Lack of run-down of smooth muscle P2X receptor currents recorded with the amphotericin permeabilized patch technique, physiological and pharmacological characterization of the properties of mesenteric artery P2X receptor ion channels

P2X(1) receptor subunit contribution to gating revealed by a dominant negative PKC mutant

The family of ATP-gated P2X receptor channels have a conserved protein kinase C site in the N-terminal intracellular domain. This site was disrupted in human P2X(1) receptors by the mutation T18A. T18A mutants were expressed at normal levels in Xenopus oocytes; however, the peak current amplitude was reduced by >99% and showed approximately 10 fold faster desensitisation in response to ATP than wild type (WT) receptors showed. P2X receptor subunits form functional trimeric channels. Co-expression of T18A and WT receptors (90:10 ratio) produced heteromeric T18A/WT channels with the rapid T18A time-course and an approximately 90-fold increase in peak current amplitude compared to T18A. Similarly, T18A dominated the desensitisation phenotype of heteromeric channels composed of T18A and slowly desensitising K68A mutants. These results suggest that phosphorylation of P2X(1) receptors has a dramatic effect on the time-course of the response and may provide a mechanism for regulating channel function.

Structure-activity relationships of diadenosine polyphosphates (Ap(n)As), adenosine polyphospho guanosines (Ap(n)Gs) and guanosine polyphospho guanosines (Gp(n)Gs) at P2 receptors in the rat mesenteric arterial bed

1. Vascular effects of diadenosine polyphosphates (Ap(n)As), adenosine polyphospho guanosines (Ap(n)Gs) and guanosine polyphospho guanosines (Gp(n)Gs), novel families of naturally-occurring signalling molecules, were investigated in methoxamine preconstricted rat isolated perfused mesenteric arterial beds. 2. Three different types of response were elicited by Ap(n)As and Ap(n)Gs. Those with a short polyphosphate chain (n=2 - 3) elicited vasorelaxation. Ap(3)A was more potent than Ap(2)A, and both were more potent than the corresponding Ap(n)G. Relaxations to Ap(3)A and Ap(3)G, but not to Ap(2)A and Ap(2)G, were blocked by endothelium removal and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a P2 receptor antagonist. 3. Longer polyphosphate chain Ap(n)As and Ap(n)Gs (n=4 - 6) elicited dose-dependent vasoconstriction followed by prolonged vasorelaxation, with a potency order for both types of response of Ap(5)A> or =Ap(6)A>Ap(4)A. A similar order and potency was observed for Ap(n)Gs. Contractions and prolonged relaxations were blocked by PPADS and P2X(1) receptor desensitization with alpha,beta-methylene ATP (alpha,beta-meATP), and were largely endothelium-independent. 4. In the presence of alpha,beta-meATP rapid relaxations to contractile Ap(n)As and Ap(n)Gs (n=4 - 6) were revealed. 5. Gp(n)Gs were virtually inactive, except for Gp(2)G which elicited vasoconstriction via PPADS- and alpha,beta-meATP-sensitive smooth muscle P2X(1)-like receptors. 6. These data show that, as with Ap(n)As, the length of the polyphosphate chain (n) is an important determinant of the activity of Ap(n)Gs at P2 receptors in the rat mesenteric arterial bed. When the chain is short (n=2 - 3) the purines elicit rapid vasorelaxation, which for Ap(3)A and Ap(3)G is mediated via endothelial P2Y(1)-like receptors. When the chain is long (n=4 - 6) Ap(n)As and Ap(n)Gs elicit vasoconstriction via P2X(1)-like receptors, followed by prolonged endothelium-independent vasorelaxation. Rapid relaxation to contractile dinucleotides (n=4 - 6) is revealed by block of vasoconstriction. Regarding the purine moiety, one adenine is crucial and sufficient for vasoactivity as Gp(n)Gs were largely inactive, and Ap(n)As and Ap(n)Gs approximately equipotent.

Agonist-stimulated internalisation of the ligand-gated ion channel P2X(1) in rat vas deferens

Using cell surface biotinylation and Western blotting, we investigated the extent to which native P2X(1) receptors in rat vas deferens are internalised after exposure to agonist. Exposure to 100 microM alpha,beta-meATP 30 min prior and during a 10 min biotinylation period resulted in a approximately 50% reduction in the amount of biotinylated P2X(1) receptor indicating that activation of the receptor by agonist induces receptor internalisation. Furthermore, biotinylation under saturating conditions suggests that once internalised, a rapid recycling of P2X(1) receptor back to the cell surface occurs. The physiological implications of these mechanisms in terms of receptor function are discussed.

Properties of P2X and P2Y receptors are dependent on artery diameter in the rat mesenteric bed

P2 receptor mediated contractile responses have been characterized in different diameter arteries from the rat mesenteric arterial vasculature (first, second to third and fifth to sixth order for large, medium and small arteries) using wire myograph and diamtrak video imaging. alpha,ss-methylene ATP (alpha,beta-meATP) evoked transient concentration-dependent contractions in mesenteric arteries with EC(50) values of 0.4, 2.5 and 107 microM for small, medium and large arteries respectively. Suramin (10 - 100 microM) produced substantial parallel rightward shifts of the concentration-response curve to alpha,beta-meATP in small and medium-sized arteries with pA(2) of 5.1. Responses in large vessels were unaffected by suramin. Immunohistochemical analysis of arterial sections revealed no substantial differences in expression patterns of P2X receptors between different sizes of artery. P2X(1) receptors were expressed at high levels, P2X(4) and P2X(5) receptors were also detected on smooth muscle. The P2X receptor response is dominated by P2X(1) receptor in small and medium arteries but the nature of the receptor mediating the suramin insensitive alpha,beta-meATP mediated response in large arteries is unclear. The P2Y receptor agonist UTP was significantly more potent in small than in medium or large arteries (EC(50) values: 15.0 microM small, 88.5 microM diamtrak medium 1.6 mM myography medium and 1.4 mM large). Responses in both small and medium-sized vessels were reduced by suramin (30 - 100 microM). The sensitivity to UTP and suramin indicates the presence of P2Y(2) receptors. This study shows that P2 receptors do not have a homogenous phenotype throughout the mesenteric vascular bed and that the properties depend on artery size.