In this study the ATP-induced (P2X) currents in isolated peritoneal macrophages of wild type (WT) and P2X(4) knockout (P2X(4)(-/-)) mice were studied by means of whole-cell patch clamp in order to (1) survey the P2X currents of native macrophages and (2) to investigate the expression of P2X(4)-like currents in the WT versus P2X(4)(-/-) mice. Three types of currents were observed in the isolated macrophages: (1) in approximately 10% of both WT and P2X(4)(-/-) macrophages a fast activating and inactivating P2X1-like current was recorded with low concentrations (0.1-1 microM) of ATP; (2) 85% of wild type and 100% of P2X(4)(-/-) macrophages exhibited a non-desensitizing P2X(7)-like current activated at high concentrations of ATP (10mM). The identity of the P2X(7) current was confirmed using the specific blocker A-740003; (3) 88.6% of the WT but none of the P2X(4)(-/-) macrophages showed a small P2X(4)-like current that desensitized slowly upon ATP application at intermediate concentrations (3-300 microM). Several observations indicated that the slowly desensitizing current in WT macrophages was P2X(4). The EC50 value of 5.3 microM ATP was as expected for P2X(4) and the current induced by 3-300 microM ATP was absent in P2X(4)(-/-) mice. Upon application of 3 microM ivermectin, a P2X(4)-selective modulator, the amplitude of this current was increased and the desensitization was inhibited in WT cells. In addition, this current was facilitated by 10 microM Zn(2+) but inhibited by Cu(2+) (in contrast to P2X(2)). We conclude that the P2X(4) and P2X(7) currents are functionally expressed in recruited peritoneal macrophages of WT mice and that the P2X(4)-like current is absent in P2X(4)(-/-) mice.
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