Burn-induced pathological changes at skeletal muscle include weakness, easy fatigability, and aberrant responses to neuromuscular relaxants. The aberrant responses are comprised of hypersensitivity and lethal hyperkalemic responses to succinylcholine and hyposensitivity to curare-like drugs. These pathological responses in muscle stimulate a denervation-like phenomenon where an increase in acetylcholine receptor (AChR) number is present.
The specific aims of the project are to test the hypothesis that the neuromuscular changes are related to (1) an increase in AChR due to burn-induced gene-mediated increases in messenger ribonucleic acid (mRNA) levels which produce AChR, (2) elevated levels of circulating stress or inflammatory hormones, catecholamines and interleukin-1 and their second messengers cyclic adenosine monophosphate (cAMP), prostaglandin E2 and their second messenger cyclic adenosine monophosphate (cAMP), prostaglandin E2 (PGE2) in muscle, and (3) depressed acetylcholinesterase enzyme activity with resultant decreased breakdown and increased levels of acetylcholine at the neuromuscular junction. The experimental design and methodology in the rat burn model, (both in vivo and in the in vitro phrenic-diaphragm) will measure functional and pharmacological responses. The responses measured will include peak developed tension, fatigue characteristics, sensitivity to d-tubocurarine and to succinylcholine including associated potassium release. Enzyme activity, cAMP, and PGE2 will be measured by radioassays and AChR concentrations will be quantitated, using specific ligand 125I-alpha-bungarotoxin. Changes in enzyme activity, cAMP, PGE2 and AChR concentrations will be correlated to the aforementioned neuromuscular responses. To differentiate burn specific from nonspecific stress or inflammation mediated neuromuscular changes, stress induced by chronic administration is isoproterenol which increases cAMP in muscle and sterile inflammation produced by chronic interleukin-1 administration which increases PGE2 in muscle will be studied in parallel in burned rodents. The hypothesis whether neuromuscular changes in burns, stress or inflammation could be ameliorated by the use of antagonists to catecholamine and prostaglandins will be examined. The specific antagonists tested will be propranolol and indomethacin respectively. To define systemic genetic responses of burn, stress or inflammation, levels of mRNA specific for AChR will be examined in these models by Northern blots using specific cDNA clones. Quantitative changes in mRNA will be correlated to AChR changes. To study the acute neuromuscular effects of circulating (protein and non-protein) plasma factors, the phrenic- diaphragm from unburned rat will be perfused with plasma from burned and control animals prior to and after differential precipitation of proteins. The broad long-term objectives are, therefore, (a) to provide information on burn-induced functional and pharmacological changes at the motor unit and (b) to characterize the mechanisms which induce these changes at the motor unit and (b) to characterize the mechanisms which induce these changes. The health-relatedness of these studies is to provide a rationale for pharmacological manipulation to prevent or treat neuromuscular disorders and for better utility of neuromuscular relaxants in burns. Knowledge of the factors which stimulate mRNA to increase AChR may have a potential application in the treatment of another pathological state, myasthenia, where AChR is decreased.
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