Effect of hemorrhage on regional morphometric indexes of cerebral capillarity

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Abstract
This study was performed to determine whether the brain can increase the number of perfused capillaries and arterioles supplying it regionally during hemorrhage. This was done using a technique to simultaneously determine total and perfused regional arteriolar and capillary morphology. Conscious Long-Evans rats served as unbled controls or were bled 65 mmHg or to 40-45 mmHg and stabilized for 30 min. Regional cerebral blood flow was determined using [14C]iodoantipyrine in half of these animals and fluorescein isothiocyanate-dextran was injected in the other half for determination of perfused cerebral microvascular morphometric indexes. The total microvasculature was labeled postmortem via an alkaline phosphatase stain. Regional cerebral blood flow was significantly increased in animals bled to 65 mmHg. During hemorrhage to 40-45 mmHg, cerebral blood flow was reduced 50% (from 59 .+-. 28 to 26 .+-. 11 ml .cntdot. min-1 .cntdot. 100 g-1, means .+-. SD) with no regional redistribution. For all treatments, total capillary density ranged from 400 to 500 capillaries/mm2, and in controls 47% were perfused. Animals bled to 65 mmHg did not mobilize their unperfused microvascular reserve even though they showed a slight tendency to do so. During hemorrhage to 40-45 mmHg, this percent increased significantly to 57% with the largest increase occurring in the pons. Approximately 51% of arterioles were perfused in controls and this was not different compared with the percent perfused during hemorrhage. Despite the overall lack of mobilization of unperfused arterioles, some regions within the brain significantly mobilized their reserves with severe hemorrhage, e.g., hippocampus (78%), hypothalamus (67%), and medulla (73%). Thus, despite a reduction in flow and hypocapnia, the brain can increase the number of perfused microvessels during hemorrhage.