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These findings raise the possibility that sole examination of CBF velocity is insufficient to characterize CBF regulation fully during orthostatic stress. Thus, the differences in the CBF responses in the ICA and VA that we report in response to HUT provide new information regarding CBF regulation during orthostatic stress. In this study, the reduction in ICA diameter was probably related to the hydrostatic pressure difference between Supine and HUT. The effect of HUT on carotid artery diameter has been examined (Steinback et al. 2005); however, data concerning the VA are lacking. The absence of a change in VA diameter during HUT may be explained by a difference in mechanical properties of the vessel for a change in hydrostatic pressure between ICA and VA. In addition, the different blood flow responses might be explained [http://www.selleckchem.com/products/azd9291.html this website] by differential autonomic innervations of the ICA and VA, or regional heterogeneity in ion channels and/or reactive oxygen species (Edvinsson et al. 1976; Andresen et al. 2006; Faraci, 2006). The posterior cerebral circulation may have less sympathetic innervation than the anterior cerebral portion (Edvinsson et al. 1976; Hamel et al. 1988). Another possible explanation for the difference between the ICA and VA in terms of blood flow responses to the HUT may be a difference in CO2 reactivity, [http://en.wikipedia.org/wiki/GPX4 GPX4] although posture might affect the cerebral CO2 reactivity (Mayberg et al. 1996). Accordingly, these factors may be linked to the lower vascular tone in the VA vascular bed during orthostatic stress, although orthostatic stress-induced reductions in CBF velocity cannot be explained only by sympathetic nerve activity (Zhang & Levine, 2007) or lower (Immink et al. 2009). An alternative explanation may be that blood flow and vascular conductance during orthostatic stress are different in the ICA and VA because they are operating in different metabolic states. As neural activity and metabolism are closely related to regional CBF, termed neurovascular coupling, [http://www.selleckchem.com/products/ch5424802.html http://www.selleckchem.com/products/ch5424802.html] the vascular beds supplying more metabolically active brain regions are likely to be dilated (Nakagawa et al. 2009). Such observations are relevant to the present study, where the territories supplied by the vertebro-basilar system (i.e. medulla oblongata, visual cortex, cerebellum and vestibular regions) are robustly and constantly activated during HUT due to sympathoexcitation, visual stimulation, postural control and gravitational stress. This would place the vertebro-basilar territories in a state of continuous vasodilatation compared with the internal carotid territories (Haubrich et al. 2004; Nakagawa et al. 2009). However, our study did not incorporate direct imaging techniques to assess regional brain metabolic rate for oxygen.

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Notekey15 : Page créée avec « These findings raise the possibility that sole examination of CBF velocity is insufficient to characterize CBF regulation fully during orthostatic stress. Thus, the differ... »