Spontaneous peristaltic airway contractions propel lung liquid through thebronchial tree of intact and fetal lung explants

Spontaneous contractions of the fetal airways are a well recognized but poo rly characterized phenomenon. In the present study spontaneous narrowing of the airways was analyzed in freshly isolated lungs from early to late gest ation in fetal pigs and rabbits and in cultured fetal mouse lungs. Propagat ing waves of contraction traveling proximal to distal were observed in fres h lungs throughout gestation which displaced the lung liquid along the lume n. In the pseudoglandular and canalicular stages (fetal pigs) the frequency ranged from 2.3 to 3.3 contractions/min with a 39 to 46% maximum reduction of lumen diameter. In the saccular stage (rabbit) the frequency was 10 to 12/min with a narrowing of similar to 30%, In the organ cultures the waves of narrowing started at the trachea in whole lungs, or at the main bronchus in lobes (5.2 +/- 1.5 contractions/min, 22 +/- 8% reduction of lumen diame ter), and as they proceeded distally along the epithelial tubes the luminal liquid was shifted toward the terminal tubules, which expanded the endbuds . As the tubules relaxed the flow of liquid was reversed, Thus the behavior of airway smooth muscle in the fetal lung is phasic in type (like gastroin testinal muscle) in contrast to that in postnatal lung, where it is tonic. An intraluminal positive pressure of 2.33 +/- 0.77 cm H2O was recorded in r abbit fetal trachea. It is proposed that the active tone of the smooth musc le maintains the positive intraluminal pressure and acts as a stimulus to l ung growth via the force exerted across the airway wall and adjacent parenc hyma. The expansion of the compliant endbuds by the fluid shifts at the air way tip may promote their growth into the surrounding mesenchyme.