Regional involvement of an endothelium‐derived contractile factor in the vasoactive actions of neuropeptide Y in bovine isolated retinal arteries

1 In vitro experiments in a microvascular myograph were designed in order to investigate the effects of human neuropeptide Y (NPY), its receptor subtype and the mechanisms underlying NPY actions in bovine isolated retinal proximal (PRA) and distal (DRA) arteries. 2 A single concentration of NPY (10 nm) induced a prompt and reproducible contraction which reached a plateau within 1–4 min, after which the response returned to baseline over the next 2 −10 min. Cumulative addition of NPY induced concentration‐dependent contractions of bovine retinal arteries, with an EC50[M] of 1.7 nm and a maximal response equal to 54 ±8% of Emax (absolute maximal contractile levels of vessels) and not different from that obtained by a single addition of the peptide. There were no significant differences in either sensitivity or maximal response to NPY between PRA and DRA. 3 Porcine NPY and the selective Y1‐receptor agonist, [Pro34]NPY, also induced concentration‐dependent contractions of the retinal arteries with a potency and maximal response not significantly different from those of human NPY; in contrast, the selective Y2‐receptor agonist, NPY(13 −36), caused only a 5% contraction at the highest concentration used. 4 Removal of extracellular Ca2+ or pretreatment with the 1,4‐dihydropyridine Ca2+‐channel blocker, nifedipine (1 μm), reduced the contractile response of 10 nm NPY to 18.4±3.3% (n = 6) and 18.6±3.9% (n = 6), respectively, of the controls. 5 Mechanical removal of the endothelium depressed the maximal contraction elicited by NPY in PRA but did not affect either sensitivity or maximal response to the peptide in DRA. In endothelium‐intact arteries, blockade of the cyclo‐oxygenase pathway with 3 μm indomethacin increased resting tension in both PRA and DRA and significantly inhibited sensitivity and maximal contraction to NPY of PRA and DRA, respectively. The thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist, SQ30741, reduced both sensitivity and maximal contraction to NPY in PRA but not in DRA. 6 In endothelium‐denuded PRA, indomethacin but not SQ30741 significantly reduced NPY maximal response and induced a marked increase in resting tension suggesting a basal release of a vasodilator prostanoid from smooth muscle cells. 7 Superoxide dismutase (SOD) (150 u ml−1) reduced the maximal contraction to NPY in PRA. Inhibition of the nitric oxide (NO) synthase with NG‐nitro‐L‐arginine (L‐NOARG) (30 μm), enhanced sensitivity and maximal contraction to NPY in both PRA and DRA. In the presence of L‐NOARG, SOD did not further inhibit NPY responses in PRA. 8 NPY (10 nm) induced a 2.9 fold leftwards shift of the noradrenaline concentration‐response curves in PRA and increased maximal response by 50 ± 16%. Neither 1 nor 10 nm NPY affected noradrenaline responses in DRA. [Pro34]NPY (10 nm), but not NPY(13–36), mimicked the potentiating effect of NPY on noradrenaline responses in PRA. 9 TXA2 analogue, U46619, at 10 nm elicited 3.6 fold leftwards shift of the noradrenaline concentration‐responses curves in PRA and increased the maximal contraction by 32±3, whereas in the presence of 1μm SQ30741, 10 nm NPY did not potentiate noradrenaline responses. 10 The present results indicate that NPY may play a role in the regulation of retinal blood flow through both a direct contractile action, independent of the vessel size and a potentiation of the responses induced by noradrenaline in the proximal part of the retinal circulation, both effects being mediated by Y1 receptors. NPY promotes Ca2+ influx through voltage‐dependent Ca2+ channels and stimulates the synthesis of contractile prostanoids in PRA and DRA, although only in PRA does the peptide trigger the release of an endothelium‐derived contractile factor which facilitates the contraction and also seems to account for the potentiating effect of NPY.