Cellular mechanisms of burn-related changes in contractility and its prevention by mesenteric lymph ligation

Pathophysiology of Severe Burn Injuries: New Therapeutic Opportunities From a Systems Perspective

Severe burn injury elicits a profound stress response with the potential for high morbidity and mortality. If polytrauma is present, patient outcomes appear to be worse. Sex-based comparisons indicate females have worse outcomes than males. There are few effective drug therapies to treat burn shock and secondary injury progression. The lack of effective drugs appears to arise from the current treat-as-you-go approach rather than a more integrated systems approach. In this review, we present a brief history of burns research and discuss its pathophysiology from a systems' perspective. The severe burn injury phenotype appears to develop from a rapid and relentless barrage of damage-associated molecular patterns, pathogen-associated molecular patterns, and neural afferent signals, which leads to a state of hyperinflammation, immune dysfunction, coagulopathy, hypermetabolism, and intense pain. We propose that if the central nervous system control of cardiovascular function and endothelial-glycocalyx-mitochondrial coupling can be restored early, these secondary injury processes may be minimized. The therapeutic goal is to switch the injury phenotype to a healing phenotype by reducing fluid leak and maintaining tissue O2 perfusion. Currently, no systems-based therapies exist to treat severe burns. We have been developing a small-volume fluid therapy comprising adenosine, lidocaine, and magnesium (ALM) to treat hemorrhagic shock, traumatic brain injury, and sepsis. Our early studies indicate that the ALM therapy holds some promise in supporting cardiovascular and pulmonary functions following severe burns. Future research will investigate the ability of ALM therapy to treat severe burns with polytrauma and sex disparities, and potential translation to humans.

Depressed Myocardial Contractility: Can It Be Rescued?

Current dogma suggests patients with advanced systolic heart failure have an irreversible depression in myocardial contractility. Recent experience with improved ventricular function during continuous flow ventricular assist devices used as destination therapy would suggest otherwise. Herein, cellular and molecular signaling involved in reversing depressed myocardial contractility would be addressed. This includes cardiomyocyte thyroid hormone signaling responsible for the reexpression of fetal gene program that preserves cell efficiency (work and energy consumed) and the rescue of an endogenous population of atrophic myocytes bordering on microdomains of fibrosis to improve contractile mass.

Cardiovascular Dysfunction Following Burn Injury: What We Have Learned from Rat and Mouse Models

Severe burn profoundly affects organs both proximal and distal to the actual burn site. Cardiovascular dysfunction is a well-documented phenomenon that increases morbidity and mortality following a massive thermal trauma. Beginning immediately post-burn, during the ebb phase, cardiac function is severely depressed. By 48 h post-injury, cardiac function rebounds and the post-burn myocardium becomes tachycardic and hyperinflammatory. While current clinical trials are investigating a variety of drugs targeted at reducing aspects of the post-burn hypermetabolic response such as heart rate and cardiac work, there is still a paucity of knowledge regarding the underlying mechanisms that induce cardiac dysfunction in the severely burned. There are many animal models of burn injury, from rodents, to sheep or swine, but the majority of burn related cardiovascular investigations have occurred in rat and mouse models. This literature review consolidates the data supporting the prevalent role that β-adrenergic receptors play in mediating post-burn cardiac dysfunction and the idea that pharmacological modulation of this receptor family is a viable therapeutic target for resolving burn-induced cardiac deficits.

Redefining the gut as the motor of critical illness

The gut is hypothesized to play a central role in the progression of sepsis and multiple organ dysfunction syndrome. Critical illness alters gut integrity by increasing epithelial apoptosis and permeability and by decreasing epithelial proliferation and mucus integrity. Additionally, toxic gut-derived lymph induces distant organ injury. Although the endogenous microflora ordinarily exist in a symbiotic relationship with the gut epithelium, severe physiological insults alter this relationship, leading to induction of virulence factors in the microbiome, which, in turn, can perpetuate or worsen critical illness. This review highlights newly discovered ways in which the gut acts as the motor that perpetuates the systemic inflammatory response in critical illness.

