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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

Hypothermia and hypoxia inhibit the Hering-Breüer reflex in the marsupial newborn

Adaptational and Evolutionary Respiratory Physiology Laboratory, Department of Zoology, La Trobe University, Melbourne, Victoria 3086, Australia

Submitted 27 April 2003 ; accepted in final form 18 December 2003

The effects of lowering body temperature (T_b) on metabolic rate, ventilation, and the strength of the Hering-Breüer expiratory promoting reflex (HB reflex; determined from an inhibitory ratio calculated from volumetric measurements of the respiratory rhythm) were examined in 18-day-old ectothermic pouch young of the tammar wallaby during normoxia or hypoxia (10% O₂). Hypoxia and hypothermia, either singularly or combined, depressed metabolic rate. At all T_b, the hypoxic hyperventilation was associated with a significant hyperpnea. At pouch T_b (36.5°C) during normoxia, inflation of the lungs with -5 or -10 cmH₂O extrathoracic pressure induced a significant HB reflex. Exposure to cold reduced the strength of the reflex, almost abolishing it at 28°C. For T_b above 28°C, the reflex in hypoxia was always less than the corresponding normoxic value. Taken in context with the changes in metabolic state that occurred, these data in the ectothermic marsupial newborn suggest that the decline in the HB reflex during moderate hypothermia is the result of a direct effect of T_b on vagal mechanisms rather than a temperature-driven decline in metabolic rate that should have acted to strengthen the HB reflex. Therefore, it seems that inputs inhibitory to breathing are more negatively affected during cold than those inputs that are excitatory.

metabolic rate; hyperventilation; breathing pattern

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