P2X receptors in cochlear Deiters' cells

1. The ionotropic purinoceptors in isolated Deiters' cells of guinea-pig cochlea were characterized by use of the whole-cell variant of the patch-clamp technique. 2. Extracellular application of adenosine 5'-triphosphate (ATP) induced a dose-dependent inward current when the cells were voltage-clamped at -80 mV. The ATP-induced current showed desensitization and had a reversal potential around -4 mV. 3. Increasing intracellular free Ca²⁺ by decreasing the concentration of EGTA in the pipette solution reduced the amplitude of the ATP-gated current. 4. The order of agonist potency was: 2-methylthioATP (2-meSATP) > ATP > benzoylbenzoyl-ATP (BzATP) > α,β-methyleneATP (α,β,meATP > adenosine 5'-diphosphate (ADP) > uridine 5'-triphosphate (UTP) > adenosine 5'-monophosphate (AMP) = adenosine (Ad). 5. Pretreatment with forskolin (10 μM), 8-bromoadenosine-3',5'-cyclophosphate (8-Br-cyclic AMP, 1 mM), 3-isobutyl-1-methylxanthine (IBMX, 1 mM) or phorbol-12-myristate-13-acetate (PMA, 1 μM) reversibly reduced the ATP-induced peak current. 6. The results are consistent with molecular biological data which indicate that P2X₂ purinoceptors are present in Deiters' cells. In addition, the reduction of the ATP-gated current by activators of protein kinase A and protein kinase C indicates that these P2X₂ purinoceptors can be functionally modulated by receptor phosphorylation.