Polydatin protects cardiac function against burn injury by inhibiting sarcoplasmic reticulum Ca2+ leak by reducing oxidative modification of ryanodine receptors

Our recent studies demonstrate that burn trauma induces leaky sarcoplasmic reticulum (SR) in heart due to excessively active ryanodine receptor (RyR) function. SR Ca(2+) leak causes partial depletion of SR Ca(2+) content and disturbances in intracellular Ca(2+) homeostasis, resulting in the pathogenesis of burn-generated cardiac dysfunction. This study investigated the role of polydatin, a resveratrol glucoside, in preventing SR leak and its therapeutic effect against burn-generated cardiac dysfunction. We found that polydatin treatment improved cardiac function impaired by burn injury of 30% of total body surface area. Parallel to the alterations in cardiac function, polydatin significantly increased the defective systolic Ca(2+) transient and contractility in burn-traumatized cardiomyocytes. Burn injury increased the occurrence of Ca(2+) sparks. The enhancement of Ca(2+) spark-mediated SR leak caused partial depletion of SR Ca(2+) content in burn-traumatized cardiomyocytes. Furthermore, we found that the content of free thiols (the number of reduced cysteines) in RyR2 in cardiomyocytes determined by the monobromobimane fluorescence of RyR2 was decreased markedly in burn-traumatized hearts. Polydatin treatment decreased intracellular reactive oxygen species levels and restored the amount of free thiols in RyR2 in burns. Concomitantly, polydatin corrected Ca(2+) spark-mediated SR leak and restored SR Ca(2+) load. The systolic Ca(2+) transient and cellular contractility were significantly increased by polydatin treatment. Taken together, the present findings provide the first evidence demonstrating that polydatin prevents enhanced Ca(2+) spark-mediated SR leak by reducing oxidative stress in RyR2 in burn-traumatized heart, leading to protection of cardiac function against burn injury.

Cellular mechanism underlying burn serum-generated bidirectional regulation of excitation-contraction coupling in isolated rat cardiomyocytes

Myocardial depressant factors have long been recognized to be present in burn serum (BS) and contribute to burn-generated cardiac contractile dysfunction. However, much of the cellular and molecular mechanism for its role in the development of the cardiac deficiency remains unknown. In this study, we investigated the effect of BS on myocardial contractility and Ca handling in single rat cardiomyocytes. The results revealed that BS (5% by volume) bidirectionally regulated cardiac excitation-contraction (EC) coupling. The action potential-elicited Ca transient and cell shortening were increased by 28.0% ± 9.7% and 34.7% ± 12.5% within 20 min after BS stimulation (the upregulation phase), but decreased by 20.5% ± 6.8% and 32.3% ± 5.1% at 60 min after BS stimulation (the downregulation phase). There was a 32.0% ± 5.8% reduction in sarcoplasmic reticulum (SR) Ca content at the downregulation phase, whereas no alteration was detected at the upregulation phase. The incidences of spontaneous Ca sparks and Ca waves were significantly increased after BS stimulation, no matter at the upregulation or downregulation phase. The hyperactive Ca sparks and Ca waves could be completely abolished by antioxidative treatment (vitamin A, 0.2 mM; and vitamin E, 1 mM) and partially reversed by NOS inhibitor L-NAME (100 μM), but not by blocking Ca influx with nifedipine (1 μM). With the normalization of Ca sparks, BS-induced alterations of action potential-elicited Ca transient and contractility were prevented by antioxidative therapy. Taken together, we propose that BS-associated bidirectional regulation of EC coupling is attributed largely to oxidative stress-induced hyperactivity of ryanodine receptors, increasing EC coupling through enhancing intracellular Ca release initially, but subsequently decreasing EC coupling by partially depleting SR Ca content through enhancement of Ca spark-mediated SR leak.

Enteral supplementation enriched with glutamine, fiber, and oligosaccharide prevents gut translocation in a bacterial overgrowth model

BACKGROUND

Normal gut flora plays an important role in the intestinal mucosal barrier function under various critical conditions. The flora may alter after severe insults, such as trauma and shock. Enteral nutrition should preserve the gut environment; however, full support is usually difficult for severely ill patients because of impaired gastrointestinal motility. Currently, we have commercial enteral supplementation product enriched with glutamine, dietary fiber, and oligosaccharide (GFO) in Japan. This study examines the hypothesis that the enteral supplementation ameliorates gut injury induced by a bacterial overgrowth model, even in small volumes and quantities.

MATERIALS

Balb/c mice received antibiotics (4 mg/mL of streptomycin) in their drinking water for 4 days to kill the normal gut flora after which they were orally inoculated with a streptomycin-resistant strain of Escherichia coli, known as E. coli C-25. The mice that were administered bacterial monoassociation received 0.5 mL of GFO twice daily (GFO group) or 10% of glucose solution (GLU group). Unsupplemented drinking water was used for control animals (control) whose gut flora was normal. The mice were killed and their mesenteric lymph nodes complex was harvested and processed to test gut bacterial translocation. The cecal population levels of bacteria and ileum histology were also examined.

