STRETCH-INDUCED MYOGENIC RESPONSES OF AIRWAYS AFTER HISTAMINE AND CARBACHOL

The purpose of the study described here was to determine the possible role of a myogenic response of bronchial smooth muscle in deep inspira tion (DI)-induced bronchoconstriction. Model experiments were performe d on sheep tracheal strips. The effect of sudden stepwise elongation o n isometric tension of tracheal muscle was studied in the absence and presence of the bronchoconstrictors carbachol (10(-8) M) and histamine (10(-4) M). In control strips tension increased rapidly with stretch and was followed by stress relaxation which corresponds to creep or br onchial dilatation. In histamine-and carbachol-treated strips a reacti ve contraction with a rhythmic pattern interrupted the process of stre ss relaxation. These responses appeared after only 20% elongation and were characteristic of a myogenic contraction which in the in vivo sit uation would correspond to a bronchoconstriction. These findings are i nterpreted as a functional transformation of multiple-to single-unit s mooth muscle due to the influence of carbachol and histamine. This sug gests that stretching (DI) of bronchial smooth muscle in the presence of carbachol and histamine induces a protracted myogenic contraction, which may explain bronchoconstriction after DI in severe asthma.