Uptake of 36Cl and 22Na by the Choroid Plexus‐Cerebrospinal Fluid System: Evidence for Active Chloride Transport by the Choroidal Epithelium

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Abstract
Cl and Na transport by the lateral ventricle (LVCP) and 4th 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 more than 5 h to reach steady-state distribution in the in vivo choroid plexuses and CSF after i.p. injection. The LVCP and 4VCP 36Cl steady-state spaces were comparable (55-56%), but the 4VCP 22Na space (39%) tended to be greater than the LVCP 22Na space (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 2 components, a fast component (t1/2 [half-time] 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 represented extracellular and erythrocyte contributions to the tissue radioactivity and 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. Cl evidently 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 (.apprx. 90 mV), which is twice that of the Cl electrochemical potential gradient (.apprx. 45 mV).