COMPARISON OF THE EFFECTS OF SYMPATHOMIMETIC AMINES UPON VENOUS AND TOTAL VASCULAR RESISTANCE IN THE FORELEG OF THE DOG

¹ Cardiovascular Research Laboratories, Department of Internal Medicine, State University of Iowa College of Medicine, Iowa City, Iowa

The effects of seven sympathomimetic amines upon total vascular and venous resistance were compared in the perfused foreleg of the dog. All the agents tested were less potent than norepinephrine with respect to total vasoconstrictor activity. The order of potency was: norepinephrine > metaraminol = phenylephrine > methoxamine > tyramine > ephedrine = mephentermine.

The pressor amines had a differential ability to constrict the veins in the foreleg. Different degrees of venoconstriction were produced for similar changes in total resistance. Norepinephrine was by far the strongest venoconstrictor agent of the seven drugs. Metaraminol had a significantly greater venoconstrictor effect than methoxamine. Methoxamine, phenylephrine, and mephentermine all produced very small venoconstrictor responses, and ephedrine appeared to be intermediate among the seven agents in its action upon veins.

The percentage contribution of the increase in venous resistance to the increase in total resistance was greater as dosage was increased. These results suggest that the sympathomimetic amines constrict the venous segment proportionately more at higher doses than at lower doses, whereas the resistance vessels are relatively more sensitive to lower doses.