Contractions of canine vascular smooth muscle cells caused by ouabain are due to release of norepinephrine from adrenergic nerve endings.

Experiments were performed to determine whether the contractions of isolated canine blood vessels caused by ouabain are due solely to the release of endogenous norepinephrine. The response of segments of splenic arteries and veins and strips of splenic capsules to ouabain were compared before and after surgical sympathectomy. Successful denervation was demonstrated by absence of a contractile response to electrical stimulation, supersensitivity to exogenous norepinephrine, an extremely low content of endogenous norepinephrine, reduced accumulation of 3H-norepinephrine, and inability of electrical stimulation and tyramine to augment the overflow of 3H-norepinephrine. Ouabain augmented the overflow of 3H-norepinephrine from control but not from denervated splenic capsule. It caused contraction of control but not of denervated splenic arteries, veins, and capsules. Studies were also conducted in the same tissues and in segments of mesenteric and femoral arteries and of circular and longitudinal segments of the portal mesenteric vein before and after chemical sympathectomy with 6-hydroxydopamine. In these tissues, the contractile responses to electrical stimulation but not exogenous norepinephrine and prostaglandin F2 alpha were abolished. In the control tissues, ouabain caused strong contractions, whereas, in the denervated tissues, only a weak or no response to the glycoside occurred. The study demonstrates that, in isolated canine blood vessels, ouabain causes contraction of smooth muscle cells by virtue of its ability to release norepinephrine from the sympathetic nerves.