Ca(2+)-activated K+ current induced by external ATP in PC12 cells.

1. The effect of external ATP on the membrane current was investigated in PC12 cells by whole-cell voltage-clamp techniques. 2. Lower concentrations of ATP (1 or 10 mumol/L) induced only an inward current at 1 mmol/L EGTA in the K+ pipette solution, while higher concentrations of ATP (100 mumol/L and 1 mmol/L) induced an outward current following the inward current. 3. Lowering the EGTA concentration in the pipette solution induced a larger outward current following ATP application. The membrane potential at which the outward current crossed with the control before ATP application was more negative at lower concentrations of EGTA in the pipette. 4. The development of the outward current was blocked by a Ca(2+)-free external solution, 5 mmol/L tetraethylammonium and a Cs+ pipette solution instead of K+, indicating that the outward current was a Ca(2+)-activated K+ current. 5. Charybdotoxin (0.1 mumol/L) and iberiotoxin (0.1 mumol/L), but not apamin (0.2 mumol/L) blocked the development of the outward current, indicating the ATP-induced outward current is a BK-type Ca(2+)-activated K+ channel current and not the SK type. 6. UTP had no effect on the membrane current, indicating that the ATP-induced current change was not mediated by P2u but by P2x purinoceptor. 7. In conclusion, stimulation of P2x purinoceptors by ATP induces a Ca(2+)-permeable inward current that results in increases in intracellular Ca2+ concentrations and activation of a BK-type Ca(2+)-activated K+ current in PC12 cells.