This study will assess the utility of using changes in myoglobin (Mb) concentration in response to chronic hypoxia as a diagnostic tool for assessing the balance between oxygen supply and demand in muscle. A mouse model of inherited hypoxic exercise tolerance (HET) will be used in which 2 inbred strains of mice and their F1 hybrid have dramatically different capacities for hypoxic exercise once they have been acclimated to hypoxia. Two genes of major effect are primarily responsible for the differences in HET, and mice in backcross generations segregate into 4 HET phenotypic classes based on the genotype of the two major loci. Experimental groups will consist of the 3 strains of mice acclimated to normoxia or hypoxia, and the 4 HET phenotypic classes, which will allow the analysis of the effects of the two major genes. The specific aims are to (1) measure [Mb] in skeletal and cardiac muscle, (2) measure Mb fractional saturation during muscle stimulation using NMR, (3) determine the maximal rate of oxygen consumption in muscle (VO2max), and (4) develop a reaction-diffusion model using these parameters that allows a quantitative evaluation of Mb function during hypoxic exercise. The general hypothesis is that Mb's position at the end of the oxygen delivery cascade and its role in mediating PO2 in the cell makes it an ideal probe of the balance between oxygen supply and demand in muscle fibers. The long term goal of the project is to understand the basis for changes in [Mb] in sufficient detail so that these changes can be interpreted with respect to whole animal and cell level O2 supply and demand. This work will have application to disease states that lead to regional hypoxia, exercise physiology, and health issues related to living at high elevations. Relevance to Public Health: In skeletal and cardiac muscle, increases in work load lead to dramatically higher rates of oxygen demand that must be supported by proportional increases in oxygen supply. However, many disease states compromise the capacity to delivery oxygen at elevated work loads leading to tissue damage. This study will assess the utility of using the muscle protein myoglobin as a sensitive indicator of the balance between oxygen supply and demand during conditions of reduced oxygen availability and elevated work load. [unreadable] [unreadable] [unreadable] [unreadable]