Functional properties of native and cloned P2X receptors

Electrophysiological experiments on dissociated smooth muscle and neurons have revealed three distinct phenotypes of P2X receptor: (1) a rapidly desensitizing, beta-methylene ATP-sensitive response typical of most smooth muscle; (2) a non-desensitizing, alpha,beta-methylene ATP-insensitive response characteristic of PC12 phaeochromocytoma cells and rat superior cervical ganglion neurons; and (3) a non-desensitizing, alpha, beta-methylene ATP-sensitive response observed in sensory neurons. All of these purinoceptors share a similar cationic and high Ca2+ permeability and sensitivity to blockade by suramin, Cibacron blue, oxidized ATP, pyridoxal-5-phosphate and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Heterologous expression of two forms of cloned P2X receptors (from rat vas deferens and PC12 cells) reveals that each cloned receptor can reconstitute native responses with remarkable fidelity. Such results suggest that homo-oligomeric channels may be formed from single subunits of the P2X receptor in smooth muscle, PC12 cells and some neurons. The third phenotype observed in native cells might result from co-assembly of subunits of the cloned receptors. However, co-expression studies show that these two forms of the P2X receptor do not heteropolymerize. Therefore, the non-desensitizing, alpha, beta-methylene ATP-sensitive response observed in sensory neurons may result from a distinct P2X receptor or from heteropolymerization of more than one distinct P2X purinoceptor.