The role of sympathetic nerves in the activation of the carotid body chemoreceptors at birth in the sheep.

Carotid body chemoreceptor afferent discharge was recorded in the sinus nerve of sheep fetuses within a few days of term, exteriorized from ewes given pentobarbitone anaesthesia, and in six newborn lambs, 7-21 h old. Chemoreceptor discharge, defined as such since it was irregular, had no relation to the heart beat and was excited by hypoxia, hypercapnia, H+ and NaCN, was recorded in only 8 out of 20 fetuses. It was abundant in both the newborn lambs and also in fetus carotid bodies studied in vitro. Although electrical stimulation of the sympathetic fibres to the carotid body excited the fetal chemoreceptors and caused a fall in carotid body blood flow, the fetal chemoreceptors still responded to natural stimuli and drugs in both fetus and lamb after the sympathetic pathway had been cut. Further, there appeared to be clear dissociation of chemoreceptor and sympathetic activation shortly after birth. These results confirm and extend earlier studies and they suggest that, although the sympathetic pathway may contribute to chemoreceptor activation at birth, other, possibly more important, factors must be involved. These are discussed.