We cultured canine kidney (MDCK) cells stably expressing aquaporin-2 (AQP2) on collagen-coated permeable membrane filters and examined the effect of extracellular ATP on arginine vasopressin (AVP)-stimulated fluid transport and cAMP production. Exposure of cell monolayers to basolateral AVP resulted in stimulation of apical to basolateral net fluid transport driven by osmotic gradient which was formed by addition of 500 mM mannitol to basolateral bathing solution. Pre-exposure of the basolateral surface of cell monolayers to ATP (100 µM) for 30 min significantly inhibited the AVP-stimulated net fluid transport. In these cells, AVP-stimulated cAMP production was suppressed as well. Profile of the effects of different nucleotides suggested that the P2Y2 receptor is involved in the action of ATP. ATP inhibited the effect of isoproterenol as well, but not that of forskolin to stimulate cAMP production. The inhibitory effect of ATP on AVP-stimulated fluid movement was attenuated by a protein kinase C inhibitor, calphostin C or pertussis toxin. These results suggest that prolonged activation of the P2 receptors inhibits AVP-stimulated fluid transport and cAMP responses in AQP2 transfected MDCK cells. Depressed responsiveness of the adenylyl cyclase by PKC-mediated modification of the pertussis-toxin sensitive Gi protein seems to be the underlyihng mechanism.