Function of Vascular Smooth Muscle and Its Sympathetic Innervation in the Newborn Dog*

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Abstract
Electrical stimula-tion of the sympathetic nerves to the hindlimbs of the newborn dog produces a change in vascular resistance that is dependent on age. Stimulation consistently elicits vasodilatation until 2 weeks of age, and this dilatation is effectively abolished by atropine. Nerve stimulation in animals older than 2 weeks produces an increase in vascular resistance that becomes progressively greater in magnitude with age. These results indicate that the cholinergic portion of the sympathetic lumbar chain is functional at an earlier time following birth than the adrenergic component. The observations confirm that systemic blood pressure increases with increasing maturity of the animal. The newborn is sub-sensitive to the constrictor action of epinephrine, norepinephrine, tyramine, and angiotensin, and the sensitivity increases with increasing age. Analysis of durations of pressor responses indicates that the younger animal is unable to terminate the action of the catecholamines and angiotensin as rapidly as the adult.