A modified method for measurement of radiotracer permeation across the rat blood--brain barrier: the problem of correcting brain uptake for intravascular tracer.

A modified technique to assess integrity of the rat blood--brain barrier is described in which a poorly permeating radiotracer is injected intravenously and its net permeation across the cerebral vasculature in 30 min is related to its time-averaged level in the circulation. At the termination of experiments, direct carotid arterial perfusion clears blood from brain in 30 s and permits measurement of the tracer level in brain parenchyma without correction for intravascular tracer. Constant-rate withdrawal of a single arterial blood sample throughout the period of tracer circulation replaces the need for repeated blood sampling and graph plotting to determine the time-integrated plasma tracer level. During the first 2-3 min following intravenous injection of [3H]mannitol or [14C]sucrose, permeation across the barrier occurred at a rate substantially greater than that measured for a 30 min circulation time. The relatively rapid uptake of these tracers into brain during the first few minutes after injection would explain in part why estimates of regional blood volume, calculated from ratios of brain vs blood tracer concentration after short circulation times, were much higher than values determined using [51Cr]erythrocytes.