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 or der to investigate the effects of human neuropeptide Y (NPY), its rece ptor subtype and the mechanisms underlying NPY actions in bovine isola ted retinal proximal (PRA) and distal (DRA) arteries. 2 A single conce ntration of NPY (10 nM) induced a prompt and reproducible contraction which reached a plateau within 1-4 min, after which the response retur ned to baseline over the next 2-10 min. Cumulative addition of NPY ind uced concentration-dependent contractions of bovine retinal arteries, with an EC(50)[M] of 1.7 nM and a maximal response equal to 54 +/- 8% of E(max) (absolute maximal contractile levels of vessels) and not dif ferent from that obtained by a single addition of the peptide. There w ere no significant differences in either sensitivity or maximal respon se to NPY between PRA and DRA. 3 Porcine NPY and the selective Y-1-rec eptor agonist, [Pro(34)]NPY, also induced concentration-dependent cont ractions of the retinal arteries with a potency and maximal response n ot significantly different from those of human NPY; in contrast, the s elective Y-2-receptor agonist, NPY (13-36), caused only a 5% contracti on at the highest concentration used. 4 Removal of extracellular Ca2or pretreatment with the 1,4-dihydropyridine Ca2+-channel blocker, nif edipine (1 mu M), 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 con trols. 5 Mechanical removal of the endothelium depressed the maximal c ontraction elicited by NPY in PRA but did not affect either sensitivit y or maximal response to the peptide in DRA. In endothelium-intact art eries, blockade of the cyclo-oxygenase pathway with 3 mu M indomethaci n increased resting tension in both PRA and DRA and significantly inhi bited sensitivity and maximal contraction to NPY of PRA and DRA, respe ctively. The thromboxane A(2) (TXA(2))/prostaglandin H-2 (PGH(2)) rece ptor antagonist, SQ30741, reduced both sensitivity and maximal contrac tion to NPY in PRA but not in DRA. 6 In endothelium-denuded PRA, indom ethacin but not SQ30741 significantly reduced NPY maximal response and induced a marked increase in resting tension suggesting a basal relea se 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-a rginine (L-NOARG) (30 mu M), enhanced sensitivity and maximal contract ion to NPY in both PRA and DRA. In the presence of L-NOARG, SOD did no t further inhibit NPY responses in PRA. 8 NPY (10 nM) induced a 2.9 fo ld 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. [Pro(34)]NPY (10 nM), but not NPY (13-36), mimicked the potentiating effect of NPY on noradr enaline responses in PRA. 9 TXA(2) 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%, whe reas in the presence of 1 mu M SQ30741, 10 nM NPY did not potentiate n oradrenaline responses. 10 The present results indicate that NPY may p lay a role in the regulation of retinal blood flow through both a dire ct contractile action, independent of the vessel size and a potentiati on of the responses induced by noradrenaline in the proximal part of t he retinal circulation, both effects being mediated by Y-1 receptors. NPY promotes Ca2+ influx through voltage-dependent Ca2+ channels and s timulates the synthesis of contractile prostanoids in PRA and DRA, alt hough only in PRA does the peptide trigger the release of an endotheli um-derived contractile factor which facilitates the contraction and al so seems to account for the potentiating effect of NPY.