Meconium aspiration produces airway hyperresponsiveness and eosinophilic inflammation in a murine model

In Vitro Action of Meconium on Bronochomotor Tonus of Newborns with Meconium Aspiration Syndrome

AIM

Here we studied the role of meconium in the respiratory system on live and exited newborns (weight 250-3000 g). Throughout this study is followed the response of tracheal rings in acetylcholine and histamine in different molar concentrations (10^-1, 10^-2, 10^-3, 10^-4 mol/dm³).

METHODS

To study the smooth tracheal musculature we used 23 tracheal preparations obtained from the newborns exited from meconium aspiration.

RESULTS

Based on the functional analysis of the tracheal specimen we have concluded that the meconium aspiration did not change the smooth musculature response on acetylcholine and histamine when compared to control group, exited from lung inflammatory processes (e.g., pneumonia, bronchopneumonia, atelectasis, cerebral hemorrhage), where tracheal smooth musculature response is significant (P for other causes is not significant (P > 0.01).

CONCLUSION

The conclusions suggest that meconium did not potentiate the constrictor action of acetylcholine and histamine in the tracheobronchial system and did not cause modulation of bronchomotor tonus in case of his aspiration. Meconium causes mild relaxation of smooth tracheal musculature with a mechanism which is not mediated by cyclooxygenase products, from tracheal epithelium or proteins. Also, direct activity in the smooth musculature of several tested acids seems to have no significant impact in increasing the tonus of respiratory airway of smooth tracheal musculature.

Experimental Models of Acute Lung Injury in the Newborns

Acute lung injury in the preterm newborns can originate from prematurity of the lung and insufficient synthesis of pulmonary surfactant. This situation is known as respiratory distress syndrome (RDS). In the term neonates, the respiratory insufficiency is related to a secondary inactivation of the pulmonary surfactant, for instance, by action of endotoxins in bacterial pneumonia or by effects of aspirated meconium. The use of experimental models of the mentioned situations provides new information on the pathophysiology of these disorders and offers unique possibility to test novel therapeutic approaches in the conditions which are very similar to the clinical syndromes. Herewith we review the advantages and limitations of the use of experimental models of RDS and meconium aspiration syndrome (MAS) and their value for clinics.

Glucocorticoids in the treatment of neonatal meconium aspiration syndrome

Meconium aspiration syndrome is a serious neonatal disease with complex pathophysiology. With respect to the contribution of meconium-induced lung edema, inflammation and vasoconstriction on the course of the disease, glucocorticoids are increasingly used in the treatment of MAS despite the fact that principal questions on the choice of GCs derivative, mode of delivery and dosing have not been answered yet. To bring a complex insight into the topic, this article reviews the pathomechanisms of MAS, mechanisms of action of GCs, as well as the advantages and disadvantages of GCs administration in experimental models and newborns with MAS.

The role of complement in meconium aspiration syndrome

The complement system is part of the host defense with a number of biological effects, most of which contribute to the inflammatory reaction by activation of cells like leukocytes and endothelial cells. An intact complement system is required for protection against infection and for maintaining internal inflammatory homeostasis. However, the system is a double-edged sword as improperly or uncontrolled activation is disadvantageous and potentially harmful for the host. Meconium aspiration syndrome (MAS) is associated with a local inflammatory reaction in the lungs, frequently described as a chemical pneumonitis. Cytokines, arachidonic acid metabolites and reactive oxygen species are involved in this reaction. We have recently documented that meconium is a potent activator of complement in vitro and in an experimental piglet model of MAS, the latter presenting with an inflammatory profile closely resembling systemic inflammatory response syndrome. We postulate that complement activation may contribute to the pathogenesis of MAS.

Pharmacotherapy for meconium aspiration

In this article we have attempted to review the current pharmacological treatment options for infants with meconium aspiration syndrome with or without persistent pulmonary hypertension. These treatments include ventilatory support, surfactant treatment and inhaled nitric oxide (INO), in addition to older and newer pharmacological treatments. These include sedatives, muscle relaxants, alkali infusion, antibiotics and the newer vasodilators. Many aspects of treatment, including ventilatory care, surfactant treatment and the use of INO, are reviewed in great detail in this issue. On the other hand, many newer pharmacological modalities of treatment described here have not been evaluated with randomized control trials. We have given an overview of these emerging therapies.

