Autonomic nervous system response to remote ischemic conditioning: heart rate variability assessment.

BACKGROUND: Remote ischemic conditioning (RIC) is a procedure applied in a limb for triggering endogenous protective pathways in distant organs, namely brain or heart. The underlying mechanisms of RIC are still not fully understood, and it is hypothesized they are mediated either by humoral factors, immune cells and/or the autonomic nervous system. Herein, heart rate variability (HRV) was used to evaluate the electrophysiological processes occurring in the heart during RIC and, in turn to assess the role of autonomic nervous system. METHODS: Healthy subjects were submitted to RIC protocol and electrocardiography (ECG) was used to evaluate HRV, by assessing the variability of time intervals between two consecutive heart beats. This is a pilot study based on the analysis of 18 ECG from healthy subjects submitted to RIC. HRV was characterized in three domains (time, frequency and non-linear features) that can be correlated with the autonomic nervous system function. RESULTS: RIC procedure increased significantly the non-linear parameter SD2, which is associated with long term HRV. This effect was observed in all subjects and in the senior (> 60 years-old) subset analysis. SD2 increase suggests an activation of both parasympathetic and sympathetic nervous system, namely via fast vagal response (parasympathetic) and the slow sympathetic response to the baroreceptors stimulation. CONCLUSIONS: RIC procedure modulates both parasympathetic and sympathetic autonomic nervous system. Furthermore, this modulation is more pronounced in the senior subset of subjects. Therefore, the autonomic nervous system regulation could be one of the mechanisms for RIC therapeutic effectiveness.

Identification of human circulating factors following remote ischemic conditioning (RIC): Potential impact on stroke.

Remote ischemic conditioning (RIC) is a procedure consisting of short cycles of ischemia applied in a limb that activates endogenous protection in distant organs, such as the brain. Despite the promising outcomes of RIC, the biochemical factors governing inter-organ communication remain largely unexplored, particularly in humans. A pilot study on 20 healthy humans was performed to identify potential circulating biochemical factors involved in RIC signalling. Blood was collected before and immediately, 4 and 22 h after the end of RIC. To characterize the responses triggered by RIC, a combination of biochemical and proteomic analysis, along with functional in vitro tests in human cells, were performed. RIC did not alter the levels of nitric oxide, bilirubin and cell-free mitochondrial DNA. In contrast, carboxyhaemoglobin levels increased following RIC at all time points and young subset, suggesting endogenous production of carbon monoxide that is a cytoprotective gasotransmitter. Additionally, the levels of glutathione and cysteinylglycine bound to proteins also increased after RIC, while glutathione catabolism decreased. Plasma proteomic analysis identified overall 828 proteins. Several steps of statistical analysis (Student's t-test, repeated measures ANOVA, with Holm corrected pairwise p-values <0.05 threshold and fold change higher or lower than 100 %) leaded to the identification of 9 proteins with altered circulating levels in response to RIC at 4h and 22h. All 9 proteins are from extracellular space or exosomes, being involved in inflammation, angiogenesis or metabolism control. In addition, RIC-conditioned plasma from young subjects protected microglial cell culture against inflammatory stimuli, indicating an anti-inflammatory effect of RIC. Nevertheless, other functional tests in neurons or endothelial cells had no effect. Overall, we present some evidence for RIC-induced anti-inflammatory and antioxidant responses in healthy human subjects, in particular in young subjects. This study is a first step towards the disclosure of signalling factors involved in RIC-mediated inter-organ communication.