MYOGENIC RESPONSE IN LARGE PULMONARY-ARTERIES AND ITS ONTOGENY

To evaluate the myogenic response and its ontogeny in large pulmonary arteries, we studied 45 newborn and 30 adult guinea pigs. Compared with the those of the adult, the newborn arterial vessels possessed a significantly (p < 0.01) smaller diameter (1153 +/- 34 versus 1656 +/- 65 mu m), static compliance (2.2 +/- 0.3 versus 4.6 +/- 0.7 mu m/mN), and active stress (3.4 +/- 0.4 versus 5.8 +/- 0.7 mN/mm(2)). Stretch-induced contraction was obtained by quick stretch of the vessel segments to 120, 140, 160, 180, or 200% of their optimal length, and the myogenic response was measured as the change in force after muscle relaxation with papaverine. A myogenic response was observed in 94% of the newborn and 93% of adult vessel segments, and significant age differences in the response were present. The magnitude of the active force generated for any stretch over 120% was significantly greater in the newborn (p < 0.01), and as a percentage of K+ (127 mM) stimulation, a 2-fold stretch of the vessels' optimal length resulted in a force of 1073 +/- 159% in the newborn compared with 51 +/- 16% in the adult (p < 0.01). The myogenic response in these large pulmonary vessels was completely suppressed by a calcium channel blocker (D-600) but unaltered by addition of a nitric oxide synthase inhibitor (N-G-methyl-L-arginine) or indomethacin. We conclude that the large pulmonary arterial vessels of the guinea pig exhibit a powerful stretch-induced myogenic response that is greater in the newborn period. Changes in vessel wall stiffness brought about by the myogenic response of large pulmonary arteries may play a role in the control of lung vascular resistance.