The contractile response of thiopental in large and small ovine airways.

The present study addresses the question of bronchial reactivity towards thiopental using pharmacological assay and ultrastructural analysis with transmission electron-microscopy in sheep airways. Smooth muscle strips of the upper, middle and lower trachea and small size bronchial rings of sheep were investigated using organ bath technique with monitoring of isometric tension in response to thiopental (10(-7)-10(-4)M), histamine (10(-4)M) and carbachol (10(-8)M). Thiopental elicited contractile responses in the large airways, with most marked contractions in the upper trachea. In the small lobar bronchi thiopental caused a biphasic response with predominant relaxation. Similar reactions could be elicited by histamine. Ultrastructural studies showed a large number of mast cells with equal distribution throughout the bronchial tree. Histamine release from bronchial tissue was detected by bioassay after thiopental challenge. It can be concluded that thiopental at a concentration of 10(-4)M releases histamine from mast cells within the airways with consequent constriction in the trachea and mainly dilatation in the smaller intralobular airways. Under these conditions therefore, one does not necessarily expect a net change of airflow resistance with thiopental anaesthesia.