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

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 mu m, 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 held 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 N-G-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 pD(2) and maximum responses of 4.72+/-0.12 and 89.6+/-3.8% (n=12), respectively. ADP and the P-2Y-agonist, 2-methylthio-ATP, induced relaxations equipotent to ATP, while the P-2X-agonist, alpha, beta-methylene ATP (10(-9)-10(-4) M), and the P-2U-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 P-1-purinoceptor antagonist, 8-phenyltheophylline (10(-5) M), while this antagonist inhibited the concentration-dependent relaxations to adenosine. In contrast, the P-2-purinoceptor antagonist, suramin (3 x 10(-5) M), markedly reduced the relaxations to EFS. 6 After desensitization of P-2X-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 P-2Y-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, pD(2) 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 P-2Y-purinoceptors. A prejunctional modulation of these relaxations by endothelial-derived NO cannot be excluded.