Two different ionotropic receptors are activated by ATP in rat microglia

1. Our aim was to assess whether ATP-induced inward currents in microglia are due to a single or more than one purinergic receptor. The ATP dose-response curve showed two components, whose presence might be due to the activation of high and low affinity receptors. 2. The P2Z/P2X7 specific receptor agonist benzoylbenzoyl-ATP (Bz-ATP) and some P2 receptor agonists were tested. The rank order of potency was Bz-ATP >> ATP = 8-methylthio-ATP (2-MeSATP) > alpha,beta-methylene ATP (alpha,beta-meATP) greater than or equal to ADP. beta,gamma-MethyleneATP (beta,gamma-meATP), UTP and adenosine were ineffective. 3. The non-specific P2 receptor antagonist suramin antagonized by 92 +/- 2% the inward current induced by 100 mu M ATP, and by 51 +/- 8 and 68 +/- 6% those induced by 3 mM ATP and 100 mu M Bz-ATP, respectively. The P2Z/P2X7 antagonist oxidized ATP (oATP) almost abolished the inward current induced by 3 mw ATP or Bz-ATP, but was ineffective against 100 mu M ATP. 4. Inward currents induced by low ATP concentrations (less than or equal to 100 mu M) were generally followed by an almost complete and irreversible desensitization, while those elicited by ATP greater than or equal to 1 mM showed only a partial decline. Interestingly,, the inward current induced by 100 mu M 2-MeSATP showed a large desensitization, while that induced by Bz-ATP did not. 5. In voltage-ramp experiments, the 100 mu M ATP-induced current exhibited a slight inward rectification more risible at negative potentials, while the 3 mM ATP-induced current did not. 6. ATP induced a fast and large increase in [Ca2+] that promptly recovered in the continuous presence of low ATP doses, but did not recover in high ATP doses. As with desensitization, the response to Bz-ATP mimicked that of high doses of ATP. 7. When Ca2+ mobilization due to P2Y receptors was blocked by thapsigargin-induced Ca2+ depletion or by pertussis toxin treatment, 10 mu M nl ATP was still able to induce a Ca2+ transient, which represented the contribution of the Ca2+ influx induced by P2X receptors. 8. In conclusion, the inward currents and a fraction of the Ca2+ transients induced by ATP in microglia are due to at least two ATP-sensitive receptor channel types, whose different proper ties and sensitivity to ATP may be associated with different functional roles.