Rate of secretion of lung surfactant before and after birth.

Using electron microscopic techniques, we defined the percentage of intracellular lamellar bodies (LB) in the process of extrusion from type II cells, designated that as an index of secretion of surfactant, and studied the rate of release of surfactant in the lungs of fetal and newborn rats. The index of secretion increased twice in perinatal period. The most dramatic increase occurred during labor which consequently declined to fetal level within 15 min of birth. The second rise began at 30 min after birth and reached maximum in 2 h. Thereafter, it gradually declined reaching its lowest at 24 h. The number of LB/type II cell decreased with an increase in the rate of release and began to rise at about 1-2 h after increase in the rate of release. It is speculated that the rise in secretion rate during labor is in response to the change in circulating hormone levels, and that for after birth in response to lung distension and air ventilation. We also used this method of analysis in lavaged fetal lungs to test our unconfirmed belief that lung lavage promotes release of intracellular LB. The results, consisting of a 5-fold increase in the index of secretion and a 20% reduction in the number of LB/cell in lavaged lungs, were in favour of the notion. This should be taken into consideration when lung lavage is used to obtain 'extracellular surfactant'.