The long-term objectives of this application are to develop a better understanding of the effects of critical- illness myopathy on diaphragm (DIA) muscle function;focusing on the molecular mechanisms of the dysfunction. Acquired muscle weakness in the Intensive Care Unit, referred to "critical-illness myopathy," occurs commonly in patients receiving prolonged mechanical ventilation combined with both neuromuscular junction blocking agents (NMBA) and corticosteroid (methylprednisolone, MP). These patients have difficulty weaning from mechanical ventilation. The mechanisms of DIA dysfunction are unknown, but are likely multifactorial;including enhanced proteolysis as a key factor. The ubiquitin-proteasome is an important proteolytic system in skeletal muscle. However, currently unknown is the role of upstream pathways -- i.e., the IGF-1/PI3K/Akt/Foxo within the DIA muscle under controlled mechanical ventilation (CMV) combined with NMBA, and MP. Our recent preliminary studies -- comparing 2 days of CMV, high-dose MP, or combined CMV-MP -- showed that DIA force and IGF-1 mRNA were reduced to the same extent, suggesting that these pathological perturbations activate a common signaling pathway. However, unknown is whether the combination of CMV-NMBA with high- or low-dose MP produces similar levels of DIA dysfunction, or results from similar mechanisms. Additionally, no information is available about whether in vivo administration of rhIGF-1 will mitigate DIA dysfunction. This application describes two specific aims to test the hypothesis that I) the effects of CMV-NMBA plus MP on DIA function is dose-dependent, with low-dose MP being protective of DIA function due both to increased protein synthesis via Akt/mTOR and decreased proteolysis via Akt/Foxo pathway. We will determine the interactive effects of 3 days of CMV-NMBA without or with high- or low-dose MP on DIA contractile, structural and molecular properties. Our hypothesis will be supported if the combined CMV-NMBA plus low-dose MP preserves DIA force production comparable to that in controls. II) rhIGF-1 mitigates DIA force-loss. During CMV-NMBA plus MP, we will compare the effects of 3 days of rhIGF-1 alone;combined rhIGF-1 plus PI3K-inhibitor (wortmannin), or combined rhIGF-1 plus mTOR- inhibitor (rapamycin) on DIA contractile, structural, and molecular properties. Our hypothesis will be supported if rhIGF-1 treatment prevents DIA force-loss, while treatments with rhIGF-1 plus either PI3K inhibitor, or mTOR inhibitor abolishes its beneficial effects. Pathogen-free New Zealand White rabbits will be studied.
In Specific Aim I, the animals will be randomly assigned into four groups receiving either 3 days of CMV-NMBA (cisatracurium);CMV-NMBA-MP3 (3 mg/kg/d), CMV-NMBA-MP30 (30 mg/kg/d) or 0 cm H2O of continuous positive airway pressure (CPAP [control]).
In Specific Aim II, groups of animals treated per Specific Aim I will be further divided into subgroups to receive rhIGF-1 alone, rhIGF-1 plus either wortmannin, or rapamycin. Functional, morphological, biochemical;cellular/molecular measurements of the DIA will be performed.
Relevance to Veterans Health One-third of patients admitted to the Intensive Care Unit (ICU) require mechanical ventilation. The major indication for the initiation of mechanical ventilation is acute respiratory failure. When post-operative causes are excluded, pneumonia, chronic obstructive pulmonary disease (COPD), and congestive heart failure comprise the top three reasons for initiating mechanical ventilation. All three diagnoses are prevalent in the veteran populations, with COPD comprising a large proportion of patients. Many of these patients require corticosteroid and mechanical ventilatory support for various durations following exacerbation of their disease. In addition, when the disease (e.g., pneumonia) evolves into or manifests as acute respiratory distress syndrome (ARDS), a paralyzing drug may be administered to facilitate mechanical ventilation. Despite the efficacy of short-term, high- or low-dose corticosteroid in COPD exacerbation, or of short-term paralyzing drug administration to improve patient-ventilator interaction, or to reduce inflammatory cytokine production, these are risk factors for acquired muscle weakness. Acquired muscle weakness is associated with prolonged weaning time and longer duration of mechanical ventilation, longer ICU and hospital stay, and increased treatment costs, not only for inpatient care, but also for outpatient rehabilitation. The proposed study will shed light on the mechanisms of early development of critical-illness myopathy, and its possible prevention with low- dose corticosteroid or potential treatment with existing drugs. The study has the potential to stimulate developments of drugs to ameliorate or prevent acquired muscle weakness.