THE VELOCITIES OF RED-CELL AND PLASMA FLOWS THROUGH PARENCHYMAL MICROVESSELS OF RAT-BRAIN ARE DECREASED BY PENTOBARBITAL

Local cerebral blood flow is lowered in many brain areas of the rat by high-dose pentobarbital (50 mg/kg). In the present study, the mechani sm of this flow change was examined by measuring the distribution of r adiolabeled red blood cells (RBCs) and albumin (RISA) in small parench ymal microvessels and calculating the microvascular distribution space s and mean transit times of RBCs, RISA, and blood. In most brain areas , pentobarbital slightly decreased the RISA space, modestly increased the RBC space, and did not alter the blood space. The mean transit tim es of RBCs, RISA. and blood through the perfused microvessels were con siderably greater in treated rats than in controls. These findings ind icate that the mechanism by which high-dose pentobarbital diminishes l ocal cerebral blood flow in rat brain is, in the main, a lowered linea r velocity of plasma and RBC flow through small parenchymal microvesse ls and not decreased percentage of perfused capillaries (capillary ret irement). This response is probably driven mainly by lowered local met abolism and may well entail a slight increase in the number of small m icrovessels that are perfused by RBCs.