Manganese Uptake and Efflux in Cultured Rat Astrocytes

Abstract: Astrocytes play a central role in manganese (Mn) regulation in the CNS. Using primary astrocyte cultures from neonatal rat brains, these studies demonstrate a specific highaffinity transport system for Mn²⁺. Saturation kinetics are clearly indicated by both l/s versus l/s plots (K_m= 0.30 ± 0.03 μMM; V_max= 0.30 ± 0.02 nmol/mg of protein/min) and plots of v versus [s]. Several divalent cations (Co², Zn²⁺, and Pb²⁺) failed to inhibit the initial rate of ⁵⁴Mn²⁺ uptake. In contrast, extracellular Ca²⁺ at 10 μM decreased ⁵⁴Mn²⁺ uptake. Exchange with extracellular Mn²⁺ was not obligatory for the efflux of ⁵⁴Mn²⁺ into extracellular medium because efflux occurred into Mn²⁺‐free extracellular medium, but efflux of ⁵⁴Mn²⁺ was enhanced when astrocytes were equilibrated in the presence of unlabeled Mn²⁺. Efflux of ⁵⁴Mn²⁺ was biphasic with both a rapid and a slow component. Efflux was most rapid during the first 10 min of incubation, with 27.5 ± 2.2% of ⁵⁴Mn²⁺ transported extracellularly, and 37.2 ± 1.2% of preloaded ⁵⁴Mn²⁺ was retained by the astrocytes at 120 min. These studies show, for the first time, that mammalian astrocytes can transport Mn via a specific transport system.