P2X receptors in cochlear Deiters' cells

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Abstract
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. 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. Increasing intracellular free Ca2+ by decreasing the concentration of EGTA in the pipette solution reduced the amplitude of the ATP‐gated current. 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). 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. 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.