Capillary Perfusion in the Brain

Abstract

The perfusion mode of cerebral capillaries is still a matter of controversy. Whereas the interrelationships between the density of perfused capillaries, local glucose utilization, and local blood flow in the brain are well known (Klein et al. 1986), the question of the normal perfusion pattern of brain capillaries and of recruitment of capillaries under conditions of high blood flow is still open. The present investigation therefore aimed to test the normal perfusion pattern of brain capillaries by fluorescent staining of capillaries and of their contents. Another question posed by this study was whether drastic increases in cerebral blood flow (CBF) induced by CO₂ are accompanied by increases in the number of perfused capillaries. Experiments were performed on awake rats to avoid an influence of anesthesia on capillary perfusion. Since the results of such investigations are critically dependent on the method used to mark the perfused capillaries, additional studies were performed to verify the applicability of the fluorescence method used in the present and the previous (Klein et al. 1986) study. In one series of experiments the densities of perfused capillaries were compared with fluoresceinisothiocyanate (FITC) coupled to globulin, as used in the present study, or to dextran, as applied in studies by Weiss and his group (Buchweitz and Weiss 1986; Francois-Dainville et al. 1986; Grover et al. 1986; Weiss et al. 1982), who came to different conclusions about capillary perfusion from their data; another series of experiments investigated the question of whether the anesthesia, which preceded all the experiments (for placing the catheters), could have influenced the results of the capillary density measurements. To this end, FITC-globulin was injected into a tail vein in naive, untreated control rats and the density of perfused capillaries measured. In the last series of experiments, the capillary structures were stained using fluorescent antibodies against the basal membrane constituent fibronectin. The counts obtained were compared with those obtained with intravascular fluorescent markers. The results showed the validity of the method used and a continuous perfusion of all the capillaries during both normocapnia and hypercapnia.