The effects of hypoxia on the metabolic and cardiorespiratory responses to shivering produced by external and central cooling in the pigeon

Respiratory, cardiovascular and blood gas responses of pigeons to spinal cord cooling (36±1°C), to ambient cooling (T _a=5°C) and to simultaneous spinal cord and ambient cooling were measured at three different levels of fractional inspired oxygen concentration (F₁O₂=0.209, 0.10 and 0.07). Shivering and the ‘extra’ \(\dot VO_2 \) provoked by ambient and/or spinal cord cooling were more or less reduced during hypoxic exposure depending on the intensity of cold stress and hypoxic states. At F₁O₂=0.10 shivering was markedly reduced and sometimes inhibited, whereas at F₁O₂=0.07 any pattern of cold tremor was inhibited. The accompanying cardiorespiratory responses were similar to those of thermoneutral controls exposed to the same F₁O₂. The amount by which \(\dot VO_2 \) was reduced in the pigeons exposed to hypoxia during ambient and/or spinal cord cooling was correlated, at both levels of hypoxia, to the thermoregulatory \(\dot VO_2 \) (viz. the ‘extra’ \(\dot VO_2 \) produced by cooling) prior to exposure to the hypoxic gas. The effect of hypoxia on shivering and associated cardiorespiratory adjustments was rapid and was completely reversible on return to air. We conclude that the thermoregulatory system in pigeons is sensitive to hypoxia, as is the case for mammals. The F₁O₂ that begins to inhibit thermoregulatory and metabolic responses to cold is lower in birds, perhaps as a result of the better ability of the bird to increase intrapulmonary gas and blood O₂ convective transports when exposed to hypoxic gas. Alternatively, this difference could be explained on the basis that birds, unlike mammals, appear to lack the capacity for the nonshivering fraction of thermogenesis which is thought to be selectively susceptible to moderate hypoxia.