Phrenic nerve activity during artificial ventilation at different body temperatures and its relationships with carotid chemosensory activity.

While the chemoreceptor discharges of carotid bodies in vitro are highly dependent on temperature, these chemoreceptors in situ contribute only moderately to the ventilatory adjustment to changing body temperature (Tb), probably because of the concomitant and reverse changes in natural chemoreceptor stimuli in closed-loop preparations. Accordingly, we studied the frequency of carotid chemosensory discharge (fx) and the phrenic integrated electroneurogram (IENGph) in pentobarbitone anesthetized cats, paralyzed with alcuronium and artificially ventilated, at three steady-state levels of Tb (35.5, 37.5 and 40.2 degrees C), modifying the frequency and volume of the ventilator to maintain PETCO2 within normal range. While fx increases along with Tb when PETCO2 is allowed to fluctuate freely, its mean basal value was not consistently different at the three Tb's studied under controlled conditions. The amplitude of IENGph was reduced and the frequency of phrenic inspiratory cycles was increased as Tb was raised from 35.5 to 37.5 degrees C and then to 40.2 degrees C. Brief 100% O2 inhalations and i.v. injections of dopamine produced minimal depressions of IENGph amplitude in hypothermia, but pronounced although similar depressions in normothermia and hyperthermia. i.v. injections of NaCN augmented fx and IENGph in dose related manner, and the relationships between both variables showed larger changes in IENGph at the hypothermic and normothermic conditions when expressed in absolute terms, but not when expressed in relative terms. Thus, the chemosensory input is not consistently modified by thermal levels under controlled ventilatory conditions, but the chemosensory drive of the ventilatory output is less pronounced in hypothermia. The chemosensory input is similarly affected by varying degrees of cytotoxic hypoxia at different Tb's, but the ventilatory output is less vigorously increased in hyperthermia, pointing to a decreased reflex gain in that condition.