A pre-junctional action of 5-hydroxytryptamine and methysergide on noradrenergic nerves in dog isolated saphenous vein.

Electrical stimulation (2 Hz for 2 min) of dog isolated saphenous vein strips pre-incubated with tritiated noradrenaline increased the overflow of tritium of which about 80% was noradrenaline. 5-Hydroxytryptamine (5-HT; 1.0 x 10-9-1.0 x 10-7 mol litre-1) and methysergide (3.0 x 10-8-3.0 x 10-6 mol litre-1) inhibited the induced overflow of total tritium by a maximum of 78 +/- 4% and 47 +/- 7% respectively (mean +/- s.e. mean, n=6 for each). Methysergide was about 30 times less potent than 5-HT and the maximum inhibition obtained was less than with 5-HT. Both compounds inhibited electrically-induced contractions and overflow of tritiated noradrenaline. Their inhibitory actions on tritium overflow were little affected by phentolamine (1.0 x 10-6 mol litre-1) or cyproheptadine (1.0 x 10-6 mol litre-1), nor was the inhibitory effect of methysergide on electrically induced contractions antagonized by atropine, mepyramine, cimetidine or propranolol. The findings suggest that the prejunctional inhibitory effect of methysergide may be mediated via stimulation of a 5-HT receptor which, unlike the D-receptor, is not blocked by cyproheptadine. The possibility that the pre-junctional 5-HT receptor in the dog saphenous vein is the same as the post-junctional receptor in this preparation is discussed.