Two different ionotropic receptors are activated by ATP in rat microglia

1. Our aim was to assess whether ATP-induced inward currents in microglia a re due to a single or more than one purinergic receptor. The ATP dose-respo nse curve showed two components, whose presence might be due to the activat ion of high and low affinity receptors. 2. The P2Z/P2X7 specific receptor agonist benzoylbenzoyl-ATP (Bz-ATP) and s ome 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-m eATP), 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 a ntagonist oxidized ATP (oATP) almost abolished the inward current induced b y 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 d esensitization, while those elicited by ATP greater than or equal to 1 mM s howed 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 th e 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 AT P doses. As with desensitization, the response to Bz-ATP mimicked that of h igh 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 o f 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 a ssociated with different functional roles.