Development of the arterial chemoreflex and turnover of carotid body catecholamines in the newborn rat.

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Abstract
1. The peripheral, arterial chemoreceptors in the carotid body are active and responsive in the fetus. At birth, when oxygenation increases, the chemoreceptors are silenced. Over the next few days the sensitivity is reset toward the adult level and the chemoreceptors influence breathing during normal conditions. In order to investigate the underlying mechanisms of this resetting we examined the strength of the chemoreflex in newborn rats and correlated this to the contents of dopamine and noradrenaline in the carotid bodies of the newborn pups and near-term fetuses. Furthermore, turnover rates of dopamine and noradrenaline were determined in newborn rats up to 1 week of age by analysis of catecholamine decreases after inhibition of synthesis with .alpha.-methyl-p-tyrosine. 2. Chemoreceptor influence was assessed by the method of ''physiological chemodenervation'' with hyperoxia of 15-20 s duration in unanaesthetized rat pups. Relative changes in ventilation elicited by hyperoxia were determined by body plethysmography. We found no change in ventilation on the day of birth either in vaginally born rats or in near-term pups delivered by Caesarean section. After 1 day there was a significant decrease in ventilation of -19.4 .+-. 2.3% (mean .+-. S.E.M.) and at 7 days of age the decrease was -28.8 .+-. 2.2%, suggesting an increasing influence from the peripheral chemoreceptors. 3. The contents of dopamine and noradrenaline were measured by high-performance liquid chromatography. Dopamine increased from 3.7 .+-. 0.4 pmol (pair of carotid bodies)-1 in the fetus to a peak of 15.9 .+-. 2.6, 6-12 h after birth followed by a decline to 7.1 .+-. 0.7 at 7 days of age. Noradrenaline levels increased from 1.3 .+-. 0.3 in the fetus to 9.6 .+-. 1.1 pmol (pair of carotid bodies)-1 after 4 days. The turnover rate of dopamine decreased from 4.4 pmol (pair of carotid bodies)-1 h-1 0-6 h after birth to 1.0 at 6-12 h of age. The turnover rate of noradrenaline also decreased over the first hours following delivery. 4. Since dopamine is an inhibitory neuromodulator in this system, we suggest that the increase in sensitivity seen after the first day of life is, at least in part, due to a decrease in the release of dopamine and thus a removal of an inhibitory mechanism.