Glutamate-sodium Cotransport in the Kidney: An Example for the Plasticity of Transport Systems
Glutamate is an important metabolic intermediate as well as a neurotransmitter. Therefore, the question of its transport across plasma membranes has generated considerable interest. In the liver, the results concerning the ability to transport glutamate have been controversial. In liver slices (Hems et al., 1968) and in the perfused liver (Ross et al., 1967) the parenchymal cell membrane was found to be relatively impermeable to glutamate, whereas sodium-dependent glutamate transport was observed in rat hepatocytes in primary monolayer culture (Gebhardt and Mecke, 1983) and in a mixed preparation of plasma membrane vesicles from rat liver (Sips et al., 1982). The discrepancy between these different results was resolved recently by Ballatori et al. (1986) who demonstrated that the sodium-L-glutamate cotransport system was only present in the canalicular domains of liver plasma membranes. Since this domain is not exposed in slices or in the intact organ the strong polarity of the hepatocyte with regard to the distribution of transport systems (Kinne, 1987) could be made responsible for the divergence in the experimental results.
KeywordsPermeability Choline Succinate Nism
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