Activation of P2X-like receptors in rat isolated 2nd order mesenteric arteries induces vasodilation: pole of EDHF Louise S. Harrington and Jane A. Mitchell. Unit of Critical Care, The National Heart and Lung Institute, Imperial College, London, SW3 6LY, UK.

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ATP is released by a range of vascular cells under inflammatory conditions. ATP induces vasodilation in some vascular beds via the release of nitric oxide (NO) and prostacyclin (PGI₂) from the endothelium, by way of P2Y receptor activation. Recently, we have shown, using rat perfused mesenteric preparations, that high doses of ATP also induce a profound vasodilator response which we characterised as EDHF-like in nature (Stanford and Mitchell, 1998; Stanford et al., 2001). Furthermore, and by contrast to the initial transient NO-mediated vasodilatation induced by ATP, the 'EDHF' component does not appear to be mediated by traditional vasodilator P2Y receptors (Gitlin et al., 2001a), but may be mediated by either a P2X receptor (Gitlin et al., 2002; Ralevic, 2002) or an unidentified purinergic receptor. In order to further understand this phenomenon we have investigated the effects of ATP and the P2X selective ligand ,ß methylene ATP on vasomotor tone of isolated 2^nd order rat mesenteric arteries.

Male Wistar rats (200 ± 15.4g) were killed by lethal exposure to CO₂ followed by cervical dislocation. The mesenteric bed was removed and 2^nd order arteries (240-250µm) isolated and mounted in wire myographs using a dissecting microscope. Tissues were immersed in physiological salt solution (PSS), equilibrated (30 min) and tensions normalised as described previously (Mulvany and Halpern, 1977). Vessels were then contracted with approximately EC₈₀ concentration of methoxamine (10^-5M). Single concentrations of either ATP, ,ß methylene ATP (10^-4M each), acetylcholine or sodium nitropruside (10-5M each) were then added to tissues. Dilator responses were calculated as a percentage of tone induced by methoxamine. In some experiments the nitric oxide synthase inhibitor, L-N^G nitro-L-arginine (L-NAME; 10^-3M), the cyclo- oxygenase inhibitor indomethacin (10^-5M), or apamin (5x10^-7M) plus charybdotoxin (10^-7M), which together inhibit EDHF responses were added.

Figure 1: (A) ATP, a,b methylene ATP, acetylcholine (ACh) or sodium nitroprusside (SNP) induced dilation; time control (T.Con.).(B) Effect of L-NAME plus indomethacin (L+I), apamin plus charybdotoxin (A+C),L+I plus A+C (L/I/A/C) or KCl on the vasodilator actions of ATP. Data is shown as the mean ± s.e.m. for n=3-8 experiments. Significance (one-way ANOVA; p<0.05) between ATP-induced response with or without drugs is denoted by* .

Both ATP and ,ß methylene ATP induced vasodilation of pre-constricted mesenteric vessels. In both cases vasodilation was insensitive to the combination of L-NAME and indomethacin, but reduced by the combination of apamin plus charybdotoxin or high KCl (124x10^-3M).

Here we have reproduced a phenomenon previously only noted in intact perfused mesenteric beds where either ATP or a selective P2X ligand induces vasodilation, and is possibly mediated by EDHF. Since P2X receptors have previously been linked to vasoconstrictor responses, these findings prompt us to re-examine the role of P2X receptors in the regulation of vasomotor tone.

Acknowledgements: This work was funded by the British Heart Foundation.

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