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Updated: Jul 6, 2026

In vitro Measurements of Tracheal Constriction Using Mice
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In vitro Measurements of Tracheal Constriction Using Mice

Published on: June 25, 2012

Human meconium has a pulmonary vascular and airway smooth muscle relaxant effect

Rogerio Tessler1, Jingyi Pan, Humberto Holmer Fiori

  • 1Department of Pediatrics, The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8.

Pediatric Research
|March 25, 2008
PubMed

Insights

Meconium acts as a pulmonary vasodilator in newborn rat lungs, contrary to previous beliefs. This study found meconium relaxes lung vascular and airway muscles in vitro.

Area of Science:

  • Neonatal Physiology
  • Pulmonary Medicine
  • Gastroenterology

Background:

  • Meconium aspiration is linked to persistent pulmonary hypertension syndrome of the newborn (PPHN), often attributed to vasoconstriction.
  • However, meconium has demonstrated a relaxant effect on rat tracheal muscle in previous studies.

Purpose of the Study:

  • To investigate the in vitro effects of meconium on both lung vascular and airway smooth muscle in newborn and adult rats.
  • To clarify the role of meconium in pulmonary vascular tone and airway reactivity.

Main Methods:

  • Isolated third-to-fourth generation pulmonary arteries and bronchi from 3-day-old and adult rats were studied.
  • Vascular and airway muscle responses to meconium were assessed in vitro, with and without pre-contraction, and in the presence of L-NAME and a superoxide scavenger.
  • The impact of meconium incubation on agonist-stimulated force and endothelium-dependent relaxation was evaluated.
  • In vivo meconium instillation followed by mechanical ventilation was performed, with subsequent in vitro assessment of pulmonary arterial muscle contraction.

Main Results:

  • Fresh homogenized meconium did not induce contraction in arterial or airway smooth muscle.
  • Meconium induced significant relaxation in precontracted pulmonary arteries, with a greater effect in newborn rats (53%) compared to adults (34%).
  • This relaxation was partially inhibited by L-NAME and enhanced by a superoxide scavenger, suggesting a role for nitric oxide and superoxide.
  • Bronchial smooth muscle relaxation to meconium was greater in adult rats than in newborns.
  • In vitro meconium incubation reduced agonist-induced force and enhanced endothelium-dependent relaxation.
  • Airway meconium instillation in vivo increased pulmonary arterial muscle contraction in response to thromboxane.

Conclusions:

  • Meconium acts as a pulmonary vasodilator in vitro, challenging the traditional view of its role in PPHN.
  • The relaxant effect on pulmonary vasculature is age-dependent and involves nitric oxide and superoxide pathways.
  • While meconium relaxes airway smooth muscle, in vivo instillation can lead to increased pulmonary arterial reactivity.

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