Thermoregulatory changes by hypoxia: lessons from the paramecium

1. In organisms ranging from paramecia to mammals, hypoxia elicits a regulated decrease in body temperature (Tb). A decrease in Tb is an important adaptation to hypoxia primarily because it lowers metabolic rate when oxygen supply is limited, thus facilitating survival. 2. Although this beneficial response is extremely widespread among taxa, little is known of the cellular mechanisms that mediate hypoxia-induced decreases in Tb. This is due, in large part, to the extreme complexity of vertebrate thermoregulatory systems. 3. The thermoregulatory system of the unicellular paramecium is much simpler than that of vertebrates, yet it responds similarly to hypoxia. Research has explored the functional importance of hypoxia-induced decreases in Tb. In addition, a number of possible mediators and signalling pathways in hypoxia-induced reductions in Tb have been assessed. 4. In Paramecium caudatum, hypoxia appears to exert its thermoregulatory effects by inhibiting oxidative phosphorylation. Decreases in intracellular [ATP] and pH may be important intermediate signals. In addition, an endogenous opioid system appears to help mediate hypoxia-induced changes in thermoregulatory behaviour.