Ca2+mobilization induced by ATP, isoproterenol and the Ca2+-ATPase inhibitor thapsigargin in the human submandibular duct cell line A253 was investigated using the Ca2+-sensitive fluorescent indicator fura-2. ATP and isoproterenol increased cytosolic free Ca2+([Ca2+]i) and subsequent exposure to thapsigargin after ATP or isoproterenol stimulation caused a further increase in [Ca2+]i. However, ATP and isoproterenol were not able to elicit a further increase in [Ca2+]iafter exposure of the cells to thapsigargin. Relatively few cells reacted to isoproterenol stimulation, but nearly all cells reacted to isoproterenol if ATP was added together with, or prior to isoproterenol stimulation. Moreover, the effect of ATP was potentiated by prior or simultaneous addition of isoproterenol. Furthermore, ATP decreased [Ca2+]iin the presence of thapsigargin probably due to agonist-induced export of intracellular calcium. The results may suggest the existence of three thapsigargin sensitive pools; one opened by ATP acting through P2-purinergic receptors and IP3, one opened by isoproterenol acting through β2-adrenergic receptors, and a third pool not sensitive to ATP or isoproterenol.