Novel variant of the P2X2 ATP receptor from the guinea pig organ of Corti

Outward rectifying potassium currents are the dominant voltage activated currents present in Deiters' cells

Supporting cells in the cochlea are thought to maintain the homeostasis of the organ of Corti and contribute to the electrical and micromechanical environment of the hair cells. Of the different types of supporting cells, Deiters' cells form a structure that holds the outer hair cells (OHCs) at their base and apex. This structure may play an important role in modifying cochlear mechanics by influencing the force produced by sound induced motion of the OHCs which in turn may be modulated by ATP acting on ligand gated cation channels on the Deiters' cells. Also, a glia-like role of buffering external K+ concentration for the Deiters' cells has been suggested. We studied Deiters' cells' electrical properties and ion conductances using the whole cell variant of the patch clamp technique since they must play an important role in the function of these cells. It was found that isolated Deiters' cells possess a large voltage activated, outwardly rectifying K+ selective conductance. Voltage activated Ca2+ currents and non-selective currents were not detected and voltage activated inward currents were very small. The outward K+ currents were found to be dependent on voltage but not on Ca2+ for their activation. Nimodipine and 4-aminopyridine (4-AP) were shown to interact directly with the K+ channels in a voltage dependent manner. It is suggested that the K+ selective channels in Deiters' cells may be similar to the Kv1.5 type channel. However, based on the voltage dependence of the channels that was described by double Boltzmann equation and on the alteration of that dependence by 4-AP, it is possible that more than one type of K+ selective channel exists.

Additional pharmacological evidence that endogenous ATP modulates cochlear mechanics

In the cochlea, outer hair cells (OHCs) generate the active cochlear mechanics whereas the supporting cells, such as Deiters' cells and Hensen's cells, may play a role in both the active and passive cochlear mechanics. The presence of receptors for adenosine triphosphate (ATP) on OHCs, Deiters' cells and Hensen's cells indicates that endogenous ATP may have a role in cochlear mechanics. To explore this possibility, the effects of the ATP antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), were studied in guinea pig both in vitro on isolated OHCs, Deiters' cells, Hensen's cells and pillar cells using the whole-cell configuration of the patch-clamp technique, and in vivo on sound evoked cochlear potentials (cochlear microphonic, CM; summating potential, SP; compound action potential, CAP) and distortion product otoacoustic emissions (DPOAEs) using cochlear perilymphatic perfusion. Results show that PPADS (100 microM) reduced the inward current evoked by 5-10 microM ATP in OHCs, Deiters' cells, Hensen's cells and pillar cells. This effect of PPADS was slow in onset and was slowly reversed to a varying degree in the different cell types. In vivo application of PPADS in increasing concentrations reduced the sound evoked CAP, SP and increased N1 latency starting at about 0.33 mM (SP) and 1 mM (CAP and N1 latency). PPADS (0.33-1 mM) reversibly suppressed the initial value of the quadratic DPOAE and reversed the 'slow decline' in the quadratic DPOAE that occurs during continuous stimulation with moderate level primaries. These results, together with the similar effects of the ATP antagonist suramin reported previously (Skellett et al., 1997), may be evidence that endogenous ATP acting on cells in the organ of Corti alters cochlear mechanics.