Calcium release from intracellular stores evoked by extracellular ATP in a Xenopus renal epithelial cell line

1. The signal transduction mechanism mediating extracellular adenosine 5'-triphosphate (ATP)-induced calcium release in a renal epithelial cell line (A6) was investigated using the whole-cell voltage-clamp technique and fura-2 fluorescence measurement. 2. ATP (10 mu M) activated calcium-dependent non-selective cation channels in cells held under voltage clamp. 3. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta X; 0.1-1.0 mM) in the pipette inhibited the ATP-activated calcium-dependent currents. With guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 0.1-1.0 mM) in the pipette, the currents were spontaneously elicited without application of ATP. Pretreatment with pertussis toxin (PTX) affected neither the ATP-activated currents nor the increase in intracellular free calcium concentration ([Cd2+](i)) evoked by ATP. 4. Intracellular application of neomycin or heparin inhibited the ATP-activated currents. Inositol 1,4,5-trisphosphate (IP3; 0.1-100 mu M) in the internal solution produced currents similar to those due to ATP activation. 5. These results suggest that a PTX-insensitive guanosine 5'-triphosphate (GTP)-binding regulatory protein (G protein) is involved in extracellular ATP-induced phosphoinositide turnover and subsequent calcium release from IP3-sensitive stores, which subsequently activates the calcium-dependent channels in AB cells.