Responses of saphenous and mesenteric veins to administration of dopamine

Others have observed that dopamine (3,4-dihydroxyphenylethylamine) constricts resistance vessels in skin, but dilates these vessels in the mesentery. We studied the effects of dopamine on cutaneous and mesenteric veins of dogs to see if this agent also produced qualitatively different effects on the tone of capacitance vessels (veins) in these vascular beds. The lateral saphenous or the left colic vein was perfused at constant flow with blood from a femoral artery. Pressures at the tip of the perfusion cannula and at the tip of a catheter 15 cm downstream were recorded continuously. Increases in the pressure gradient between these two points indicated venoconstriction; decreases indicated venodilatation. Dopamine and norepinephrine injected into the perfusion tubing caused constriction of both veins. The constriction was antagonized by blockade of alpha receptors. A dilator action of dopamine was not seen, even after alpha receptor blockade or in the presence of increased venous tone produced by serotonin, norepinephrine, or nerve stimulation. Reserpine and cocaine did not alter responses to dopamine in the saphenous vein; this suggests that the venoconstrictor action of dopamine results mainly from a direct effect on alpha receptors and that uptake into sympathetic nerve endings may not be important in regulating the amount of dopamine available to receptors in the saphenous vein.