Defining the lower limits of blood-brain barrier permeability: factors affecting the magnitude and interpretation of permeability-area products.

Experimental alteration in the restricted permeability of the blood-brain barrier to polar, blood-borne molecules is often quantitated in the rat with use of 14C-sucrose or 3H-mannitol delivered as a test substance into the circulation. The underlying principle is to relate the quantity of saccharide that has permeated into brain parenchyma, after an arbitrary time period, to some index of the circulating tracer level. This study indicates that to correct the radioactivity level in the brain tissue for intravascular tracer, it is an erroneous practice to estimate the latter as the product of tissue blood volume and the tracer concentration measured in a systematic blood sample. Dissected brain tissue was found to have a lower hematocrit and thereby larger plasma/tracer compartment per unit blood volume than femoral arterial blood. It is further shown that, although commercially supplied stocks of 14C-sucrose or 3H-mannitol may contain only small quantities of radioactive impurities, their inclusion in injectates and preferential uptake into brain may cause significant overestimation of permeability to the parent tracer. It is also confirmed that magnitude of permeability-area (PA) products for permeation of purified sucrose or mannitol into brain varies inversely with the length of time allotted for tracer circulation in the bloodstream. This finding is at variance with the assumptions of a two-compartment (plasma/brain) diffusion model underlying such measurements and supports a recently published model for blood-to-brain transfer based on multiple uptake compartments in brain parenchyma. The factors compromising PA measurement identified in this study may partly underlie variations in PA values published from several laboratories that had been attributed to genetic differences in laboratory rats.