Increased relaxation of immature airways to beta2-adrenoceptor agonists is related to attenuated expression of postjunctional smooth muscle muscarinic M2 receptors.
Spontaneous or agonist-induced contraction of airway smooth muscle can be observed very early in fetal life, thus explaining the possible occurrence of bronchospasm in very low birth weight infants within the first days of life. In an attempt to better manage such bronchospasms, the aim of the present study was to investigate the age-specific modifications in airway smooth muscle relaxation to beta2-agonists and muscarinic antagonists using a combination of functional and molecular techniques. In the rat, isometric relaxation to the beta2-agonist salbutamol was examined in tracheae; we also examined muscarinic receptor expression (M2R and M3R mRNA levels) in airway smooth muscle by immunochemistry, Western blotting, and real-time PCR. Compared with adults, salbutamol-induced relaxation was twofold greater in immature rat isolated tracheae preconstricted by carbachol. This effect was associated with a lower expression of M2R in the smooth muscle of immature animals (sixfold and almost twofold as assessed by immunochemistry and Western blotting, respectively). Real-time PCR data indicate that changes in M2R expression according to age occurred at a posttranscriptional level. In adult airways, there was a significantly greater functional efficacy of M2R blockade by methoctramine compared with that shown in immature rats. Because of the limited availability of human neonate lung tissue, only the molecular part of the study was performed, and we observed a qualitatively similar effect, i.e., a lower M2R expression in the neonatal airway smooth muscle, although this was quantitatively smaller. We conclude that beta2-agonist-induced relaxation is enhanced in immature compared with adult airways as a result of greater postjunctional M2R expression in adult airway smooth muscle. This finding may be of importance in the clinical management of bronchoconstriction in neonates.
Fayon
Michael
M
Laboratoire de Physiologie Cellulaire Respiratoire, INSERM E356, Université Bordeaux 2, 146 rue Léo Saignat, 33000 Bordeaux, France. michael.fayon@chu-bordeaux.fr
Dumas De La Roque
Eric
E
Berger
Patrick
P
Begueret
Hugues
H
Ousova
Olga
O
Molimard
Mathieu
M
Marthan
Roger
R
D058672:Adrenergic beta-2 Receptor Antagonists; D000375:Aging / Q000502:physiology*; D000420:Albuterol / Q000494:pharmacology*; D000818:Animals; D000831:Animals, Newborn; D002478:Cells, Cultured; D006801:Humans; D007231:Infant, Newborn; D007537:Isometric Contraction / Q000187:drug effects / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D043585:Receptor, Muscarinic M2 / Q000378:metabolism*; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D014132:Trachea / Q000187:drug effects / Q000502:physiology*
D058672:Adrenergic beta-2 Receptor Antagonists; D043585:Receptor, Muscarinic M2; D018343:Receptors, Adrenergic, beta-2; D000420:Albuterol