Effects of hyperthermia and stimulation of the hypothalamus on the activity of the phrenic nerve in hypo- normo- and hypercapnic rabbits.

Experiments were carried out on male rabbits, anesthetized with urethane, bilaterally vagotomized, paralysed and artificially ventilated. The preoptic area (POA) was stimulated at three different levels of CO2: in hypo- normo- and hypercapnia under normothermia, moderate hyperthermia and hyperthermia. Changes in central respiratory activity (monitored by phrenic nerve activity) due to CO2 levels, stimulation of the POA during normothermia and progressively increased body temperature were investigated. Results showed that: (i) When body temperature was raised and during stimulation of the POA, the frequency of phrenic nerve volleys, tidal volume eq. and minute ventilation eq. were increased. (ii). The acceleration of the respiratory rhythm was reached by shortening of both respiratory phases, inspiratory and expiratory. However, the expiratory phase changed more than the inspiratory one and there was a close proportional relationship between inspiratory and expiratory durations. (iii) Together with the increase in the respiratory drive resulting in the combination of influences of both factors, the temperature and CO2, the effects of POA stimulation diminished gradually.