Human Meconium has a Pulmonary Vascular and Airway Smooth Muscle Relaxant Effect.

Meconium aspiration is believed to cause persistent pulmonary hypertension syndrome of the newborn (PPHN) via vasoconstriction, whereas meconium has a relaxant effect on rat tracheal muscle. We evaluated the meconium effect on lung vascular and airway muscle. Three days old and adult rat 3-4 generation arteries and adjacent bronchi were studied in vitro. Fresh homogenized meconium did not induce arterial or airway muscle contraction. In pre-contracted arteries, meconium induced muscle relaxation that was greater (P<0.01) in the newborn (53+/-5%), when compared with adult vessels (34+/-3%). This relaxant response was partially abrogated (P<0.01) by L-NAME (28+/-4%) and enhanced by a superoxide scavenger (55+/-4%). Pre-contracted bronchial muscle relaxed to meconium in vitro and the magnitude of response was greater in the adult when compared with the newborn (P<0.01). In vitro incubation with meconium (3 h) reduced agonist-stimulated force and enhanced endothelium-dependent relaxation (P<0.01). Airway meconium instillation followed by mechanical ventilation enhanced thromboxane-induced newborn rat pulmonary arterial muscle contraction in vitro (P<0.01). We conclude that meconium is a pulmonary vasodilator in vitro and the pathogenesis of PPHN possibly depends on the release of vasoconstrictors following its aspiration.

Tessler R, Pan J, Fiori HH, Belik J.

Department of Pediatrics [J.P., J.B.], The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8; Department of Pediatrics [R.T., H.H.F], Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil, 90680000.