INTERACTIONS BETWEEN CELLULAR RESPIRATION AND THERMOREGULATION IN THE PARAMECIUM

An important adaptation to hypoxia is a regulated reduction in body temperature because it lowers metabolic rate when oxygen supply is limited. Although this beneficial response occurs in organisms ranging from protozoans to mammals, little is known of the cellular mechanisms responsible for the hypoxia-induced reduction in temperature. Using the unicellular protozoan, Paramecium caudatum, we showed that inhibition of oxidative phosphorylation with sodium azide (NaN3) under normoxic conditions mimics the thermoregulatory effects of hypoxia, causing this species to select a lower temperature in a thermal gradient (P < 0.0001). Under control conditions, selected temperature (T-sel) was 28.3 +/- 0.3 degrees C. NaN3 concentrations of 0.1 mM and above significantly reduced T-sel (P < 0.0001). Ten millimolar NaN3 produced the maximal reduction in T-sel, 11.4 degrees C, and the dose that produced 50% of the maximal response was 0.7 mM. The reduction in temperature was beneficial because both O-2 consumption and survival were significantly less affected by NaN3 at lower temperatures. These results suggest that Og does not directly affect thermoregulation in the paramecium. Rather, the hypoxia-induced reduction in T-sel results from inhibition of oxidative phosphorylation.