ATP-INDUCED CURRENT IN ISOLATED OUTER HAIR-CELLS OF GUINEA-PIG COCHLEA

1. Electrical and pharmacologic properties of ATP-induced current in outer hair cells isolated from guinea pig cochlea were investigated in the whole-cell recording mode by the use of a conventional patch-clamp technique. 2. Under current-clamp conditions, rapid application of ATP depolarized the outer hair cells resulting in an increase in conductance. The ATP-induced response did not show any desensitization during a continuous application. 3. At a holding potential of -70 mV, the ATP-induced inward current increased in a sigmoidal fashion over the concentration range between 3 μM and 1 mM. The half-maximum concentration (EC50) was 12 μM and the Hill coefficient was 0.93. 4. The ATP-induced current had a reversal potential near 6 mV, which was close to the theoretical value (1 mV) calculated from the Goldman-Hodgkin-Katz equation for permeable intra- and extracellular cations. 5. In the current-voltage (I-V) relationship for the ATP response a slight inward-going rectification was observed at more positive potentials than the reversal potential. 6. The substitution of extracellular Na+ by equimolar choline+ shifted the reversal potential of the ATP-induced current to more negative values. The substitution of Cs+ in the internal solution by N-methyl-D-glucamine+ (NMG+) shifted it in the positive direction. The reversal potential of ATP-induced current was also shifted to positive values with increasing extracellular Ca2+ concentration. A decrease of intracellular Cl- by gluconate- did not affect the revesal potential, thereby indicating that the ATP-induced current is carried through a large cation channel. 7. Calculated relative permeability ratios were P(Na)/P(Cs) = 1.08 and P(Ca)/P(Cs) = 3.02. The sequence of relative permeabilities for the large cation channel was in the order Ca2+ >> Li+ > Na+ > Cs+ >> choline+. 8. The relative potency of ATP agonists was ATP >> ADP > adenylylimido 5'-diphosphate lithium salt (AMP-PNP) > α,β-methylene adenosine 5'-triphosphate lithium salt (α,β-meATP) = β,γ-methylene adenosine 5'-triphosphate sodium salt (β,γ-meATP), indicating the involvement of the P2 subtype of purinergic receptors in outer hair cells. 9. Quinidine, d-tubocurarine and phentolamine antagonized the ATP-induced current in a concentration-dependent manner.