PREVIOUS observations showed that the rate at which the central nervous system incorporates phosphate from the plasma is slow, and the total amount taken up by the brain is relatively small. There seems to be a protective mechanism which prevents the brain from exchanging unlabeled phosphate for labeled phosphate as rapidly as other organs do. This barrier is already present in the embryonic state, although it reaches its full development only after birth.1
In order to by-pass the barrier, radioactive phosphate was injected into the cerebrospinal fluid directly; the result was a rapid uptake of the isotope by the brain.* The pituitary gland and the tuber cinereum are not protected by the blood-brain barrier, and their exchange of phosphate with the plasma differs essentially from that of the central nervous system proper.4 Other non-nervous elements of the brain, such as the pineal gland, choroid plexus, and area postrema,
BAKAY L. STUDIES ON BLOOD-BRAIN BARRIER WITH RADIOACTIVE PHOSPHORUS: IV. Spatial Aspects of Phosphate Exchange Between Plasma and Brain. AMA Arch NeurPsych. 1954;71(6):673–683. doi:10.1001/archneurpsyc.1954.02320420001001
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