A comparison of the effects of 5- and 6-hydroxydopamine on the isolated canine saphenous vein.

We studied and compared the effects of 5- and 6-hydroxydopamine (5- or 6-OHDA) on venous tone and on adrenergic and nonadrenergic responses of the isolated canine saphenous vein. In the tissue bath, both 5- and 6-OHDA caused increases in tone and abolished adrenergic responses to transmural nerve stimulation. Effects of 6-OHDA were blocked by desmethylimipramine. 6-OHDA reduced norepinephrine levels of venous strips to 15% of those in control strips. Neither 5- nor 6-OHDA markedly altered peak contractile responses to norepinephrine but 6-OHDA prolonged the response to norepinephrine. Both compounds reduced responses to tyramine, with 6-OHDA producing a greater reduction than 5-OHDA. Large contractile responses to 5-OHDA were blocked only partially by treatment of strips with 6-OHDA, suggesting that contractile responses to 5-OHDA were mediated by direct and indirect actions. These results suggest differences in the actions of 5- and 6-OHDA on adrenergic terminals and smooth muscle in saphenous veins. The results of the experiments suggest that both 5- and 6-OHDA may be useful pharmacological interventions for the study of adrenergic mechanisms in isolated blood vessels.