Comparative Changes in Segmental Vascular Resistance in Response to Nerve Stimulation and to Norepinephrine

The effects of stimulating sympathetic nerve fibers and of intra-arterial norepinephrine on vascular pressures and resistances in muscle and paw of perfused forelegs of dogs were compared by means of small vessel cannulations and during constant blood flow. The following conclusions may be drawn: 1. Sympathetic nerve stimulation causes constriction of the large artery segment predominantly. The constriction appears to be most intense at the level of the carpus. 2. The large artery constriction at the level of the carpus diverts blood flow away from the paw and into more proximal muscular parts of the foreleg. 3. Changes in segmental vascular resistances during nerve stimulation differ in paw and muscle. In the paw, venous segments constrict markedly whereas small vessel segments usually dilate and may constrict minimally; but in the muscle, arterial and small vessel segments constrict more than venous segments. 4. The effects of norepinephrine differ from those of nerve stimulation. Norepinephrine causes constriction of small vessels in paw predominantly. Constriction of both small vessel and venous segments in paw exceeds constriction of corresponding segments in muscle.