ATP has been suggested to act as a neurotransmitter or a neuromodulator in the cochlea. The responses to ATP in different cell types of the cochlea vary in terms of the rate of desensitization and magnitude, suggesting that there may be different subtypes of P2X receptors distributed in the cochlea. Recently three ionotropic P2X2 receptor splice variants, P2X2-1, P2X2- 2, and P2X2-3, were isolated and sequenced from a guinea pig cochlear cDNA library. To test the hypothesis that these different splice variants could be expressed as functional homomeric receptors, the three P2X2 receptor variants were individually and transiently expressed in human embryonic kidney cells (HEK293). The biophysical and pharmacological properties of these receptors were characterized using the whole cell patch- clamp technique. Extracellular application of ATP induced an inward current in HEK293 cells containing each of the three splice variants in a dose- dependent manner indicating the expression of homomeric receptors. Current- voltage (I-V) relationships for the ATP-gated current show that the three subtypes of the P2X2 receptor had a similar reversal potential and an inward rectification index (I50 (mV)/I(-50 mV)). However, the ATP-induced currents in cells expressing P2X2-1 and P2X2-2 variants were large and desensitized rapidly whereas the current in those cells expressing the P2X2-3 variant was much smaller and desensitized slower. The order of potency to ATP agonists was 2-MeSATP > ATP > α,β -MeATP for all three expressed splice variants. The ATP receptor antagonists suramin and PPADS reduced the effects of ATP on all three variants. Results demonstrate that three P2X2 splice variants from guinea pig cochlea, P2X2-1, P2X2-2, and P2X2-3, can individually form nonselective cation receptor channels when these subunits are expressed in HEK293 cells. The distinct properties of these P2X2 receptor splice variants may contribute to the differences in the response to ATP observed in native cochlear cells.
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