Estimation of cerebral extraction of circulating compounds by the brain uptake index method: influence of circulation time, volume injection, and cerebral blood flow.

The Oldendorf "brain uptake index" method for estimating and characterizing the transport of circulating substances across the blood-brain barrier in the rat has been widely applied. The present study illustrates that minimizing the volume injected into the circulation to 10 microliters or less is necessary. Otherwise the flow artefacts, induced by the injected volume and the rate of injection, will influence the index. This principle is also relevant to techniques using intracarotid injections for the direct study of cerebral blood flow. The study also elucidates the need for reducing the circulation time after injection to about 5 sec, to avoid artefacts due to metabolism and back diffusion to the circulation of the substances studied.