The role of nitric oxide synthase and heme oxygenase in the protective effect of hypothermia in ischemia-reperfusion injury

Ischemia Time in Extremity Allotransplantation: A Comprehensive Review

In response to the widespread occurrence of limb loss and the transformative potential of extremity vascularized composite allotransplantation (VCA), we examine the impact of warm and cold ischemia duration on limb survival and functional recovery. Our insights into warm ischemia are largely derived from relevant literature on replantation and revascularization. Studies indicate that achieving reperfusion within 5 to 6 hours of warm ischemia is critical for limb survival, and within 3 hours for curbing significant functional deficits. For limbs preserved in static cold conditions, as is standard practice in VCA, reperfusion should be attained within 10 to 12 hours of cold ischemia. However, our analysis exposes a lack of data on extremity functional recovery following cold ischemia, particularly in humans or large animal models. This underscores a gap in the literature that could guide clinical ischemia management in VCA if addressed. We anticipate optimal functional recovery between 3 and 6 hours of cold ischemia, as supported by outcomes in rats. Prolonged ischemia times are also associated with graft rejection, posing unique challenges to VCA. Tissues exhibit diverse responses, with muscle and nerve being highly susceptible to ischemic damage, and skin acquiring heightened immunogenicity. Ischemia management emerges as a focus for future policy and research initiatives. On the horizon, exploring updated transplantation protocols, vascular shunts, stabilizing perfusion solutions, and subnormothermic machine perfusion could mitigate ischemic damage and enhance clinical outcomes in extremity VCA.

Pharmaceutical Preconditioning With Nitric Oxide Synthase and L-Arginine in Ischemic Tissues

BACKGROUND

Nitric oxide (NO) is a multifunctional signaling molecule involved in regulating vascular tone and tissue oxygenation. It is also an important cytoprotective agent against ischemia-reperfusion injury (IRI). Enhancing NO bioavailability via exogenous NO synthases (NOSs) and L-arginine promotes conversation to NO, circumventing the problem of nonfunctioning NOSs under hypoxic and acidic conditions. In this study, the authors evaluated the therapeutic efficacy of neuronal, inducible, and endothelial NOS and L-arginine on reperfusion-induced skin flap alterations.

METHODS

The vascular pedicle isolated rat skin flap model was used and underwent 3 hours of ischemia. At 30 minutes before ischemia, normal saline, endothelial-, inducible-, and neuronal NOSs (1/2 IU) and L-arginine (100 mg/kg body weight) were administered by means of intravenous infusion. The IRI-induced alterations were measured 5 days after the operation.

RESULTS

The 3 isoforms of NOS increased the flap vitality rate (VR) from 10% to 23% compared with the control group. L-Arginine treatment also increased the VR by approximately 15%. The combination of L-arginine with NOS resulted in even higher flap VRs. The best results could be achieved with the combination of endothelial NOS (2 IU) and L-arginine.

CONCLUSIONS

Modulation of NO bioavailability via exogenous application of NOSs and L-arginine significantly improved VRs in a skin flap rat model. This pharmacologic preconditioning has the potential to attenuate IRI-induced alterations in skin flaps.

Burn Injury-Associated MHCII+ Immune Cell Accumulation Around Lymphatic Vessels of the Mesentery and Increased Lymphatic Endothelial Permeability Are Blocked by Doxycycline Treatment

It is theorized that toxic agents are transported from the hyperpermeable gut of burn victims through the lymph, to the systemic circulation, causing global injury. We believe that immune cells respond to leakage of "toxic lymph" following trauma causing the attraction of these cells to the perilymphatic space. To test this, we utilized a model of burn on rats to examine changes in a single immune cell population associated with mesenteric lymphatic dysfunction. We examined the ability of serum from these animals to increase permeability in lymphatic endothelial monolayers and disrupt cellular junctions. We also treated burn animals with doxycycline, an inhibitor of microvascular permeability, and observed the effects on immune cell populations, morphometry, and lymphatic endothelial permeability. Burn injury increased the number of MHCII⁺ immune cells along the vessel (>50%). The size and shape of these cells also changed significantly following burn injury. Serum from burn animals increased lymphatic endothelial permeability (∼1.5-fold) and induced breaks in VE-cadherin staining. Doxycycline treatment blocked the accumulation of immune cells along the vessel, whereas serum from doxycycline-treated animals failed to increase lymphatic endothelial permeability. The size of cells along the vessel in doxycycline-treated burn animals was not affected, suggesting that the cells already present on the lymphatic vessels still respond to substances in the lymph. These findings suggest that factors produced during burn can induce lymphatic endothelial barrier disruption and lymph produced during traumatic injury can influence the attraction and morphology of immune cell populations along the vessel.

The effects of hypothermia and L-arginine on skeletal muscle function in ischemia-reperfusion injury

OBJECTIVES

Ischemia-reperfusion (I/R) injury can have detrimental effects on skeletal muscle. We have shown that vessel permeability can be minimized in a hypothermic setting and also by administering the nitric oxide synthase (NOS) stimulator, L-arginine, at physiologic temperatures. The purpose of this study was to examine and compare skeletal muscle contractility after an I/R insult during hypothermic conditions, warm conditions, and also with the administration of L-arginine at physiologic temperatures. We hypothesized that hypothermia and L-arginine administration will also demonstrate protective effects to skeletal muscle contractility.

METHODS

Using Sprague-Dawley rats, the extensor digitorum longus muscle was rotated on its vascular pedicle to a thermo-controlled stage. Ischemia was established using an atraumatic femoral artery tourniquet. Reperfusion was performed under control and experimental conditions including local hypothermia and intravenous L-arginine. After harvesting experimental muscles, contractility was then quantified by using a tissue bath stimulator with force transducers.

RESULTS

Warm reperfusion resulted in marked decrease in muscle contractility compared with sham animals. Local hypothermia showed statistically significant preservation of contractility compared with the sham group. This protective effect was recapitulated by the application of NOS inducers (L-arginine) at warm conditions.

CONCLUSIONS

These findings demonstrate that hypothermia and L-arginine are protective of skeletal muscle contractility after an I/R injury. The results presented may have profound effects on future therapeutic recommendations and suggest possible pathways for clinical intervention to modulate I/R injury, which is commonplace in orthopaedic trauma and reconstructive surgery.

Facts and fiction: the impact of hypothermia on molecular mechanisms following major challenge

Numerous multiple trauma and surgical patients suffer from accidental hypothermia. While induced hypothermia is commonly used in elective cardiac surgery due to its protective effects, accidental hypothermia is associated with increased posttraumatic complications and even mortality in severely injured patients. This paper focuses on protective molecular mechanisms of hypothermia on apoptosis and the posttraumatic immune response. Although information regarding severe trauma is limited, there is evidence that induced hypothermia may have beneficial effects on the posttraumatic immune response as well as apoptosis in animal studies and certain clinical situations. However, more profound knowledge of mechanisms is necessary before randomized clinical trials in trauma patients can be initiated.