the same cell induced a [Ca(2+)]i rise with a pattern similar to that induced by AII. AII increased the formation of Ins(1,4,5)P3 by about 3.0 fold, while the NPY-induced [Ca(2+) formation was very small. U73122 completely inhibited not only Ins(1,4,5)P3 synthesis, but also Ca(2+) mobilization induced by either agonist. The effect of U73122 on the NPY-induced Ca(2+)i increase was about 10-fold more potent that on the AII-induced one. U73343, an inactive analog of U733343, had no influence on any of the AII- and NPY-mediated effects. Herbimycin A completely inhibited the platelet-derived growth factor-induced [Ca(2+]i increase but had no effect on the NPY-induced [Ca(2+)]i increase, indicating that phospholipase C-gamma 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-beta, which may account for the hypersensitivity of the NPY effect to U73122.