P2X receptors are trimeric, ATP-gated cation channels. In mammals seven P2X subtypes have been reported (P2X1-P2X7), as well as several variants generated by alternative splicing. Variants confer to the homomeric or heteromeric channels distinct functional and/or pharmacological properties. Molecular biology, biochemical, and functional analysis by electrophysiological methods were used to identify and study a new variant of the P2X1 receptor named P2X1b. This new variant, identified in rat optic nerve, was also expressed in other tissues. P2X1b receptors lack amino acids 182 to 208 of native P2X1, a region that includes residues that are highly conserved among distinct P2X receptors. When expressed in Xenopus oocytes, P2X1b was not functional as a homomer; however, when co-expressed with P2X1, it downregulated the electrical response generated by ATP compared with that of oocytes expressing P2X1 alone, and it seemed to form heteromeric channels with a modestly enhanced ATP potency. A decrease in responses to ATP in oocytes co-expressing different ratios of P2X1b to P2X1 was completely eliminated by overnight pretreatment with apyrase. Thus, it is suggested that P2X1b regulates, through a use-dependent mechanism, the availability, in the plasma membrane, of receptor channels that can be operated by ATP.
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