Fetal nicotine exposure increases airway responsiveness and alters airway wall composition in young lambs

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Abstract

To test the hypotheses that fetal nicotine exposure alters airway wall composition and enhances the airway response to inhaled methacholine (MCh), lambs were exposed during the last fetal trimester to (1) a low dose (LN) (n = 13, 0.5 mg/kg/d (maternal weight) of free base nicotine, (2) a moderate dose (MN) (n = 10, 1.5 mg/kg/d) or (3) saline (n = 14). Studies were performed at postnatal days 12, 26 and 52. Prenatal nicotine exposure induced a dose- and age-related hyper-responsiveness to MCh in the proximal airways. Moment analysis of nitrogen decay curves showed no nicotine or MCh effects on ventilation homogeneity or gas-mixing efficiency in the distal airways during MCh inhalations suggesting a bimodal response. Fetal nicotine exposure increased epithelial mucosubstance volume in central (LN, MN) and distal bronchi (LN), increased smooth muscle volume in distal bronchi and bronchioles (LN) and decreased bronchiolar diameter (MN). In conclusion, third trimester nicotine exposure causes hyperreactive proximal airways and alters proximal airway wall composition associated with airflow limitation.

Introduction

There is substantial evidence for a relationship between passive tobacco smoke exposure of the fetus and newborn and subsequent development of respiratory problems in infancy and childhood (Le Souef, 2000, Stocks and Dezateux, 2003, Moshammer et al., 2006). There are several studies of lung function in early infancy showing an association between maternal smoking during pregnancy and signs of flow limitations (Hanrahan et al., 1992, Tager et al., 1995, Brown et al., 1995, Jones et al., 2000). Furthermore, detrimental influences on lung development from exposure to prenatal tobacco smoke, e.g., lung hypoplasia and altered parenchymal structure have been shown in rats (Collins et al., 1985, Ji et al., 1994, Joad et al., 1999), mice (Blacquière et al., 2009) and in rhesus monkeys (Wang et al., 2008, Yu et al., 2008). The effects of prenatal exposure to nicotine have also been investigated in monkeys (Sekhon et al., 1999, Sekhon et al., 2001), sheep (Sandberg et al., 2004, Sandberg et al., 2007) and in rats (Maritz et al., 1993a, Maritz et al., 1993b). The topic of nicotine effects on lung development has recently reviewed by Maritz (2008).

Increased airway responsiveness early in life can be a predisposing factor for reduced lung function and development of wheezing problems and asthma later in life (Turner et al., 2002, Stocks and Dezateux, 2003). It has been proposed that this predisposition may be a result of the complex interaction between genetic susceptibility and fetal as well as early neonatal exposure to environmental factors such as tobacco smoke (Sheikh et al., 1999, Jaakkola and Gissler, 2004, Tepper et al., 2005, Turner et al., 2005, Goksor et al., 2007).

Elliot et al. (2001) have shown that 3-week-old guinea pigs exposed to tobacco smoke before birth have increased airway responsiveness associated with decreased alveolar attachment points. However, the relationship between prenatal smoke exposure and subsequent airway hyperresponsiveness, wheezing, and childhood asthma has not been clearly established in humans.

No association between airway responsiveness and maternal smoking in pregnancy was found in a large cohort of infants by Palmer et al. (2001). However, other investigators (Gilliland et al., 2001, Dezateux et al., 2001, Tepper et al., 2005) have proposed that the increased airway responsiveness early in life associated with maternal smoking during pregnancy is related to reduced airway calibre rather than increased smooth muscle tone in the airways.

Nicotine is the main ingredient in tobacco smoke thought to be related to human disease (Slotkin, 1998). Nicotine binds to abundant nicotinic acetylcholine receptors in the fetal lung, and it is likely that this interaction underlies many of the effects of maternal smoking on lung development (Sekhon et al., 2001). However, it is not known whether prenatal exposure to nicotine specifically causes airway hyperresponsiveness to methacholine, nor is it known whether prenatal nicotine exposure reduces airway diameter of small airways, i.e., distal bronchi and bronchioles.

In a previous study in which young lambs were exposed to nicotine prenatally, we found altered lung function during their first 2 months of life (Sandberg et al., 2004). Lambs exposed to nicotine showed disparate effects on airway function with signs of proximal airway obstruction, while distal airway function seemed unaffected or improved. We interpreted those results as being caused by nicotine-induced structural changes of reduced proximal airway calibre and accelerated maturation of the acinar part of the lung.

In this study, we tested the hypotheses that in utero exposure to nicotine increases airway responsiveness to methacholine (MCh) inhalation and alters proximal airway wall composition that could result in airflow limitation in young lambs. We also wished to determine whether these effects are influenced by the dose of prenatal nicotine exposure. Because we expected a disparate impact by nicotine on proximal and distal airway development as well as function over the seven-week study period, we used lung function methods that distinguish between those two areas of the lung.

Section snippets

Subjects

Thirty-seven lambs of mixed breed born by spontaneous vaginal delivery were studied three times during the first two months of life. Twenty-three of these lambs were exposed to nicotine prenatally, thirteen (8 female and 5 male) with a low dose (LN) and ten (4 female and 6 male) with a moderate dose (MN). Fourteen lambs (7 female and 7 male) served as controls (C). All lambs were twins except for one singleton lamb used for the low dose nicotine group. The LN, MN and C lambs were born at

Baseline values

Most baseline variables were comparable in the three groups on the three study occasions (Table 1). However, there were differences between the groups for RL and airway conductance normalized to lung volume (specific conductance, sG) at 52 days of age (Kruskal–Wallis test). At that age, RL was significantly higher in the MN group compared with C, and sG was significantly lower in MN compared with C.

Airway response to MCh

Airway resistance increased significantly from baseline in response to MCh inhalations in all

Discussion

The major results in this study were that lambs exposed to nicotine in utero exhibited increased airway responsiveness when studied during the first 4 weeks after birth. This increase was most pronounced in lambs exposed to a moderate dose of nicotine.

However, there was no effect from fetal nicotine exposure on peripheral airway function detectable by our technique. Prenatal nicotine exposure altered airway wall composition in distal bronchi and bronchioles and reduced bronchiolar diameter,

Acknowledgements

The authors thank Rao Gaddipati, M.S. and Charles A Dematteo Jr for skilled technical assistance, Mauli Shah, Marie Suffia and Monica Stupaczuk for tissue preparation, sampling and morphometric analysis, Drs. Patrick Arbogast and James Christopher Slaughter for very helpful biostatistical advice, Dr. Neal Benowitz for performing nicotine and cotinine determinations and Dr. Beverly Mellen for performing the randomization. Supported by a grant from the Smokeless Tobacco Research Council (#0765).

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