We assessed the dynamics of neuroendocrine (NE) cell numbers, the intensity of specific 5-HT fluorescence, and the arterial medial thickness in the lungs of neonatal rabbits in normoxia and acute and chronic hypoxia. Hypoxic neonates had significantly higher NE cell numbers and medial thickness of the pulmonary arteries at 5 days of age than did normoxic controls; 1- and 3-day-old young that died in hypoxia also had significantly higher cell numbers and medial thickness than did hypoxic survivors. A decline in these cell numbers was noted between 1 and 5 days of age among normoxic young, whereas there was no significant change among hypoxic young. Medial thickness was unchanged among normoxic young but increased between 1 and 5 days of age among hypoxic survivors. A 1-day exposure to normoxia of hypoxic young four days postpartum caused a decrease in NE cell numbers and medial thickness to more normal values. Serotonin (5-HT) fluorescence intensity levels of groups of NE cells or neuroepithelial bodies (NEBs) in this group were equal to those of normal controls although these levels were decreased in early chronic hypoxia. Medial thickness and NE cell numbers were inversely correlated with serotonin levels, suggesting that serotonin may be associated with medial hypertrophy and presence of argyrophil material. Medial thickness was positively correlated with NE cell numbers. The above findings led to the following summary: pulmonary NE cells respond to changes in airway oxygen levels; hypoxia or decreased oxygen is associated with decreased cellular 5-HT content and an increase in NE cell numbers by argyrophil stain and medial thickness of pulmonary artery walls. The change to normoxia from hypoxia results in higher cellular 5-HT content and decreased NE argyrophil cell numbers along with reduced pulmonary artery wall medial thickness.
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