Effects of anions on ATP-activated nonselective cation current in NG108-15 cells

Extracellular ATP induces a nonselective cation current and elevates intracellular Ca2+ concentration via P-2Z receptors in NG108-15 cells. We found that the ATP-induced nonselective cation current became larger in methanesulfonic acid (MS-) than in Cl- external solution. We therefore examined the effects of various external anions on the ATP-induced cation current with the use of the whole cell patch-clamp technique. The concentration-response curves for ATP were obtained in different anionic external solutions. The maximum current density (I-max) and the concentration of agonist that gives 50% of maximum response (EC50) value of ATP were obtained by fitting the curves with the use of the Hill coefficient of 2. The apparent I-max decreased in the order of aspartic acid (Asp(-)) > MS- > F- > Cl- > Br- greater than or equal to I-. The apparent EC50 values for ATP were shifted to the right in the sequence of Asp(-) < F- < MS- < Br- < Cl- < I-. Thus both I-max and EC50 values were affected by anions, indicating that anions are mixed type inhibitors of the ATP-induced current. The shift of the EC50 values of ATP indicates that anions interfere with ATP binding to the receptor. External Cl- was a noncompetitive inhibitor with respect to external Na+, a major cation carrying the ATP-induced current. We conclude that extracellular anions inhibit the ATP-induced nonselective cation current at least partly by interfering with ATP binding to the P-2Z receptor, which is associated with the nonselective cation channels.