The effects of pentobarbitone and chloralose anaesthesia on the vagal component of bronchoconstriction produced by histamine aerosol in the anaesthetized dog.

1 Total lung resistance (R(L)) and dynamic lung compliance (C(dyn)) were measured in dogs anaesthetized with pentobarbitone or chloralose and subjected to aerosols of histamine during 4 successive inspirations.2 Histamine caused concentration-dependent increases in R(L) and decreases in C(dyn). A significant vagal component was involved, but only when chloralose was employed and then only in the R(L) response.3 The resting values of R(L) and C(dyn) were similar regardless of which anaesthetic was used and remained essentially the same if the vagi were cooled.4 Electrical stimulation of the efferent vagi caused large increases in R(L) of dogs given chloralose and these effects were attenuated by the administration of pentobarbitone. Such stimulation was relatively ineffective in dogs given pentobarbitone alone.5In vitro, electrical field stimulation caused contractions of dog trachealis muscle. The responses were reduced by pentobarbitone in concentrations approximating to plasma levels in the anaesthetized dogs (1 to 5 x 10(-4) M), but the effects of exogenous acetylcholine were unaltered. The inhibition was dose-dependent, reversed by washing and unaltered by hexamethonium.6 The results suggest that pentobarbitone inhibits the vagal component of histamine-induced bronchoconstriction in the dog by an action on the efferent pathway. Furthermore, pentobarbitone acts either by blocking transmission along postganglionic parasympathetic nerves or by preventing the release of acetylcholine from the nerve endings in the lung.