Effects of bambuterol and terbutaline on isolated rat's tracheal smooth muscle

Terbutaline (Bricanyl) and its prodrug Bambuterol (Bambec) are highly potent β₂-adrenoceptor agonists often used in asthma patients. Terbutaline in the form of inhaled short-acting bronchodilator relieves asthmatic symptoms. However, the effects of bambuterol given intratracheally have rarely been explored. To verify the effect of bambuterol and terbutaline, which act on the tracheal smooth muscle directly in vitro, we used our preparation to test the effects of bambuterol on isolated rat's tracheal smooth muscle compared with terbutaline. The following assessments of bambuterol and terbutaline were performed: (1) effect on tracheal smooth muscle resting tension; (2) effect on contraction caused by 10^-6 M methacholine as a parasympathetic mimetic; (3) effect of the drugs on electrically induced tracheal smooth muscle contractions. The results indicated that adding bambuterol induced a significant further contraction to 10^-6 M methacholine-induced contraction when the preparation was increased to 10 ^-4 M. But terbutaline elicited a relaxation response at a dose of 10^-6 M or more. Both bambuterol and terbutaline could inhibit electrical field stimulation (EFS) induced spike contraction. Terbutaline had an anti-cholinergic effect that could relieve asthmatic symptoms. But the effect of bambuterol acting on tracheal smooth muscle directly was controversial.