Comparative changes in segmental vascular resistance in response to nerve stimulation and to norepinephrine.

• The comparative effects of circulating norepinephrine and of stimulation of sympathetic nerves on veins, arteries and small vessels of the same vascular bed are of particular interest because norepinephrine is a neurotransmitter. Both similarities and differences between the effects of norepinephrine and nerve stimulation have been reported. Kelly and Visscher demonstrated that stimulation of sympathetic nerves caused large increments in small vein pressure and suggested, as did Davis in a more recent report, that nerve stimulation causes proportionately more venous than arterial constriction in the hind paw of the dog. Haddy et al. observed that infusions of norepinephrine may increase small vein pressure in the forepaw of the dog and increase the rate of weight gain of the foreleg; they suggested that norepinephrine also may cause, at times, venous constriction which is relatively greater than arteriolar constriction. Shadle et al. came to a similar conclusion. Mellander and Folkow found similarities between responses to norepinephrine and to nerve stimulation, but their