REACTIVITY TO ALPHA-AGONISTS IS HEIGHTENED IN IMMATURE PORCINE PULMONARY-ARTERIES

Background. Pulmonary hypertension after cardiopulmonary bypass is a c ommon problem in pediatric cardiac operations. This study tested the h ypothesis that there is a difference between adult and immature pulmon ary artery constrictor and dilator responses. Methods. Reactivity of p ulmonary artery ring segments from 22 mature (15 to 19 weeks) and 15 i mmature pigs (4 to 5 weeks) was tested in a vessel myograph. Potassium as a receptor-independent vasoconstrictor and phenylephrine as an alp ha-receptor-mediated vasoconstrictor were used to assess smooth-muscle vasoconstriction. To assess endothelial cell function (nitric oxide p roduction and secretion), we used increasing concentrations of bradyki nin or acetylcholine. Sodium nitroprusside was used to produce maximum smooth-muscle relaxation at the end of each experiment. Results. The data demonstrated maturation-independent endothelium and smooth-muscle -mediated vasodilatation. Pulmonary artery ring segments from immature pigs had significantly less KCl constriction compared with mature pig s (p < 0.05). In contrast, pulmonary ring segments from immature pigs demonstrated enhanced alpha-receptor-mediated constriction compared wi th mature pigs. Conclusions. These data may explain perioperative pulm onary vasoconstriction in pediatric patients.