Saturable Transport of Manganese(II) Across the Rat Blood‐Brain Barrier

Unanesthetized adult male rats were infused intravenously with solutions containing 54Mn (II) and one of six concentrations of stable Mn(II). The infusion was timed to produce a near constant [Mn] in plasma for up to 20 min. Plasma was collected serially and on termination of the experiment, samples of CSF, eight brain regions, and choroid plexus (CP) were obtained. Influx of Mn (JMn) was calculated from uptake of 54Mn into tissues and CSF at two different times. Plasma [Mn] was varied 1,000‐fold (0.076–78 nmol/ml). Over this plasma concentration range, JMn increased 123 times into CP, 18–120 times into brain, and 706 times into CSF. CP and brain JMn values fit saturation kinetics with Km (nmol/ml) equal to 15 for CP and 0.7–2.1 for brain, and Vmix (10–2 nmol · g−1· s−1) of 27 for CP and 0.025–0.054 for brain. Brain JMn except at cerebral cortex had a nonsaturable component. CSF JMn varied linearly with plasma [Mn]. These findings suggest that Mn transport into brain and CP is saturable, but transport into CSF is nonsaturable.