Uptake of 36Cl and 22Na by the choroid plexus-cerebrospinal fluid system: evidence for active chloride transport by the choroidal epithelium.

Cl and Na transport by the lateral ventricle (LVCP) and fourth ventricle (4VCP) choroid plexuses were examined by kinetic analysis of 36Cl and 22Na uptake into the choroid plexus-CSF system of the adult rat. Both radioisotopes required most than 5 h to reach steady-state distribution in the in vivo choroid plexuses and CSF after intraperitoneal injection. Whereas the LVCP and 4VCP 36Cl steady-state spaces were comparable (55--56%), the 4VCP 22Na space (39%) tended to be greater than the LVCP 22Na spaces (36%). No evidence for inexchangeable Cl or Na was found for the choroid plexuses; the radioisotopic and chemical spaces were not significantly different. Choroid plexus 36Cl and 22Na uptake curves were resolved into two components, a fast component (t1/2 0.02--0.05 h) and a slow component (t1/2 0.85--1.93 h). By analysis of the distribution of [3H]inulin, [3H]mannitol, and 51Cr-tagged erythrocytes within the choroid plexuses, the fast component of 36Cl and 22Na uptake was found to represent extracellular and erythrocyte contributions to the tissue radioactivity, whereas the slow component represented isotope movement into the epithelial cell compartment. The calculated cell [Cl] of LVCP and 4VCP, 67 mmol/kg cell water, was 3.9 times greater than that predicted by the membrane potential for passive distribution. It is postulated that Cl is actively transported into the choroid epithelial cell across the basolateral membrane; the energy source for active Cl transport may be the Na electrochemical potential gradient (approximately 90 mV), which is twice that of the Cl electrochemical potential gradient (approximately 45 mV).