The pulmonary neuroendocrine system and drainage of the fetal lung: effects of serotonin

The neuroendocrine system of the lungs is maximally developed and activated at birth, but has no clear function. Here, one of its products, serotonin, was tested for an ability to stop lung fluid production or activate reabsorption. Lungs from fetal guinea pigs (61 +/- 2 days of gestation) were supported in vitro for 3 h; lung liquid production was monitored by a dye dilution method. Initial studies on 36 young fetuses (61 +/- 1 days of gestation) showed that untreated controls produced fluid at 1.17 +/- 0.23 ml.kg-1.h-1, with no significant change over 3 h (ANOVA; regression analysis); those given 10(-8) M serotonin during the middle hour showed no significant changes, but those given 5 x 10(-8), 10(-7), 10(-6), or 10(-5) M serotonin reduced production significantly (P < 0.01 to P < 0.0005). Responses were linear up to 10(-7) M (threshold, 10(-9) M) and then become maximal at 50% reduction. However, responses increased with age. Comparison of 40 fetuses divided into groups of 60-61 or 65-67 days of gestation showed a large and significant increase in responses in the older fetuses (P < 0.01), where half the preparations reabsorbed fluid. Serotonin receptors were involved, since 10(-6) M cyproheptadine abolished responses (based on 24 preparations). Amiloride-sensitive Na+ channels were involved, since 10(-6) M amiloride abolished responses (based on 24 preparations). These results, in combination with earlier results from somatostatin and dopamine, together with histochemical and clinical observations, strongly suggest that the neuroendocrine system of the lungs may find a function in clearing fluid from the lungs at time of birth.