Modulation by purines of calcium-activated non-selective cation channels in the outer hair cells of the guinea-pig cochlea

1. The cell-attached and cell-free configurations of the patch-clamp technique were used to investigate whether external ATP and its derivatives modulate channel activity in outer hair cells freshly isolated from the guinea-pig cochlea. 2. Submicromolar concentrations of ATP stimulated a non-selective cation channel with a conductance of about 25 pS. The ATP-elicited stimulation was partly blocked by the membrane-permeant blocker 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), and mimicked by the calcium ionophore, ionomycin, suggesting that the channel activated by ATP is identical to a previously reported calcium-activated non-selective (CAN) cation channel. 3. The P-2X agonist beta,gamma-methylene-ATP (beta,gamma-MeATP, 10 mu M) and the P-2Y agonist 2-methyl- thio-ATP (2-MeSATP, 1 mu M) both activated CAN channels. The effect of ATP was inhibited by the P-2 antagonist suramin but not by the P-2Y antagonist Reactive Blue 2. These results suggest that both purinergic receptors are involved in the ATP-evoked response and that internal calcium acts as a second messenger for opening CAN channels. 4. In contrast, adenosine inhibited CAN channels. This effect was reproduced by the A(2) agonist 5'-N-ethylcarboxyamidoadenosine (NECA) and the permeant cAMP analogue 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not by the A(1) agonist N-6-cyclohexyladenosine (CHA). CAN channels were also inhibited when the catalytic subunit of protein kinase A was applied internally on inside-out patches, suggesting that adenosine A2 receptor downregulates CAN channels via a cAMP-dependent phosphorylation.