Summary Isolated equine digital veins were examined in vitro to study the importance of the endothelium in the responses to both vasodilator and vasoconstrictor agents and to characterise the endothelial‐derived mediators involved. Carbachol (Cch; 1 μM) and bradykinin (Bk; 1 nM) caused relaxation of U44069‐induced tone by 79.5 ± 0.35 % and 73.7 ± 4.0 % respectively. Mechanical removal of the endothelium completely prevented relaxant responses to Cch and to Bk showing they were mediated by the endothelium. Treatment of veins with NG‐nitro‐L‐arginine methyl ester (L‐NAME; 30 and 300 μM) inhibited vasorelaxant responses to both Cch and Bk whereas the cyclooxygenase inhibitor, ibuprofen (10 μM) had no inhibitory effect. The inhibitory action of L‐NAME on the relaxations produced by Cch was partly reversed by L‐arginine (3 and 10 mM). Cch‐relaxations were potentiated in the presence of super oxide dismutase (15 units/ml) and inhibited by methylene blue (10 μM). The vasorelaxant effects of ATP (0.01 μM to 0.1 μM) were not dependent on the presence of the endothelium and the selective P2y receptor agonist, 2‐methylthio‐ATP proved to be ineffective as a vasodilator. Removal of the endothelium did not enhance the vasoconstrictor effects of the α1 adrenoceptor agonist phenylephrine (0.01 μM to 0.1 μM) and treatment with L‐NAME (300 μM) did not change the vasoconstrictor responses to 5‐HT (1 nM to 10 μM) or the α2 adrenoceptor agonist BHT‐920 (1 nM to 1 μM). These data suggest the L‐arginine nitric oxide pathway is of primary importance in mediating endothelium dependent relaxations to Cch and BK in equine digital veins but that basal release of nitric oxide is insufficient, under the conditions used in the present study, to modulate responses to vasoconstrictor agents. No functional evidence has been found for P2y‐receptors, α2‐adrenoceptors or 5‐HT receptors on endothelial cells of equine digital veins linked to nitric oxide production.

Summary Isolated equine digital veins (EDVs) were used to study β‐adrenoceptor mediated vasodilation and to examine isoxsuprine's vasodilatory mechanism of action. When the blood vessel wall tension was raised with potassium chloride solution (KCl; 59 mmol/l), the order of vasodilator potency of β‐agonists was: isoprenaline > fenoterol > noradrenaline > dobutamine > isoxsuprine. The β2‐selective adrenoceptor antagonist, ICI 118551 (1 nmol/1) caused a 6.74 and 6.65‐fold parallel shift to the right in the dose response curves to fenoterol and noradrenaline respectively. Propranolol (10 nmol/l) inhibited the vasodilatory action of isoprenaline in a competitive manner (19.6 ± 6.4‐fold parallel shift to the right) but was much less effective as an inhibitor of isoxsuprine's vasodilatory action. Isoprenaline and fenoterol were just as effective as vasodilators when blood vessel wall tension was raised with KCl, the thromboxanemimetic U44069 (9, 11‐dideoxy‐9α, 11α‐epoxymethano‐prostaglandin F2α; 30 nmol/l) or the α1‐adrenoceptor agonist, phenylephrine (0.3 μmol/l). In addition, fenoterol's relaxation of U44069‐induced tone was competitively inhibited by the β2‐selective adrenoceptor antagonist ICI 118551 (8.4 ± 0.9‐fold parallel shift to the right). By contrast, isoxsuprine was 81.9 times more potent as a vasorelaxant of phenylephrine‐induced tone when compared with KCl‐induced tone and proved completely ineffective as a vasodilator of U44069‐induced tone. When dose response curves to α‐adrenoceptor vasoconstrictor agonists were obtained in the presence of isoxsuprine (0.1 μmol/l), competitive antagonism occurred with methoxamine and noncompetitive antagonism with BHT‐920. These data suggest EDVs possess β2‐adrenoceptors mediating vasodilation. Isoxsuprine is an α1‐selective adrenoceptor antagonist but has very low potency and efficacy as a β‐adrenoceptor agonist in this functional bioassay. Indeed, much of the vasodilatory action of isoxsuprine may be due to a mechanism which does not involve β‐adrenoceptors.