Studies of meconium-induced lung injury: inflammatory cytokine expression and apoptosis

To review current literature related to cellular mechanisms of meconium-induced lung injury (MILI). Review of published experimental in vitro and in vivo MAS studies using human and animal lung cells. We found that meconium induces expression of cytokines and angiotensin II (ANG II)-induced apoptotic process in the lung cells. We postulate that inflammatory cytokines induce ANG II expression, which causes apoptotic cell death after binding to its AT1 receptors. We also demonstrated expression of serpins associated with meconium instillation into the lungs. Serpins are proteins that inhibit cellular proteases and elastases. Expression of serpins may be an attempt to recover lung from these injurious effects. In summary our studies show that whereas meconium induces inflammatory cytokines and subsequent cell apoptosis, the lung cells also try to protect themselves by inducing serpins. The balance of these interactions will determine the residual damage. We believe these new findings are very important in understanding of MILI.

Inflammatory Response and Apoptosis in Newborn Lungs after Meconium Aspiration

An important feature of meconium-instilled newborn lungs is an inflammatory response and apoptotic cell death. It was recently demonstrated by our group and supported by several other investigators in a relatively short period of time. Apoptosis exists also in healthy lungs, but in meconium-instilled lungs its level is usually dramatically higher. Apoptosis is characterized by loss of cell function, decrease in cell size, and its morphology. Apoptosis plays an important role in normal cell life, but increased levels of apoptosis induce great damage for any tissues. Apoptosis in the lungs has been greatly overlooked for the past decade, and meconium-induced apoptosis is a relatively new event and not effectively studied at the present time. This Review summarized current knowledge regarding meconium-induced inflammation and apoptosis in newborn lungs.

Airway responsiveness after acute exposure to urban particulate matter 1648 in a DO11.10 murine model

Enhanced airway responsiveness (AR) is a well-established characteristic of asthma that epidemiological evidence has linked with inhalation of ambient particulate matter (PM). To determine whether acute exposure to urban particulate matter PM1648 can exacerbate airway responsiveness and alter the early inflammatory state, a unique murine model was created using DO11.10 mice, transgenic for a T cell receptor recognizing ovalbumin(323-339). Because these mice are sensitive to ovalbumin, immunization procedures involving adjuvant or long aerosolization procedures are not necessary and, therefore, allow for the study of an acute AR response to particulate and antigen in young animals. AR was assessed by barometric whole body plethysmography and measured by enhanced pause (Penh). PM1648 and ovalbumin were administered intranasally 72 and 4 h before to AR assessment, respectively. A dose-response relationship between PM1648 and Penh was determined, and doses at or above 500 microg had Penh values significantly higher than saline controls. Penh values of control particle titanium dioxide (TiO(2)) were similar to saline controls demonstrating no nonspecific particulate effect on AR. Lung lavage at time of AR assessment showed no significant inflammation due to particulate exposure or ovalbumin alone; however, PM1648/ovalbumin and TiO(2)/ovalbumin combinations resulted in significant neutrophilia. In addition, treatment with polymyxin B to remove surface-bound endotoxin did not significantly affect Penh levels. These results indicate that PM1648 specifically increases AR in a dose-dependent manner and that this exacerbation is not a direct response to increased neutrophil concentration, particle-bound endotoxin or nonspecific particle effects.

Innovative neonatal ventilation and meconium aspiration syndrome

Respiratory failure remains a major cause of morbidity and mortality in the neonatal population. Infants with hypoxemic respiratory failure because of meconium aspiration syndrome (MAS), persistent pulmonary hypertension of the newborn (PPHN), and pneumonia/sepsis have a potential for increased survival with extracorporeal membrane oxygenation (ECMO). Other treatment options previously limited to inotropic support, conventional ventilatory management, respiratory alkalosis, paralysis and intravenousvasodilators have been replaced by high-frequency oscillatory ventilation (HFOV), surfactant, and inhaled nitric oxide (iNO). HFOV has been advocated for use to improve lung inflation while potentially decreasing lung injury through volutrauma. Other reports describe enhanced efficacy of HFOV when combined with iNO. Subsequent to studies reporting surfactant deficiency or inactivation may contribute to neonatal respiratory failure exogenous surfactant therapy has been implemented with apparent success. Recent studies have shown that iNO therapy in the neonate with hypoxemic respiratory failure can result in improved oxygenation and decreased need for ECMO. In this article, the authors place in context of a system-based strategy the prenatal, natal and postnatal management of babies delivered through meconium stained amniotic fluid (MSAF) so that adverse outcomes are minimized, and the least number of babies require innovative ventilatory support. At Pennsylvania Hospital, over a six-year period (1995 to 2000), 14.5% (3370/23,175 of live births babies were delivered with MSAF. These data show that 4.6% (155/3370) of babies with MSAF sustained MAS. Overall, 26% (40/155) of babies with MAS needed ventilatory support (or 0.17% of all live-births); of these only 20% (8/40 or 0.035% of live births) needed innovative ventilatory support. None died or needed ECMO. These data describe the impact of a system-based approach to prevent and manage adverse outcomes related to MSAF at regional Level III perinatal center.