THE MYOGENIC RESPONSE OF ARTERIAL VESSELS IS INCREASED IN FETAL PULMONARY-HYPERTENSION

The stretch-induced myogenic response (MR) of large-capacitance pulmon ary arteries were studied in normal and pulmonary hypertensive fetuses as well as normal newborn and adult sheep. Pulmonary hypertension in the fetus was induced by ligation of the ductus arteriosus for 12 d. T he MR was obtained by stretching the vessel segments in vitro from the ir resting diameter (no load) to the diameter at which the muscle fibe rs were at optimal length (L(o)), and the response was measured as a p ercentage of force obtained after supramaximal electrical stimulation (P-o). In five control and four pulmonary hypertensive fetuses, the MR was also obtained after a stretch of 140% of L(o). The pulmonary hype rtensive fetal arteries had a lower stress (1.3 +/- 0.4 versus 4.0 +/- 0.5 mN/mm(2); p < 0.001) and shortening capacity compared with the fe tal control (5.1 +/- 1.6 versus 9.9 +/- 0.8% of L(o); p < 0.01). The M R was observed in 21% of the control and 30% of the experimental fetus es, and it was of greater magnitude in the latter (14.8 +/- 1.9 of P-o versus 34.3 +/- 2.5%, respectively; p < 0.01). When stretched to 140% of L(o), the MR was also greater in the experimental (514 +/- 148% of P-o) than the control fetuses (142 +/- 68; p < 0.05). Postnatally, th e MR was present in 67% of the newborn and 15% of the adult pulmonary artery segments, and the response was greatest in the newborn (23.1 +/ - 4.2% of P-o) compared with the adult (2.3 +/- 0.8; p < 0.01). These data indicate that the stretch-induced MR of large-capacitance pulmona ry arteries of the sheep shows significant developmental differences p ostnatally and is greatly enhanced in the fetus after pulmonary hypert ension. We speculate that large-capacitance arteries play a role in th e control of pulmonary vascular resistance in pulmonary hypertension a s a result of wall stiffness changes in response to intravascular pres sure-induced MR.