Functional and biochemical evidence for heteromeric ATP-gated channels composed of P2X1 and P2X5 subunits

The mammalian P2X receptor gene family encodes two-transmembrane domain nonselective cation channels gated by extracellular ATP. Anatomical localization data obtained by in situ hybridization and immunocytochemistry have shown that neuronal P2X subunits are expressed in specific but overlapping distribution patterns. Therefore, the native ionotropic ATP receptors diversity most likely arises from interactions between different P2X subunits that generate hetero-multimers phenotypically distinct from homomeric channels. Rat P2X1 and P2X5 mRNAs are localized within common subsets of peripheral and central sensory neurons as well as spinal motoneurons. The present study demonstrates a functional association between P2X1 and P2X5 subunits giving rise to hybrid ATP-gated channels endowed with the pharmacology of P2X1 and the kinetics of P2X5. When expressed in Xenopus oocytes, hetero-oligomeric P2X1+5 ATP receptors were characterized by slowly desensitizing currents highly sensitive to the agonist alpha,beta-methylene ATP (EC50 = 1.1 microM) and to the antagonist trinitrophenyl ATP (IC50 = 64 nM), observed with neither P2X1 nor P2X5 alone. Direct physical evidence for P2X1+5 co-assembly was provided by reciprocal subunit-specific co-purifications between epitope-tagged P2X1 and P2X5 subunits transfected in HEK-293A cells.

Molecular cloning and regional distribution of a human proton receptor subunit with biphasic functional properties

Small changes of extracellular pH activate depolarizing inward currents in most nociceptive neurons. It has been recently proposed that acid sensitivity of sensory as well as central neurons is mediated by a family of proton-gated cation channels structurally related to Caenorhabditis elegans degenerins and mammalian epithelial sodium channels. We describe here the molecular cloning of a novel human proton receptor, hASIC3, a 531-amino acid-long subunit homologous to rat DRASIC. Expression of homomeric hASIC3 channels in Xenopus oocytes generated biphasic inward currents elicited at pH <5, providing the first functional evidence of a human proton-gated ion channel. Contrary to the DRASIC current phenotype, the fast desensitizing early component and the slow sustained late component differed both by their cationic selectivity and by their response to the antagonist amiloride, but not by their pH sensitivity (pH50 = 3.66 vs. 3.82). Using RT-PCR and mRNA blot hybridization, we detected hASIC3 mRNA in sensory ganglia, brain, and many internal tissues including lung and testis, so hASIC3 gene expression was not restricted to peripheral sensory neurons. These functional and anatomical data strongly suggest that hASIC3 plays a major role in persistent proton-induced currents occurring in physiological and pathological conditions of pH changes, likely through a tissue-specific heteropolymerization with other members of the proton-gated channel family.

Central P2X4 and P2X6 channel subunits coassemble into a novel heteromeric ATP receptor

Ionotropic ATP receptors are widely expressed in mammalian CNS. Despite extensive functional characterization of neuronal homomeric P2X receptors in heterologous expression systems, the subunit composition of native central P2X ATP-gated channels remains to be elucidated. P2X4 and P2X6 are major central subunits with highly overlapping mRNA distribution at both regional and cellular levels. When expressed alone in Xenopus oocytes, P2X6 subunits do not assemble into surface receptors responsive to ATP applications. On the other hand, P2X4 subunits assemble into bona fide ATP-gated channels, slowly desensitizing and weakly sensitive to the partial agonist alpha,beta-methylene ATP and to noncompetitive antagonists suramin and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulfonic acid. We demonstrate here that the coexpression of P2X4 and P2X6 subunits in Xenopus oocytes leads to the generation of a novel pharmacological phenotype of ionotropic ATP receptors. Heteromeric P2X4+6 receptors are activated by low-micromolar alpha, beta-methylene ATP (EC50 = 12 microM) and are blocked by suramin and by Reactive Blue 2, which has the property, at low concentrations, to potentiate homomeric P2X4 receptors. The assembly of P2X4 with P2X6 subunits results from subunit-dependent interactions, as shown by their specific copurification from HEK-293 cells transiently transfected with various epitope-tagged P2X channel subunits. Our data strongly suggest that the numerous cases of neuronal colocalizations of P2X4 and P2X6 subunits observed in mammalian CNS reflect the native expression of heteromeric P2X4+6 channels with unique functional properties.

