Effect of ‘y-Aminobutyric Acid on Neurally Mediated Contraction of Guinea Pig Trachealis Smooth Muscle1

To determine whether-y-aminobutyric acid (GABA) affects the contractile properties of airway smooth muscle and, if so, what the mechanism of action is, the authors studied guinea pig tracheal rings under isometric conditions in vitro. GABA and related substances, badofen and muscimol, had no effect on the resting tension but reversibly depressed contractions induced by electrical field stimulation in a dose-dependent fashion, ICso values (mean ± S.E.) being 5.6 ± 1.4 x 10 M, 6.8 ± 0.9 x 10-6 M and 8.5 ± 1.5 x 10 M, respectively. In contrast, GABA did not alter the response to exogenous acetyicholine or the nonadrenergic noncholinergic inhibitory component. Pretreat-ment of tissues with bicuculline antagonized the inhibitory effect of GABA as well as that of baclofen. This inhibitory effect was not mOdified by propranolol, phentolarnine, hemicholinium-3 o