Evoked local cerebral blood flow induced by somatosensory stimulation is proportional to the baseline flow

The purpose of this study was to determine the relationship between the inc rease in local cerebral blood flow during neuronal activation (evoked LCBF) and the baseline flow level. We measured the hemodynamics in alpha -chlora lose-anesthetized rats using laser-Doppler flowmetry during somatosensory s timulation under the hypocapnic, normocapnic and hypercapnic conditions. Th e baseline levels of LCBF and red blood cell (RBC) velocity under hypocapni a (PaCO2 = 26.4 +/- 1.1 mmHg) were, respectively, 10 and 11% lower than tho se under normocapnia (PaCO2 = 34.2 +/- 1.4 mmHg) (P < 0.01). The evoked res ponse magnitude of LCBF and RBC velocity under hypocapnia were, respectivel y, 22 and 18% lower than those under normocapnia. There was no significant difference in the baseline level and evoked response magnitude of RBC conce ntration. On the other hand, the baseline levels of LCBF, RBC velocity and RBC concentration under hypercapnia (PaCO2 = 73.4 +/- 13.3 mmHg) were, resp ectively, 47, 24 and 14% higher than those under normocapnia (PaCO2 = 34.7 +/- 2.5 mmHg) (P < 0.01). The evoked response magnitude of LCBF, RBC veloci ty and RBC concentration under hypercapnia were, respectively, 96, 82 and 6 2% greater than those under normocapnia. After. normalization with respect to each baseline level, there was no significant difference in normalized e voked response magnitude of LCBF, RBC velocity and RBC concentration, eithe r between hypocapnic and normocapnic conditions or between hypercapnic and normocapnic conditions, indicating that evoked LCBF is proportional to the baseline flow. These results suggest that the amount of evoked LCBF is not determined by the demand for metabolic substrates. (C) 2000 Elsevier Scienc e Ireland Ltd and the Japan Neuroscience Society. All rights reserved.