Evidence for different 5-HT1B/1D receptors mediating vasoconstriction of equine digital arteries and veins.

5-hydroxytryptamine (5-HT) is a potent vasoconstrictor of equine digital arteries and veins which may play a role in the ischaemic disease, laminitis. The present investigation compared the properties of 5-HT1B/1D receptors in arteries with those in veins using isolated rings of equine digital blood vessels. The 5-HT1B/1D receptor-selective agonists, anpirtoline and sumatriptan were 17.9 and 10 times more potent and produced 4.1 and 5.6 times greater maximum contractions, respectively, in veins when compared to arteries. Other agonists tested were of equal potency and produced the same maximum responses in veins and arteries. Propranolol competitively inhibited 5-HT1B/1D receptor mediated responses in arteries, with a pKB of 6.7, but had no significant effects on responses in veins at 1 microM. Metergoline competitively inhibited 5-HT1B/1D receptor mediated responses in veins, with a pKB of 8.1, but had no significant effect in arteries at 0.1 microM. These data suggest that 5-HT1B/1D receptors mediating vasoconstriction in equine digital arteries are pharmacologically different to those found in digital veins.