Production of Endothelins by the Ventilatory Muscles in Septic Shock

Circulating endothelin-1 (ET-1) concentration increases significantly in animal models of sepsis. The main mechanism responsible for this rise in ET-1 levels is believed to be upregulation of ET-1 synthesis in various organs, such as the lungs and heart. In this study we investigated whether ET-1 is synthesized in the ventilatory muscles and whether this synthesis is regulated in septic shock. Conscious rats were injected with Escherichia coli endotoxin (lipopolysaccharide [LPS]) and killed 6, 12, and 24 h later. A fourth group of rats was injected with normal saline and served as a control. The diaphragm was excised at the end of the experiment and quickly frozen. Diaphragmatic ET-1 level was measured with radioimmunoassay, and messenger RNA (mRNA) expression of ET-1 precursor prohormone (preproET-1), preproET-3, and endothelin-converting enzyme was measured with reverse transcription-polymerase chain reaction. LPS injection elicited an early (within 6 h) and prolonged rise in diaphragmatic ET-1 concentration. In addition, mRNA levels of preproET-1 and preproET-3 rose by about 4- and 3-fold within 6 to 12 h of LPS injection, whereas mRNA of endothelin-converting enzyme increased by more than 10-fold and peaked within 24 h of LPS injection. Immunostaining with anti-ET-1 antibody revealed positive ET-1 staining in the endothelium and somatic muscle fibers of septic diaphragms. These results indicate that diaphragmatic muscle fibers synthesize significant amounts of ET-1 in septic shock and that the rise in ET-1 production is due to upregulation of ET precursors and the converting enzyme.