INFLUENCE OF REARING TEMPERATURE ON LUNG DEVELOPMENT FOLLOWING METHIMAZOLE TREATMENT OF POSTNATAL LAMBS

This study examines the effect of ambient temperature on lung developm ent over the first month of postnatal life in lambs treated with a dru g which inhibits thyroid hormone synthesis (methimazole, 50 mg day(-1) (kg body wt(-1))). Twin lambs were hand-reared at a fixed level of nu trition in either a warm (WR, 25 degrees C) or cool (CR, 10-15 degrees C) ambient temperature. Plasma thyroid hormone concentrations were si gnificantly (P < 0.05) lower in WR lambs, but there were no difference s between WR and CR groups in thyroid gland weight or hepatic iodothyr onine 5'-deiodinase activity at 8 or 29 days of age. Lung weights and the ratio of lung weight to O-2 consumption were similar in WR and CR groups at 8 days but both were significantly lower (P< 0.01) in CR lam bs at 29 days. Lung weight at 29 days was significantly (P = 0.002) co rrelated with plasma triiodothyronine concentration in CR but nor WR l ambs, and laryngeal braking of expiratory airflow was observed in thre e of seven CR lambs but not in WR lambs. Four WR, but no CR, lambs exh ibited a rapid deterioration in health or died unexpectedly between 14 and 21 days of age. These WR lambs were characterized by having small lungs and slower growth rates. It is concluded that in WR lambs chron ically treated with methimazole, both lung development and the ability of an individual to adapt effectively to methimazole treatment are co mpromised. In CR lambs, although lung growth may be reduced this does not appear to compromise O-2 consumption or health.