Large pulmonary arteries and the control of pulmonary vascular resistance in the newborn.

In the immediate neonatal period the pulmonary vascular resistance is higher than later in life. The role of maturational differences in the smooth muscle mechanical properties of large capacitance vessels in this response has not been previously studied. To this end, we studied the smooth muscle isometric and isotonic mechanical properties, as well as the myogenic response of large extralobar pulmonary arteries in newborn and adult guinea pigs. Compared with the adult, the newborn pulmonary vascular smooth muscle generates less force and has a similar shortening capacity but longer isometric and isotonic relaxation half times. The myogenic response could be elicited in 80% of the newborn and 70% of the adult vessels. A 20% increase in vessel diameter resulted in force generation equivalent to 46 +/- 5.1% of maximal isometric tension in the newborn. Such a response was significantly greater than observed for the adult vessels (14 +/- 2.8%; p < 0.001). Our results showed significant maturational differences in the mechanical properties of the large pulmonary artery smooth muscle in the newborn. We speculate that the presence of myogenic response in large pulmonary capacitance vessels and the observed greater magnitude of stretch-induced force generation in the newborn may play an important role in the maintenance of a higher pulmonary vascular resistance in the neonatal period.