HEAT STRESS (HS)-RELATED LOSSES CONTINUE TO ERODE THE U.S. SWINE INDUSTRY'S PROFITABILITY AT AN ACCELERATING RATE. DEVELOPING INTERVENTIONS THAT TARGET UNDERLYING TISSUE AND CELLULAR DAMAGE HAS BEEN LIMITED BY A RUDIMENTARY UNDERSTANDING OF HS-MEDIATED CELLULAR DYSFUNCTIONS. EMERGING DATA FROM SKELETAL MUSCLE POINT TOWARD MITOCHONDRIAL DYSFUNCTION AS CRITICAL TO THIS PROCESS, WHICH IS OFTEN TRIGGERED BY CA2+ DYSREGULATION. THE LARGE PROPORTION OF SKELETAL MUSCLE AND ITS HIGH METABOLIC ACTIVITY EMPHASIZES ITS IMPORTANCE AS A KEY MEDIATOR OF SYSTEMIC DYSFUNCTION CAUSED BY ENVIRONMENT-INDUCED HS. CONSEQUENTLY, GIVEN THE PERSISTENT, RECURRING, AND SEVERE NATURE OF HS-MEDIATED PRODUCTION LOSSES, THERE IS A CRITICAL NEED TO PROTECT SKELETAL MUSCLE DURING PROLONGED HYPERTHERMIA. OUR PRELIMINARY DATA INDICATE FREE RADICAL INJURY AND APOPTOTIC SIGNALING IN AN OXIDATIVE (MITOCHONDRIA-RICH) MUSCLE-SPECIFIC MANNER, WHILE GLYCOLYTIC (MITOCHONDRIA-POOR) MUSCLE WAS LARGELY SPARED, IMPLICATING MITOCHONDRIA IN THIS PROCESS. OTHERS HAVE REPORTED ALTERED MITOCHONDRIAL ARCHITECTURE AND MITOCHONDRIAL DYSFUNCTION, IN VIVO. ADDITIONALLY, WE DETERMINED THAT AUTOPHAGY, A PROTEOLYTIC PROCESS ALLOWING REMOVAL OF LARGE PROTEIN AGGREGATES AND ORGANELLES (INCLUDING MITOCHONDRIA), BY LYSOSOMAL DEGRADATION BECOMES DYSREGULATED WITH HYPERTHERMIA AND THUS, PROMOTES THE ACCUMULATION OF DAMAGED MITOCHONDRIA. OUR CENTRAL HYPOTHESIS IS THAT IMPROVED CA2+ HOMEOSTATIC CONTROL WILL ATTENUATE MITOCHONDRIAL DYSFUNCTION AND SUBSEQUENT OXIDATIVE STRESS AND IMPAIRED SKELETAL MUSCLE METABOLISM DURING HS. TO TEST THIS HYPOTHESIS, IN AIMS 1 AND 2, WE WILL MANIPULATE CA2+ REGULATION IN SKELETAL MUSCLE, IN VIVO (PIGS AND MICE) AND IN VITRO (MUSCLE CELL CULTURE) DURING HS. THIS WORK IS IMPORTANT BECAUSE HS CURRENTLY REPRESENTS A SIGNIFICANT AND EXPANDING IMPEDIMENT TO EFFICIENT MEAT PRODUCTION AND COST THE U.S. SWINE INDUSTRY NEARLY $1 BILLION ANNUALLY. COLLECTIVELY, OUR FINDINGS WILL PROVIDE GREAT INSIGHT INTO MECHANISTIC CHANGES IN SKELETAL MUSCLE CAUSED BY HEAT STRESS. IN ADDITION, THESE DATA MAY CONTRIBUTE DIRECTLY TO THE DEVELOPMENT OF NOVEL AND EFFECTIVE THERAPEUTIC INTERVENTIONS THAT WILL MITIGATE HS-MEDIATED LOSSES IN GROWTH EFFICIENCY AND THUS MAINTAIN OR IMPROVE PRODUCER ECONOMIC VIABILITY EVEN IN THE FACE OF GROWING ENVIRONMENTAL THREATS.