Gray and white matter brain-blood transfer constants by steady-state tissue clearance in cat

Capillary transfer constants for gray matter have been measured by others from steady-state tissue clearance during ventriculocisternal perfusion. Similar studies in white matter, however, are complicated by the bulk flow of interstitial fluid (ISF). Recently we determined the velocity of bulk flow of ISF under normal conditions. We now report capillary transfer constants in gray and white matter by steady-state tissue clearance in the cat. Adults cats underwent a 2, 3, or 4 h ventriculocisternal perfusion with artificial cerebrospinal fluid containing [3H]sucrose and either [14C]urea or [14C]ethylene glycol. Diffusion coefficients and velocity of bulk flow were determined from tissue concentrations of the extracellular marker, sucrose. Steady-state tissue concentrations of urea and ethylene glycol were used to calculate transfer of those compounds from the brain to the blood. Urea reached steady-state by the third hour; capillary transfer constants were similar in gray and white matter. Ethylene glycol reached steady-state by the second hour; however, capillary transfer was more rapid in the gray matter than in white.