Effects of lidocaine on rat's isolated tracheal smooth muscle

Lidocaine has been used as an injection or spray solution to protect spasm reaction during tracheal intubation and bronchoscopy. The effect of lidocaine for local anesthesia is well known. However, the effect of the drug to tracheal smooth muscle is not well explored. We used our preparation to test the effectiveness of lidocaine on rat's isolated tracheal smooth muscle. The following assessments of lidocaine were performed: (1) effect on tracheal smooth muscle resting tension; (2) effect on contraction caused by 5 x 10(-6) M methacholine as a parasympathetic mimetic; (3) effect of lidocaine on electrically induced tracheal smooth muscle contractions. The addition of lidocaine induced a relaxation response to methacholine-induced contraction. Low doses of the drug resulted in a slight decrease in contraction and higher doses relaxed the trachea much more quickly. At 10(-3) M of lidocaine, almost 51% of 5 x 10(-6) M methacholine-induced contraction was decreased. Using the same concentration, the drug also could totally inhibit the electrical field stimulation-induced spike contraction. Lidocaine elicited a negligible response to the resting tension of trachea as the concentration increased. The study indicated that lidocaine could cause bronchodilation not only from blocking parasympathetic tone, but also from directly antagonizing the effect of cholinergic receptors.