P2X2 receptor mediates stimulation of parasensory cation absorption by cochlear outer sulcus cells and vestibular transitional cells

Cochlear outer sulcus cells (OSC) and vestibular transitional cells (VTC) are part of the parasensory epithelium in the inner ear and are located in homologous positions between the sensory hair cells and the cation secretory epithelial cells in the cochlea and the vestibular labyrinth. OSC are known to sustain a reabsorptive transepithelial current and to contain an immunoreactivity for P2X(2) purinergic receptors. This study addresses whether OSC and VTC share functional similarities and extends this hypothesis to the question of whether both cell types contain functional P2X(2) receptors. The current density (I-sc) was recorded with the vibrating probe technique and was found to be similar in VTC and OSC. Both gadolinium and flufenamic acid reduced I-sc in VTC, as reported previously for OSC. I-sc was stimulated by extracellular ATP but not by selective agonists of P2Y receptors. Purinergic receptor agonists increased I-sc with a potency order of ATP>2'- and 3'-O-(4-benzoyl-benzoyl) adenosine 5'- triphosphate much greater than alpha,beta -methyleneadenosine 5'- triphosphate in both OSC and VTC. In the presence of suramin (100 muM) or gadolinium (100 muM), the responses of ATP were inhibited significantly in both OSC and VTC. This pharmacological profile is consistent with that of the P2X(2) receptor. These results demonstrate that VTC participate in vestibular parasensory cation absorption and that both OSC and VTC regulate their parasensory cation flux via P2X(2) receptors, which would regulate the endolymphatic concentration of the current-carrying ion species in auditory and vestibular transduction.