The goal of the proposed research is to determine whether variations in the cholesterol content of plasma membranes from various cell types of the rainbow trout account for the thermal adaptation of membrane function characteristic of these animals (i.e., the ability of trout and other cold-blooded animals ?those animals at the same temperature as their environment! to maintain similar levels of membrane-related activities ?such as nutrient and ion transport! at different body temperatures, even though membrane structure and function are markedly perturbed by temperature), and the means by which the cholesterol content of these membranes is modified in response to temperature change. The results of the proposed experiments will provide a broadly applicable definition of the role of cholesterol in membrane structure and function. The proposed experiments will determine the effects of growth temperature on i) the plasma membrane content of cholesterol; ii) the distribution of cholesterol between the plasma membrane and intracellular stores; iii) the capacity for cholesterol biosynthesis in liver and intestine; and iv) rates of cholesterol transport from its site of synthesis into the plasma membrane. In addition, the influence of modulation in cholesterol content on various membrane functions (membrane fluidity, permeability, and rates of transport) will be assessed. The rainbow trout is an ideal animal model in which to address these basic questions in membrane biology because: 1) trout remain active and behave normally over a wide range of temperatures, indicating that they acclimate well to variations in temperature; 2) in marked contrast to homeotherms (e.g., humans or, more generally, mammals) the membrane lipid composition can be readily manipulated in a noninvasive and physiologically meaningful context, simply by altering the water temperature, thus providing a powerful tool for probing the relationships between membrane structure and function not available in higher animals; and 3) the membranes of vertebrate, cold- blooded animals are more similar to those of higher vertebrates with respect to both lipid composition and diversity than are those of microorganisms, which have also been extensively employed to study this problem.