Acid-base balance in the developing marsupial: from ectotherm to endotherm

Marsupial joeys are born ectothermic and develop endothermy within their mother's thermally stable pouch. We hypothesized that Tammar wallaby joeys would switch from α-stat to pH-stat regulation during the transition from ectothermy to endothermy. To address this, we compared ventilation (V̇e), metabolic rate (V̇o₂), and variables relevant to blood gas and acid-base regulation and oxygen transport including the ventilatory requirements (V̇e/V̇o₂ and V̇e/V̇co₂), partial pressures of oxygen (Pa_O2), carbon dioxide (Pa_CO2), pHa, and oxygen content (Ca_O2) during progressive hypothermia in ecto- and endothermic Tammar wallabies. We also measured the same variables in the well-studied endotherm, the Sprague-Dawley rat. Hypothermia was induced in unrestrained, unanesthetized joeys and rats by progressively dropping the ambient temperature (Ta). Rats were additionally exposed to helox (80% helium, 20% oxygen) to facilitate heat loss. Respiratory, metabolic, and blood-gas variables were measured over a large body temperature (Tb) range (∼15–16°C in both species). Ectothermic joeys displayed limited thermogenic ability during cooling: after an initial plateau, V̇o₂ decreased with the progressive drop in Tb. The Tb of endothermic joeys and rats fell despite V̇o₂ nearly doubling with the initiation of cold stress. In all three groups the changes in V̇o₂ were met by changes in V̇e, resulting in constant V̇e/V̇o₂ and V̇e/V̇co₂, blood gases, and pHa. Thus, although thermogenic capability was nearly absent in ectothermic joeys, blood acid-base regulation was similar to endothermic joeys and rats. This suggests that unlike some reptiles, unanesthetized mammals protect arterial blood pH with changing Tb, irrespective of their thermogenic ability and/or stage of development.