Involvement of ATP in the non‐adrenergic non‐cholinergic inhibitory neurotransmission of lamb isolated coronary small arteries

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Abstract
1. The involvement of non-adrenergic non-cholinergic (NANC) transmitters, such as nitric oxide (NO) and adenosine 5'-triphosphate (ATP), in the neurogenic relaxation of lamb coronary small arteries was investigated in vessel segments with an internal lumen diameter of 200-550 microns, isolated from the left ventricle of the heart, and suspended for isometric tension recording in microvascular myographs. 2. In both endothelium-intact and -denuded coronary small arteries treated with phentolamine (3 x 10(-6) M), propranolol (3 x 10(-6) M), and atropine (10(-6) M) and contracted to 3 x 10(-7) M of the thromboxane analogue U46619, electrical field stimulation (EFS) evoked frequency-dependent relaxations, which were markedly reduced in the presence of tetrodotoxin (10(-6) M). 3. Exogenous NO added as acidified sodium nitrite (10(-6)-10(-3) M) and L-nitrosocysteine induced potent relaxations of lamb coronary small arteries. However, both inhibition of NO synthase with NG- nitro-L-arginine (L-NOARG, 3 x 10(-5) M), and mechanical endothelial cell removal increased rather than inhibited relaxations to EFS. In small arteries processed for NADPH-diaphorase histochemistry, activity was only observed within endothelial cells. 4. In arteries contracted to U46619, exogenously added ATP caused concentration-dependent relaxations with pD2 and maximum responses of 4.72 +/- 0.12 and 89.6 +/- 3.8% (n = 12), respectively. ADP and the P2Y-agonist, 2-methylthio-ATP, induced relaxations equipotent to ATP, while the P2x-agonist, alpha, beta-methylene ATP (10(-9)-10(-4) M), and the P2U-agonist, UTP (10(-9)-10(-4) M) only caused small transient relaxations at the highest concentrations (10(-4) and 10(-3) M). 5. ATP and EFS-induced relaxations were unchanged in the presence of the P1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M), while this antagonist inhibited the concentration-dependent relaxations to adenosine. In contrast, the P2-purinoceptor antagonist, suramin (3 x 10(-5) M), markedly reduced the relaxations to EFS. 6. After desensitization of P2x-purinoceptors with alpha, beta-methylene ATP (2 x 10(-5) M), the relaxations to exogenous added ATP were enhanced, but this procedure did not influence the relaxations to EFS. In contrast, the P2y-purinoceptor antagonist, basilen blue E-3G (3 x 10(-5) M, earlier named reactive blue 2) significantly inhibited the concentration-relaxation curves to ATP and almost abolished the EFS-induced relaxations. 7. Mechanical removal of the endothelium significantly inhibited ATP-induced maximal relaxations without affecting sensitivity, pD2 and maximum relaxations being 4.72 +/- 0.12 and 89.7 +/- 3.8% (n = 10), and 5.45 +/- 0.38 and 48.0 +/- 8.6% (P < 0.05, paired t test, n = 10) in endothelium-intact and -denuded coronary small arteries, respectively. However, incubation with L-NOARG did not change relaxations elicited by ATP. 8. The present study suggests that in NANC conditions neurogenic relaxations of coronary small arteries are mediated by ATP, which relaxes coronary small arteries through P2Y-purinoceptors. A prejunctional modulation of these relaxations by endothelial-derived NO cannot be excluded.