This application proposes to examine the regulation of glycogen phosphorylase expression in cardiac and skeletal muscle. Expression of glycogen phosphorylase genes is differentially regulated during muscle development. The brain (or fetal) form is the predominate phosphorylase expressed in fetal muscle. During terminal differentiation, the muscle isozyme replaces the brain type in adult skeletal muscle and is expressed concurrently with brain phosphorylase in adult heart. Many of the contractile genes and metabolic genes expressed during terminal differentiation of skeletal muscle are also expressed in cardiac muscle. However, the ability of regulatory sequences defined in skeletal muscle to modulate cardiac gene expression has not been frequently studied because of the lack of a convenient cell system. this application proposes to employ a newly-characterized cardiac cell system for this purpose. In order to gain insight into mechanisms controlling differentiation in cardiac and skeletal muscle, it is necessary to examine the regulation of a number of muscle-specific genes and their products. Phosphorylase represents an excellent model system for studying mechanisms of isozyme switching. It plays a key role in anaerobic metabolism in both skeletal and cardiac muscle and is one of the most abundant enzymes in skeletal muscle. Deletion analysis and mutagenesis studies have identified a 43 base pair region of the muscle glycogen phosphorylase promoter which confer differentiation-specific expression on a reporter gene in muscle cells. The role of this sequence and that of a potential negative regulatory element will be further characterized. The ability of defined regions of the muscle phosphorylase 5' flanking DNA to activate reporter gene activity will be examined in cardiac cells. Because of the central role of phosphorylase for the production of energy for contraction under anaerobic conditions, the study of phosphorylase gene expression provides not only the opportunity to delineate skeletal vs. cardiac-specific regulatory properties of gene expression, but also serves as a model to initiate studies of alterations in expression of key metabolic enzymes that may result from perturbation of the cellular environment, e.g. hypoxia. We will now be able to develop an experimental system to study the interplay of metabolic response and gene regulation in cardiac cells. These studies will aid in the elucidation of the mechanisms controlling muscle gene expression and may provide insight into the mechanisms of muscle response to metabolic stress.