Injury, ranging from a paper cut that barely splits the skin to a high-speed motor vehicle accident that rips through muscle and bone, causes a disruption of tissue compartments. This breach of compartments disposes tissue to four principle problems: 1) bleeding, 2) susceptibility to infection, 3) hypoxia, and 4) tissue dysfunction. The acute phase response (APR) is the physiologic system that resolves these four problems of injury. It is divided into two temporally distinct phases: survival and repair. The survival phase utilizes coagulation and inflammation to temporarily seal off breached compartments in order to stop bleeding and prevent the spread of infection. Once the life-threatening problems of hemorrhage and susceptibility to infection have been resolved, the body enters the repair phase, where it works to restore the disrupted blood supply and regenerate damaged tissue. These processes, coupled together, replace the temporary sealant and restore the injured tissue to its original form and function.
Plasmin is the key fibrinolytic protease that transitions the APR from survival to repair. Plasmin is traditionally known for its ability to degrade fibrin clots. Previous work has demonstrated that plasmin also activates many targets outside of fibrin degradation that are essential for musculoskeletal tissue healing. Plasmin initiates the repair of injured tissue by removing the temporary fibrin sealant required for hemostasis, which allows access for repair machinery to enter the site of injury. Subsequently, plasmin activates proteases that promote angiogenesis and cell differentiation to stimulate tissue regeneration. Without adequate plasmin activity, the body fails to transition from survival to repair and is unable to reconstitute the compartments damaged by injury.
All tissue injury activates the acute phase response. Exercise is a form of contained tissue injury that harnesses the beneficial effects of the acute phase response to build muscle and promote overall health. Given that the controlled injury of exercise minimizes compartment disruption, there is a limited need to mount a response to hemorrhage or containment of infection (survival). Exercise increases the metabolic requirement of muscle to the point that it outstrips the supply of oxygenated blood and results in sustained hypoxia. In response to hypoxia, plasmin stimulates angiogenesis and activates cells involved in tissue regeneration (repair). In exercise physiology, if major injury is avoided, the body thereby bypasses the survival phase and transitions into an early and prolonged repair phase.
Despite studies demonstrating alterations in the APR with exercise, the relationship between the intensity of exercise and the extent of APR activation is not yet well defined \[8-10\]. Our aim is to gain a better understanding of the precise changes in the APR in order to maximize the reparative potential of exercise. We hypothesize that moderate intensity exercise selectively enhances plasmin-mediated fibrinolysis (repair) without inducing the procoagulant arm of the APR (survival). Additionally, we propose that the systemic activation of fibrinolysis achieved in exercise promotes global tissue health at spatially distinct locations in addition to the site of initial tissue injury (hypoxic muscle). To evaluate this, we will define the precise temporal changes in the APR in healthy individuals participating in graded intensities of exercise (rest, light, and moderate). Through elucidation of the precise temporal activation of the APR as it relates to the intensity of exercise, it may allow future studies to develop an exercise regimen that harnesses the reparative potential of fibrinolysis while avoiding the potentially harmful activation of the procoagulant survival phase of the APR.
The purpose of this study is to detail the precise temporal changes in the APR that occur in response to exercise in order to determine the types of exercise that confer maximal reparative fibrinolysis. Published research and preliminary studies conducted in our lab suggest that different types of exercise will preferentially activate fibrinolysis over coagulation, thereby promoting improved global tissue health \[8\]. As such, measuring markers of the APR in healthy individuals 1) at rest, 2) walking (light intensity exercise), 3) running (moderate intensity exercise), and 4) following endurance running (a marathon) will allow us to establish a baseline for the temporal changes in the APR that avoid activation of the procoagulant survival phase while maximizing the repair phase.
Specific aims
1. To measure the acute phase response fibrinolysis, plasminogen consumption, and inflammatory profiles of healthy individuals before and after graded exercise (at rest, light intensity, medium intensity) and after prolonged exercise at medium intensity as defined by changes in fibrinolysis, plasminogen consumption, and inflammatory response.
2. To track the APR through modulated exercise in order to determine the type of exercise that enhances physiologic benefit and limits harm.
