Circulating 5-hydroxytryptamine concentrations in preterm newborns.

Alterations in circulating 5-hydroxytryptamine (5-HT) concentrations play a role in the pathophysiology of respiratory failure in adults. We undertook a study to develop a micromethod and measure circulating free 5-HT concentrations in preterm newborns with and without respiratory distress. Forty-six samples of platelet-poor plasma were obtained from 29 preterm newborns with varying degrees of respiratory distress. Samples were taken on days 2-3 and 6-7 of life. For measuring 5-HT concentrations we used a precolumn sample enhancement technique followed by ion exchange HPLC with electrochemical detection. The assay allowed detection of extremely small (50 pg) amounts of 5-HT from small (0.2 ml) amounts of blood. The mean 5-HT concentration on days 2-3 was 1.77 +/-0.74 ng/ml (mean +/- 95% confidence limits) and on days 6-7 was 0.69 +/- 0.23 ng/ml. This represented a significant fall in 5-HT concentrations (P = 0.01). All of 16 paired serial samples fell with time (P = 0.006). We conclude that platelet-poor plasma 5-HT concentrations in premature newborns are low, that there is a significant decline in these values over the first week of life, and that, in contrast with adults, the presence of respiratory failure is not associated with increased free 5-HT concentrations. The low 5-HT concentrations seen in newborns may reflect the ability to increase pulmonary uptake.