Transport Capacity of Median Eminence I. Ammo Acid Transport

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Abstract
Evidence is accumulating that suggests the importance of the median eminence as an area of transfer for substances between the cerebrospinal fluid and the pituitary portal vessels. Therefore, we have investigated the ability of isolated median eminence of mink and rat to transport the non-utilizable amino acid, α-aminoisobutyric acid (AIB). Median eminence was found to accumulate AIB by a single, sodium-dependent active mechanism having an apparent Km of 0.77 mM and a Vmax of 22.2 µmoles/ml intracellular water/h. AIB accumulation increased in linear fashion for 20 min, and reached equilibrium in 60 min at a distribution ratio of 25–30 (0.01 mM AIB starting concentration). Active transport was markedly diminished or abolished by incubation at 0 °C or in the absence of extracellular sodium and by treatment with ouabain (10–3 M),iodoacetate (10–3 M),or sodium fluoride (10–3 M),These data support the conclusion that median eminence is metabolically very active, but do not indicate the cellular component(s) responsible for this striking in vitro transport activity.

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