Kelley, R. C. | Search bioRxiv

bioRxiv2022

Skeletal muscle Nox4 knockout prevents and Nox2 knockout blunts loss of maximal diaphragm force in mice with heart failure with reduced ejection fraction

Patients with heart failure with reduced ejection fraction (HFrEF) experience diaphragm weakness that contributes to the primary disease symptoms of fatigue, dyspnea, and exercise intolerance. Weakness in the diaphragm is related to excessive production of reactive oxygen species (ROS), but the exact source of ROS remains unknown. NAD(P)H Oxidases (Nox), particularly the Nox2 and 4 isoforms, are important sources of ROS within skeletal muscle that contribute to optimal cell function. There are reports of increased Nox activity in the diaphragm of patients and animal models of HFrEF, implicating these complexes as possible sources of diaphragm dysfunction in HFrEF. To investigate the role of these proteins on diaphragm weakness in HFrEF, we generated inducible skeletal muscle specific knockouts of Nox2 or Nox4 using the Cre-Lox system and assessed diaphragm function in a mouse model of HFrEF induced by myocardial infarction. Diaphragm maximal specific force measured in vitro was depressed by ~20% with HFrEF. Knockout of Nox4 provided full protection against the loss of maximal force (p < 0.01), while the knockout of Nox2 provided partial protection (7% depression, p < 0.01). Mitochondrial respiration measured in permeabilized diaphragm muscle bundles increased with HFrEF or the knockout of Nox4 from skeletal muscle fibers (p < 0.05). Knockout of Nox2 from skeletal myofibers improved survival from 50 to 80% following myocardial infarction (p = 0.026). Our findings show an important role for skeletal muscle NAD(P)H Oxidases contributing to loss of diaphragm maximal force in HFrEF, along with systemic pathophysiological responses following myocardial infarction.

physiology↗

bioRxiv2021

Cardiac and respiratory muscle responses to dietary N-acetylcysteine in rats consuming a high-saturated fat, high-sucrose diet

BACKGROUNDExertional dyspnea is a significant clinical concern in individuals with overweight or obesity. The pathophysiology of dyspnea is multifactorial and complex. Previous data suggest that diaphragm and cardiac abnormalities should be considered as likely contributors to obesity-related exertional dyspnea. Additionally, oxidative stress is a causative factor in the general etiology of obesity as well as skeletal and cardiac muscle pathology. Thus, this preclinical study aimed to define diaphragm and cardiac morphological and functional alterations following an obesogenic diet in rats and the therapeutic potential of an antioxidant supplement, N-acetylcysteine (NAC). METHODSMale Wistar rats ([~]7 weeks old) consumed ad libitum either lean (20% protein, 70% carbohydrate, 10% fat) or high-saturated fat, high-sucrose (HFHS, 20% protein, 35% carbohydrate, 45% fat) diets for [~]22 weeks. Rats receiving HFHS diet were randomized to drink control water or water with NAC (2 mg/ml) for the last eight weeks of the dietary intervention: Lean, HFHS, and HFHS+NAC (n = 8 per group). We evaluated diaphragm bundles (in vitro function and histology) and hearts (weights and echocardiography) for all groups. RESULTSFinal body weights of HFHS rats, but not HFHS+NAC rats, were significantly higher than Lean controls. Neither HFHS diet nor NAC supplementation affected diaphragm specific force (N/cm2), peak power (W/kg), or morphology. In cardiac muscle, right and left ventricle weights (normalized to tibia length) of HFHS rats were greater than those of Lean controls and HFHS+NAC rats. Cardiac functional abnormalities were also present in HFHS rats, with left ventricular fractional shortening (%) and posterior wall maximal shortening velocity (cm/s) increasing compared to Lean controls, but HFHS+NAC rats did not demonstrate these markers of hypercontractility. HFHS rats showed an elevated deceleration rate of early transmitral diastolic velocity (E/DT) consistent with diastolic dysfunction, but NAC eliminated this effect. CONCLUSIONOur data suggest that an HFHS diet does not compromise diaphragm muscle morphology or in vitro function, suggesting other possible contributors to breathing abnormalities in obesity (e.g., neuromuscular transmission abnormalities). However, an HFHS diet resulted in cardiac hypertrophy, hypercontractility, and diastolic dysfunction. Supplementation with NAC did not affect diaphragm morphology or function but attenuated cardiac abnormalities in the HFHS diet. Our findings support future studies testing NAC supplementation in clinical trials of humans with obesity.

physiology↗