Ischaemia/reperfusion and permanent ischaemia differentially affect haemoglobin properties - Possible influence of oxidative stress and adaptation to acute hypoxia.

Cerebral ischaemia is an acute state characterised by a severe decrease in the supply of oxygen (O2) to the brain, resulting in the death of neurons and glial cells. Despite multiple studies investigating the processes that lead to cell damage under ischaemic stroke, there is still a lack of information about the changes in blood properties under various phases of transient ischaemia with restoration of blood flow or under prolonged ischaemia. Blood, as a source of O2, might either contribute to the brain damage owing to the development of oxidative stress, or might prevent cell death by the modulation of O2 bioavailability to cells. Here, we studied hydrogen peroxide (H2O2) generation in mitochondria of neurons in vivo using the H2O2-sensitive biosensor HyPer7, and blood properties [haemoglobin (Hb) oxygenation and Hb affinity to O2, estimated with Raman microspectroscopy (RS)] in two experimental rat models: transient 60-min ischaemia induced by the occlusion of the middle cerebral artery (MCAO) followed by 48 h reperfusion, and permanent 48-h MCAO-induced ischaemia. We found that an increase in the amount of H2O2 synthesised in neuronal mitochondria under the reperfusion period correlated with a decrease in blood oxygenation level, whereas permanent ischaemia did not affect the amount of oxyhaemoglobin but led to an increase in the affinity of Hb to O2. We hypothesise that H2O2 may initiate processes that lead to increased O2 penetration to the brain tissue under reperfusion, and that increased Hb affinity to O2 may be an adaptive reaction of the blood system to acute prolonged ischaemia.