Elimination of phenolsulfonphthalein from the cerebrospinal fluid via capillaries in central nervous system in cats by active transport.

It was recently proposed that organic anions, such as cerebral acidic metabolites and phenolsulfonphthalein (PSP), are eliminated from cerebrospinal fluid (CSF) by diffusion into the central nervous system (CNS) and further by active transport into capillaries. To test this hypothesis, PSP was injected into cisternal CSF and its distribution into various parts of the CNS was measured 1 and 3 h later in control cats and those pretreated with probenecid, which blocks active transport of organic anions into capillaries. PSP in tissue shows an intensive pink color when exposed to 1 N NaOH. Planimetric analysis of color pictures of coronal CNS slices showed that at the first hour, diffusion and distribution of PSP into the CNS in both groups of animals was similar, while at the third hour, a great reduction of PSP distribution in the CNS in control and only a slight reduction in probenecid pretreated cats was observed. The results support the hypothesis that active transport across the capillary wall in the CNS is the main avenue for elimination of cerebral acidic metabolites from both CSF and CNS and in such a way that central homeostasis is maintained.