Adaptation of the brain's microcirculation to prolonged hypobaric hypoxia

Resumen

We review some of the effects of moderate hypobaric hypoxia (0.5 atmosphere) on the brain microcirculation of adult rats. Brain vascuiarity increases after prolonged hypoxia, but the time course of the adaptive response and its reversibility, were not known. We found no significant increase in brain vascularity after 4 days of hypoxia but noted a significant increase of about 33% after 1 week. Cerebral vascutarity continued to increase, attaining a maximum of about 70%, over control values afler 2 weeks of hypoxia, but without significant further increase between 2 and 3 weeks of hypoxia. Three weeks of normoxia afler 3 weeks of hypoxia reversed the hypoxia-induced cerebral hypervascularity. The cerebral hypervascularity induced by hypoxia was associated with an increased density of the glucose transported protein (GLUT-1) in isolated cerebral microvessels. This increased expression of GLUT-1 in cerebral microvessels was evident at 1 week of hypoxia, remained constant after 3 weeks of hypoxia, and was not reversed after 3 weeks of normoxic recovery. Thus, unlike the hypoxia-induced increased brain vascularity, the hypoxia-induced increased density of GLUT-1 in isolated cerebral microvessels is not readily reversible. The combination of increased brain vascularity and increased GLUT-1 expression in cerebral microlvessels of rats subjected to hypobaric hypoxia is associated with increased blood-to-brain glucose transport, increased brain concentrations of glucose and lactate, and increased regional cerebral, metabolic rate for to prolonged hypoxia and suggest increased glycolysis in the rat brain after moderate and prolonged hypoxia.