Effects of labetalol on in vitro preparations of the trachea and lung strips and on ventilation overflow in vivo in guinea pigs.

The responses of guinea pig central and peripheral airways to labetalol (both alpha- and beta-adrenoceptor blocker with intrinsic sympathomimetic activity) were studied using in vivo and in vitro preparations of the trachea and lung strips. The ventilation overflow enhanced by infusion of histamine in anesthetized guinea pigs was much more aggravated by labetalol and propranolol. The beta-adrenoceptor blocking potency of labetalol and propranolol was not significantly different between the in vitro tracheal preparations and the lung strips. Isoproterenol and salbutamol relaxed the tracheal preparations more markedly than they did the lung strips. Histamine and acetylcholine contracted the lung strips more remarkably than they did the trachea. Labetalol relaxed the trachea more markedly than it did the lung strips by its intrinsic sympathomimetic activity. These results suggest that the peripheral airways are more sensitive to constrictors and the central airways are more sensitive to relaxants such as beta-stimulants, and that the different responsiveness between the two airways to labetalol and histamine might have produced the different in vivo observations.