Regulation of body temperature is a basic and widespread physiological process occurring in species from protozoans to mammals. Maintenance of body temperature within a certain range is essential for proper functioning of most biological processes. An important characteristic of thermoregulation is that numerous stimuli elicit changes in the regulated temperature (e.g. fever, hypoxia). However, little is known of the precise mechanisms that elicit adaptive changes in thermoregulation. In vertebrates, this is due in large part to the extreme complexity of their thermoregulatory systems. Invertebrates, such as the paramecium, have a much simpler system, but little is known about those systems. The goal of this research is to develop the paramecium as a model to understand how thermoregulatory cells produce adaptive changes in the regulated temperature. This goal will be accomplished by testing the following 4 hypotheses. 1) Hypoxia lowers the temperature from which both increases and decreases in temperature cause membrane depolarization. 2) The hypoxia-induced decrease in T(s) is mediated by an increase in intracellular [H+]. 3) Paramecia will increase their selected temperature (develop a fever) in response to pathogenic bacterial infection with Holospora obtusa. 4) Survival is increased at higher temperatures during infection with Holospora obtusa. Hypothesis 1 will be tested by measuring intracellular potential with a microelectrode and investigating changes in this potential as a function of both temperature and PO2 Hypothesis 2 will tested by measuring the effects of P02 on [H+]i by fluorescence video microscopy analysis of cells containing the fluorescent pH indicator, carboxyseminapthylrhodafluor-1 (SNARF-1). In additional experiments, attempts will be made to block the hypoxia-induced reduction in T(s) by increasing [H+]i with methylamine chloride. Hypothesis 3 will bc tested by determining T(s) in a thermogradient for both infected and non-infected paramecia. Hypothesis 4 will be tested by measuring the effects of temperature on survival of infected paramecia. These studies will provide useful information to assess the feasibility of using Paramecium caudatum as a model for adaptive changes in thermoregulation at the cellular level.