Effects of extracellular ATP on Ca2+ mobilization in Madin Darby canine kidney (MDCK) cells.

Previous studies have demonstrated that ATP activates calcium-dependent K+ channels thereby leading to plasma membrane hyperpolarization and chloride secretion in MDCK cells. However, the signaling pathway involved in regulating ATP-evoked Ca2+ mobilization in MDCK cells remains unclarified. The present study was performed to elucidate the mechanisms underlying ATP-evoked calcium mobilization and inositol phospholipid turnover in MDCK cells. Our results indicate that ATP exerted its effects by activating the P2u subtype purinoceptor because both ATP and UTP but not adenosine caused Ca2+ mobilization. ATP induced [Ca2+]i increase in a dose-dependent manner with a maximal and half-maximal effect at 100 microM and 5 microM, respectively. In the absence of external Ca2+, both UTP and ATP were capable of mobilizing Ca2+ from internal stores through an IP3-sensitive pathway and the response for the former was larger than the latter. While in the presence of external Ca2+, the magnitude of UTP-induced [Ca2+]i increase was similar to that of ATP. These observations suggest that other mechanisms involved in addition to P2u subtype receptors in regulating ATP-induced Ca2+ response. The Ca2+ transients observed in Ma(2+)-depleted media were greater than those seen in Mg(2+)-contained media. ATP-induced Na+ influx were evidenced by SBFI loaded MDCK cells in Mg(2+)-depleted media. These data support that non-selective ATP4-membrane pores were formed under the application of ATP. The ATP-induced Ca2+ rise was suppressed by either omega-conotoxin or La3+ but was unaffected by the L-type Ca2+ channel blockers, verapamil and nifedipine. The deprivation of external Na+ also enhanced the ATP-induced Ca2+ rise. These data indicate that a P2 receptor-mediated cation channel was also involved under the stimulation of ATP.