The development of sympathetic innervation and the functional state of the cardiovascular system in newborn dogs.

The present study in dogs indicates that the peripheral sympathetic fibers develop mostly after birth and reach a full maturity at about 2 months of life. The norepinephrine content of the heart, spleen, intestine, salivary glands, and adrenal glands increased from birth to 56 days of age. In contrast, the content of the stellate ganglia decreased during this period. In most of the organs studied, the uptake of [3H] norepinephrine developed in parallel with the norepinephrine content, except in the right atrium and salivary glands where it was fully developed soon after birth. During development, the systemic blood pressure increased from 40 to 100 mm Hg. Bilateral adrenal vessel clamping failed to induce a fall in blood pressure in growing dogs which indicates that the adrenal medulla or the baroreceptors did not fully compensate for the lack of peripheral sympathetic fibers and for the lower blood pressure in newborn animals. Although cardiac norepinephrine content was still very low in 10-day-old animals, cardiovascular responses to direct and reflex sympathetic stimulation were similar to those observed in 56-day-old animals. These results indicate that the sympathetic nervous system becomes functional before the fibers reach their full maturity.