Objective: We studied adaptation of diaphragm and orofacial muscles as well as hormonal responses to forced oral breathing (lasting for only 4 days) following reversible bilateral nasal obstruction performed on day 8 post-natal male rats.
Design: Muscle myosin heavy chain (MHC) composition and hormone levels were analysed during two periods: 1 and 3 days after obstruction (days 9 and 11 post-natal), and following 3 months recovery with nasal breathing (90 days, adult).
Results: Diaphragm muscle showed significant increases in adult isoforms (MHC 1, 2a) in oral breathing group versus control. We observed increases in MHC neonatal and adult type 1 isoforms in muscles involved with oral breathing, masseter superficialis and anterior digastric. No changes were observed in the levator nasolabialis muscle involved with nasal breathing. Reversible nasal obstruction was associated with reduced growth of the olfactory bulbs lasting into adulthood, and an initial decrease in lung growth followed by recovery at 90 days. Adrenal hypertrophy was observed after 1 day of nasal obstruction and lasted into adulthood. The "stress" hormone response was variable, increased (over 1000%) during the obstruction but normal by adulthood. An increase in plasma testosterone was observed during the obstruction, and a decrease in thyroid hormone levels throughout.
Conclusions: Very short term nasal obstruction, i.e. forced oral breathing, leads to long term hormonal changes and respiratory muscle fibre adaptation.
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