Sensory presynaptic and widespread somatodendritic immunolocalization of central ionotropic P2X ATP receptors

Recent evidence suggests that extracellular ATP plays a neurotransmitter role in the central nervous system. Its fast ionotropic effects are exerted through a family of P2X ATP-gated channels expressed in brain and spinal cord. To determine the physiological significance of central ATP receptors, we have investigated the localization of a major neuronal P2X receptor at the cellular and subcellular levels using affinity-purified antibodies directed against the C-terminal domain of P2X4 subunit. Subunit-specific anti-P2X4 antibodies detected a single band of 57,000 +/- 3000 mol. wt in transfected HEK-293 cells and in homogenates from adult rat brain. The strongest expression of central P2X receptors was observed in the olfactory bulb, lateral septum, cerebellum and spinal cord. P2X4 immunoreactivity was also evident in widespread areas including the cerebral cortex, hippocampus, thalamus and brainstem. In all regions examined, P2X receptors were associated with perikarya and dendrites where they were concentrated at the level of afferent synaptic junctions, confirming a direct involvement of postsynaptic ATP-gated channels in fast excitatory purinergic transmission. Moreover, P2X4-containing purinoceptors were localized in axon terminals in the olfactory bulb and in the substantia gelatinosa of nucleus caudalis of the medulla and dorsal horn of the spinal cord, demonstrating an important selective presynaptic role of ATP in the modulation of neurotransmitter release in central sensory systems.

Primary structure and expression of a naturally truncated human P2X ATP receptor subunit from brain and immune system

A novel member of the ionotropic ATP receptor gene family has been identified in human brain. This 422 amino acid long P2X receptor subunit has 62% sequence identity with rat P2X5. Several characteristic motifs of ATP-gated channels are present in its primary structure, but this P2X5-related subunit displays a single transmembrane domain. Heterologous expression of chimeric subunits containing the C-terminal domain of rat P2X5 leads to the formation of desensitizing functional ATP-gated channels in Xenopus oocytes. The developmentally regulated mRNA, found in two splicing variant forms, is expressed at high levels in brain and immune system.

First-pass metabolism of lidocaine in the anesthetized rabbit. Contribution of the small intestine

It is assumed that, in vivo, the liver is the organ responsible for the first-pass metabolism of lidocaine. To assess in vivo whether the intestine and the lungs contribute with the liver to the first-pass metabolism of lidocaine, groups of anesthetized rabbits (N = 6/group) received lidocaine into the thoracic aorta (10 mg/kg), a jugular vein (10 mg/kg), a mesenteric vein (20 mg/kg), and into the duodenum (20 mg/kg). Serial blood samples were withdrawn from the abdominal aorta. The area under lidocaine plasma concentration-time curve [AUCL(O-infinity)] corrected by the dose, when injected into the jugular vein, was equal to that estimated when injected into the thoracic aorta, but was larger than the AUCL(O-infinity) corrected by the dose of lidocaine injected into a mesenteric vein [i.e. 0.0047 +/- 0.0005 vs. 0.0030 +/- 0.0004 min/ml, respectively (p < 0.05)]. Moreover, the latter was greater (p < 0.05) than the AUCL(O-infinity) corrected by the dose of lidocaine instilled into the duodenum (0.0024 +/- 0.0003 min/ml). Liver and intestinal extractions of lidocaine were 36 and 20%, respectively. Oral systemic bioavailability was 0.49. The metabolites of lidocaine, monoethylglycinexylidide (MEGX) and glycinexylidide (GX), were only detected when lidocaine was administered before the liver or the intestine. In in vitro studies, incubation of lidocaine in the supernatant of the 10,000g of epithelial cells of the intestine, liver, and lungs decreased lidocaine concentrations at the following rates: liver > intestine approximately lungs. MEGX and GX could be measured in the liver, but only MEGX in the small intestine and lung. It is concluded that the intestine contributes with the liver to the first-pass metabolism of lidocaine.