In cells expressing different receptors linked to Ins(1,4,5)P(3) formation, maximal stimulation of any one of them often releases all the Ins(1,4,5)P(3)-sensitive Ca(2+) stores, suggesting that Ins(1,4, 5)P(3) is used similarly by many receptors. In single HEK-293 cells, ATP and carbamylcholine (CCh) stimulated Ca(2+) release from intracellular stores via a pathway that was entirely dependent on Ins(1,4,5)P(3). After stimulation with maximal concentrations of ATP or CCh in Ca(2+)-free medium, there was no response to a second stimulation with the same agonist, indicating that each agonist had emptied the Ins(1,4,5)P(3)-sensitive stores to which it had access. However, the Ca(2+) release evoked by the second agonist was unaffected by prior stimulation with the first. We conclude that Ins(1,4,5)P(3) mediates the effects of both receptors, but Ins(1,4, 5)P(3) is more versatile than hitherto supposed, because the spatial organization of the signalling pathways apparently allows Ins(1,4, 5)P(3) made in response to each agonist to interact with different Ins(1,4,5)P(3) receptors.
