Recent studies have suggested that respiratory muscle dysfunction due to excessive respiratory muscle loading and/or shock (septic, cardiogenic) may contribute to the development of respiratory failure in patients with cardiopulmonary diseases. The mechanisms by which these various stresses produce alterations in muscle function are, however, poorly understood. While studies of other vital organs have suggested that many forms of stress act to induce free radical mediated tissue damage, the role of free radicals in mediating diaphragmatic dysfunction has not been examined. The purpose of the present proposal is to examine the role of free radicals in producing diaphragmatic dysfunction in response to three clinically important stresses; fatiguing levels of rhythmic contraction, protracted ischemia, and sepsis. All experiments will be performed in animal models employing cats and dogs. In Objective I we will examine the role played by free radicals in modulating the development of diaphragmatic fatigue. We will examine the effect of several free radical scavengers (SOD, catalase, DMSO) on measures of diaphragm contractile performance (twitch kinetics, force frequency relationship, action potential transmission) and oxygen utilization (blood flow, oxygen extraction, oxygen consumption) over time during fatiguing contractions. Two indices of free radical generation (tissue conjugated diene and MDA levels) will also be examined in these studies. We will study this issue both in the electrically stimulated diaphragm (this permits precise control of diaphragmatic activation) and in the spontaneously driven diaphragm during inspiratory loading. Objective II studies will examine the effect of several free radical scavengers on the diaphragmatic response to endotoxin. These studies will assess changes in contractile function, vascular reactivity, oxygen extraction, and tissue MDA and conjugated diene levels over time in endotoxin treated animals with and without free radical scavenger pretreatment. In Objective III we will examine the role of free radicals in inducing ischemia/reperfusion injury in the diaphragm. These studies will characterize the injury induced by ischemia/reperfusion (i.e. effects on strength, fatiguability, vascular reactivity, oxygen extraction, alterations in tissue MDA and conjugated diene levels) and the effect of free radical scavengers on this form of injury. The stresses to be examined in these experiments represent important clinical causes of diaphragmatic dysfunction and respiratory failure. The studies outlined in this grant may provide important information regarding the pathogenesis of these forms of diaphragmatic dysfunction and may lead to new therapeutic approaches.