Therapeutic strategies designed to attenuate ischemic injury to the nervous system have generally focused on limiting the induction and/or progression of cellular mechanisms of injury. These strategies have taken various forms, but they commonly involve either the suppression of metabolic activity or the inhibition of specific injury cascades. An alternative approach has been to enhance metabolic supply to areas of partial perfusion to sustain essential cellular functions. This approach typically entails an increase in systemic oxygen supply (hyperoxygenation) under hyperbaric and more recently normobaric conditions. Each of these strategies holds considerable promise and warrants continued and careful development.
The current study describes a novel therapeutic approach that is related conceptually to hyperoxygenation therapy. Trans-sodium crocetinate is a modified carotenoid compound that is capable of enhancing the diffusivity of small molecules in aqueous solutions. This compound and its parent compound crocetin have previously been shown to increase tissue oxygenation in multiple organ systems.40,45,48
In a model of hemorrhagic shock, TSC both improved oxygen consumption and increased survival.45
Observations presented herein support these previous findings by demonstrating that TSC treatment increases tissue oxygen levels in an area of partial perfusion in the brain. To our knowledge, this is the first evidence of a medicinally induced increase in tissue oxygenation in the penumbra of a focal ischemic event. While the mechanistic underpinnings of enhanced tissue oxygenation were not the subject of the current study, previous studies demonstrate that TSC increases hydrogen bonding among water molecules, leading to a reduction in chaos in the aqueous solution.30
This process is known as “structure-building” and in aqueous solutions (for example, plasma and interstitial fluids) increased structure can facilitate the diffusivity of small molecules, including oxygen and glucose. Inasmuch as the plasma phase is a critical site of resistance for the delivery of blood-borne oxygen to tissue,26,27,65
it is postulated that enhanced diffusivity of metabolic substrates from the vasculature contributes to the TSC-induced increase in tissue oxygenation in the ischemic penumbra.
A central goal of the current study was to evaluate the potential protective actions of TSC against ischemic injury to the brain. In a rat model of permanent focal ischemia, TSC was effective in reducing infarct volume when treatment was initiated soon after the onset of ischemia. This effect was dose dependent, with significant protection in the dosage range of 0.023–0.229 mg/kg and optimal protection at a dosage of 0.092 mg/kg TSC. The mechanism underlying the reduced efficacy of TSC at the highest dosages is at present unknown and remains a matter for future investigation. The protective effect of TSC at its optimal dosage represented a reduction in infarct volume of approximately 60%, which rivals maximal protective effects of other interventions (for example, hypothermia) observed in the 3-vessel model of focal ischemia.18
A key a priori concern about any therapy involving increased oxygenation is that elevated levels of tissue oxygen could aggravate outcomes during the reperfusion phase that follows transient ischemia. Considerable evidence supports the concept that an oxidative burst and free radical generation play critical roles in ischemia-reperfusion injury. Reperfusion in the presence of elevated tissue oxygen could conceivably amplify damaging effects of oxidative injury. This issue was evaluated using 2 approaches. First, the hyperoxia response during postischemic reperfusion was examined in TSC-treated animals. Second, the impact of TSC on cerebral infarction was assessed subsequent to ischemia-reperfusion. Rather than potentiating postischemic hyperoxia, treatment with TSC actually blunted postischemic hyperoxia. We speculate that this reduction in the reperfusion-hyperoxia reflects a healthier state of the penumbral tissue, owing to metabolic substrate supplementation produced by TSC during ischemia. Consistent with the “healthier tissue” concept is the observation that TSC significantly reduced postischemic injury after ischemia-reperfusion. Treatment with TSC reduced cerebral infarct volume by approximately 45% when initiated 10 minutes after the onset of ischemia. Thus, even though the levels of tissue oxygen were elevated at the onset of reperfusion, tissue hyperoxia was blunted during reperfusion, and tissue survival was improved by TSC.
It is important to consider the possibility that TSC could exert its protective actions via mechanism(s) other than the facilitation of small molecule diffusion. For instance, some carotenoid compounds possess free radical scavenging activity, an effect that could provide a protective influence against ischemic neural injury. While TSC is capable of scavenging free radicals, this effect only achieves significance at dosages much higher than those required for TSC to enhance diffusivity and improve survival.56
Other mechanisms by which TSC could theoretically exert protection include effects on blood flow and/or cellular metabolism. These possibilities have been examined previously with the structurally similar parent compound of TSC, crocetin. However, no significant effects on blood flow,24
oxygen solubility in blood,17
or oxidative phosphorylation17
were observed in response to treatment with crocetin. Although the effect of TSC on blood viscosity has not been studied, it has been shown to produce a slight increase in the viscosity of water,56
which would not be predicted to increase blood flow. Thus, the evidence available to date is inconsistent with the concept that an alternative mechanism to enhanced diffusivity is responsible for the protective actions provided by TSC.
Taken together, the present findings indicate that TSC could be of value for limiting intraoperative ischemic injury or other anticipated forms of ischemic challenge. Based on its presumptive mechanism of action, TSC may be capable of inhibiting a broad range of neural injury mechanisms by reducing both the intensity of the metabolic challenge during partial ischemia and by attenuating reperfusion-induced oxidative injury.