Differential effects of furosemide on porcine bronchial arterial and airway smooth muscle

Furosemide attenuates airway obstruction in asthmatic subjects when adminis tered as an aerosol pretreatment. This protective effect of furosemide coul d be related to relaxation of bronchial smooth muscle or to increased bronc hial blood flow. To determine whether furosemide dilates bronchial smooth m uscle, isometric contractile responses in distal bronchi from young pigs we re studied. In bronchial smooth muscle rings that were precontracted with 1 0(-5) M acetylcholine, significant relaxation occurred with 10(-8) to 3 x 1 0(-6) M isoproterenol but not with 10(-8) to 10(-3) M furosemide. In contra st, bronchial arteries that were precontracted with either 10(-4) M norepin ephrine or 10(-8) M vasopressin significantly relaxed in response to 10(-4) to 3 x 10(-3) M and 10(-3) to 3 x 10(-3) M furosemide, respectively. We co nclude that furosemide, under the described experimental conditions, relaxe s airway vascular smooth muscle but not bronchial smooth muscle. These resu lts are consistent with previous suggestions that inhaled furosemide increa ses blood flow to airway tissues (Gilbert IA, Lenner KA, Nelson JA, Wolin A D, and Fouke JM. J Appl Physiol 76: 409-415, 1994).