The mechanism of rapid shallow breathing due to histamine and phenyldiguanide in cats and rabbits.

In anaesthetized cats and rabbits we analyzed the rapid shallow breathing following exposure to histamine aerosol (mainly an irritant receptor stimulant) and i.v. injection of phenyldiguanide (mainly a J receptor stimulant). Both drugs caused a marked leftward displacement of the tidal volume (VT) vs inspiratory time (TI) relationship (Hering-Breuer threshold curve) without a corresponding increase in inspiratory flow rate so that inspiration was cut off at a lower VT and TI. The leftward displacement of the VT vs TI relationship occurred with a great shortening of the duration of inspiration during occluded breaths (T0I) accompanied by a shortening of the expiratory phase (T0E). These parameters monitored the central respiratory rhythm in absence of the phasic lung volume related vagal loop. It is suggested that the increased central respiratory frequency was due to the augmented firing of fibers from stimulated irritant and J receptors. Stimulation of these endings also caused the TE vs TI relationship to become steeper in cats and to be displaced downwards in rabbits.