RESULTS

The incidence and magnitude of gut translocation to the lymph nodes complex in the GLU group were significantly higher than those in the control (p < 0.01). Treatment with GFO prevented the gut translocation although animals in the GFO group had same level of the cecal bacterial population. Histologic findings in the ileum were not different between the GLU and GFO.

CONCLUSION

GFOs supplement prevented gut translocation for bacterial overgrowth even in small volumes and quantities. The intestinal histologic findings could not explain the protective mechanisms of GFO. Further studies may be needed to elucidate the benefit of the partial enteral nutrition.

Mesenteric lymph duct ligation prevents trauma/hemorrhage shock-induced cardiac contractile dysfunction

Clinical and experimental studies have shown that trauma combined with hemorrhage shock (T/HS) is associated with myocardial contractile dysfunction. However, the initial events triggering the cardiac dysfunction are not fully elucidated. Thus we tested the hypothesis that factors carried in intestinal (mesenteric) lymph contribute to negative inotropic effects in rats subjected to a laparotomy (T) plus hemorrhagic shock (HS; mean arterial blood pressure of 30-40 Torr for 90 min) using a Langendorff isolated heart preparation. Left ventricular (LV) function was assessed 24 h after trauma plus sham shock (T/SS) or T/HS by recording the LV developed pressure (LVDP) and the maximal rate of LVDP rise and fall ( +/- dP/dt(max)) in five groups of rats: 1) naive noninstrumented rats, 2) rats subjected to T/SS, 3) rats subjected to T/HS, 4) rats subjected to T/SS with mesenteric lymph duct ligation (T/SS+LDL), or 5) rats subjected to T/HS+LDL. Cardiac function was comparable in hearts from naive, T/SS, and T/SS+LDL rats. Both LVDP and +/- dP/dt(max) were significantly depressed after T/HS. The T/HS hearts also manifested a blunted responsiveness to increases in coronary flow rates and Ca(2+), and this was prevented by LDL preceding T/HS. Although electrocardiograms were normal under physiological conditions, when the T/HS hearts were perfused with low Ca(2+) levels ( approximately 0.5 mM), prolonged P-R intervals and second-degree plus Wenckebach-type atrioventricular blocks were observed. No such changes occurred in the control or T/HS+LDL hearts. The effects of T/HS were similar to those of the Ca(2+) channel antagonist diltiazem, indicating that an impairment of cellular Ca(2+) handling contributes to T/HS-induced cardiac dysfunction. In conclusion, gut-derived factors carried in mesenteric lymph are responsible for acute T/HS-induced cardiac dysfunction.

Oxidative stress-induced leaky sarcoplasmic reticulum underlying acute heart failure in severe burn trauma

Burn trauma causes cardiac dysfunction. However, much of the underlying cellular and molecular mechanisms remain elusive. In the present study, we demonstrate the roles of excessive sarcoplasmic reticulum (SR) Ca(2+) leakage and oxidative stress in burn-associated acute heart failure. In cardiomyocytes from failing rat hearts 12 h after full-thickness cutaneous burn of about 40% of the total body surface area, we found that Ca(2+) transients and contractility were impaired, but the triggering L-type Ca(2+) channel current density was unaltered, giving rise to a significantly reduced gain of excitation-contraction coupling. This deficiency in SR Ca(2+) release was accompanied by a reduction in Ca(2+) content in the SR. Surprisingly, the frequency of spontaneous Ca(2+) sparks was increased by 1.4-fold; Ca(2+) tolerance test (10 mM extracellular Ca(2+)) further showed 2.0- and 1.5-fold more frequent Ca(2+) waves and Ca(2+) sparks, respectively. Myofilament sensitivity to Ca(2+), however, seemed to be unaffected. These results suggest hyperactivity of the ryanodine receptor (RyR) Ca(2+) release channel and a leaky SR in burn. Importantly, pretreatment with antioxidant vitamins C and E seemed to prevent burn-induced RyR hypersensitivity and SR leakage and thereby normalize Ca(2+) transients and contractility. Concomitantly, the in vivo cardiac functions were also more tolerant of traumatic burn. Collectively, our findings suggest that SR leakage due to oxidative stress is likely a major candidate mechanism underlying burn-associated acute heart failure. Antioxidant therapy in burn trauma provides cardioprotection, at least in part, by protecting RyR's from oxidative stress-induced hypersensitivity.