Increased narrowing of bronchial segments from immature pigs

Bronchial narrowing was investigated to determine whether changes in smooth muscle force, described previously in different aged pigs, are associated with differences in airway narrowing. The sensitivity of bronchial segments from suckling and young pigs was compared by perfusion through the lumen with a Krebs solution at a pressure of 5 cmH2O, measuring the reduction in flow in response to carbachol and histamine. Segments of the same internal diameter (i.d. approx. 2.5 mm) from each age were used by selecting proximal segments from suckling and distal segments from young pigs. The sensitivity to carbachol or histamine was the same in smooth muscle strips from proximal and distal bronchi in each age. Furthermore, segments from either age had a similar pressure-volume relationship between -10 and 30 cmH2O. However, concentration-flow curves showed that the airways from sucklings were five times more sensitive than airways from young animals to carbachol (p less than 0.01) and, less consistently, to histamine (p greater than 0.05), when flow was reduced by 50% of maximum. Flow was abolished by maximum concentrations of carbachol at both ages whereas histamine stopped flow in the young segments and reduced it by 80% in the suckling age group. Data indicate a greater sensitivity of bronchial narrowing to carbachol in the intact airway--this is consistent with a greater force production in suckling pig airway smooth muscle. These findings support a postnatal development of airway function, as suggested from clinical observations and provocation studies in humans.