MODULATION BY PURINES OF CALCIUM-ACTIVATED NONSELECTIVE CATION CHANNELS IN THE OUTER HAIR-CELLS OF THE GUINEA-PIG COCHLEA

1. The cell-attached and cell-free configurations of the patch-clamp t echnique were used to investigate whether external ATP and its derivat ives modulate channel activity in outer hair cells freshly isolated fr om the guinea-pig cochlea. 2. Submicromolar concentrations of ATP stim ulated a non-selective cation channel with a conductance of about 25 p S. The ATP-elicited stimulation was partly blocked by the membrane-per meant blocker 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), an d mimicked by the calcium ionophore, ionomycin, suggesting that the ch annel activated by ATP is identical to a previously reported calcium-a ctivated 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 t he P-2Y antagonist Reactive Blue 2. These results suggest that both pu rinergic receptors are involved in the ATP-evoked response and that in ternal calcium acts as a second messenger for opening CAN channels. 4. In contrast, adenosine inhibited CAN channels. This effect was reprod uced by the A(2) agonist 5'-N-ethylcarboxyamidoadenosine (NECA) and th e 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 protei n kinase A was applied internally on inside-out patches, suggesting th at adenosine A2 receptor downregulates CAN channels via a cAMP-depende nt phosphorylation.