Early effects of mechanical ventilation on isotonic contractile properties and MAF-box gene expression in the diaphragm

Abstract

This study aimed to determine the time-dependent effects of diaphragmatic inactivity on its maximum shortening velocity (V(max)) and the muscle atrophy F-box (MAF-box, atrogin-1) gene expression during controlled mechanical ventilation (CMV). Twenty-four New Zealand White rabbits were grouped into 1 day, 2 days, and 3 days of CMV and controls in equal numbers. The in vitro isotonic contractile properties of the diaphragm were determined. In addition, myosin heavy chain protein and mRNA, myosin light chain, MAF-box mRNA, and volume density of abnormal myofibrils were measured. Tetanic force decreased, and V(max) increased from control of 6.4 to 6.6, 7.7, and 8.1 muscle lengths per second after 1, 2, and 3 days of CMV, respectively (P < 0.02). The increased V(max) compensated for the decreased tetanic force; consequently, compared with the controls, maximum power output was unchanged after 3 days of CMV. V(max) correlated with the volume density of abnormal myofibrils [y = 0.1x + 5.7 (r = 0.87, P < 0.01)]. In the diaphragm, MAF-box was overexpressed (355% of control) after 1 day of CMV, before the evidence of structural myofibril disarray. In conclusion, CMV produced a time-dependent increase in V(max) that was associated with the degree of myofibrillar disarray and independent of changes in myosin isoform expression. Furthermore, CMV produced an increase in MAF-box mRNA levels that may be partially or completely responsible for the degree of myofibrillar disarray resulting from CMV.

Effects of Acute Administration of Corticosteroids during Mechanical Ventilation on Rat Diaphragm

“…All three major proteolytic systems (lysosomal proteases, proteasomes, and calpains) are activated in the diaphragm secondary to CMV (12). Studies indicate that prolonged CMV promotes increased diaphragmatic 20S proteasome activity (10), increased mRNA expression levels of two muscle specific E3 ligases, Murf-1 and Mafbx (13,14), and increased cathepsin B and calpain activity (15). In addition, caspase-3 activation was shown to be a requirement for mechanical ventilator-induced myofiber atrophy and myonuclear apoptosis (16).…”

Ventilator-induced diaphragm dysfunction: cause and effect

“…Specifically, increased muscle contractile activity promotes increased protein synthesis, whereas muscle inactivity results in decreased rates of protein synthesis (13,54). The initial decrease in diaphragm protein synthesis that occurs during full support MV is likely due to decreased protein translation because myosin heavy chain mRNA levels remain unchanged (99,120). Indeed, the rate of protein synthesis in muscle is largely controlled by the efficiency of translation, which is regulated at the level of initiation (52).…”

Acute effects of high‐dose methylprednisolone on diaphragm muscle function

“…Diaphragm muscle strips were stretched similar to preparation for immunohistochemistry 31 and then fixed in 6.5% glutaraldehyde in a 0.1 M sodium cacodylate buffer as previously described. 44 Following fixation, dehydration, and polymerization, 1-m-thick sections were cut on an LKB Ultratome III using a diamond knife and stained with 0.25% toluidine blue solution. After muscle fibril orientation was determined under light microscopy, blocks were reoriented, and ultrathin sections (50 -70 nm) were cut transversely to the muscle fiber axis.…”