Effect of atrial natriuretic factor on permeability of the blood-cerebrospinal fluid barrier.

The demonstration of 125I-labelled atrial natriuretic factor (ANF)-binding sites on choroid plexus suggests a physiological role of ANF on the blood-cerebrospinal fluid barrier. This ultrastructural study was undertaken to determine whether ANF (0.5 microgram) alters the permeability of rat blood-cerebrospinal fluid barrier under steady states. Horseradish peroxidase (HRP) was used as a marker of protein permeability and ionic lanthanum as a marker of ionic permeability. HRP was not observed in the walls of choroid plexus vessels of control rats at 3 or 6 min, while at 12 min HRP was present in vessel walls and occasionally in continuity in the adjacent intercellular space between choroidal epithelial cells. In ANF-treated rats, HRP was observed in vessel walls and in the intercellular space between the choroidal epithelial cells up to the apical tight junctions at 3 min, indicating an accelerated passage of tracer. Although HRP was never observed beyond the apical tight junctions in control or test animals, at 6 min test rats showed ionic lanthanum within these junctions in focal areas and in continuity in the adjacent ventricular cavity. These studies demonstrate that ANF causes accelerated passage of both HRP and ionic lanthanum from blood into choroid plexuses with passage of ionic lanthanum into the ventricular cavity through the apical tight junctions of choroidal epithelial cells. The latter is in keeping with the known function of ANF in regulating water and electrolyte fluxes.