Inflation reflex in the rat.

The Breuer-Hering inflation reflex [BHIR] was elicited in conscious and anaesthetized rats by inflating the lungs with constant pressures of 5--20 cm H2O. The reflex was elicited well in conscious animals, but even with the maximum stimulus [inflation of 20 cm H2O, corresponding to about 4.5-fold the tidal volume] the duration of apnoea did not exceed 4 control respiration cycles. In anaesthetized animals, the same stimulus let to apnoea lasting 180--400 control respiration cycles on the average, according to the type and depth of general anaesthesia. The duration of apnoea in occlusion of the air passages in the expiratory position increased with the depth of anaesthesia, while in occlusion of the air passages at the peak of inspiration it was shortened. Stimulation of chemoreceptors [inhalation of a mixture 4% CO2 in O2 or of 8% O2 in N2] did not influence the elicitability or duration of the BHIR, nor did cooling the rats to 28 degrees C or heating them to 38 degrees C. The mean respiration frequency was 98 c/min in unanaesthetized rats, 96 c/min in urethane anaesthesis and 79--48 c/min in halothane anaesthesia, according to the depth of anaesthesia. Bilateral cervical vagotomy reduced mean respiration frequency to 35.6 c/min in conscious rats and to 31 c/min in urethane-anaesthetized animals. The results indicate the existence of species-related differences between basic regulatory mechanisms in the rat and certain other mammals.