Isolated Brain Microvessels as In Vitro Equivalents of the Blood‐Brain Barrier: Selective Removal by Collagenase of the A‐System of Neutral Amino Acid Transport

Abstract

On treatment with collagenase, brain microvessels, together with several protein components, lose some enzymatic activities such as alkaline phosphatase and gamma-glutamyltranspeptidase, whereas no change occurs in the activities of 5'-nucleotidase and glutamine synthetase. The energy-requiring "A-system" of polar neutral amino acid transport is also severely inactivated, whereas the L-system for the facilitated exchange of branched chain and aromatic amino acids is preserved. In the collagenase-digested microvessels, this leads to loss of the transtimulation effect of glutamine on the transport of large neutral amino acids, because such transtimulation is due to a cooperation between the A- and L-systems. By contrast, NH4+ maintains (and even enhances) its ability to stimulate the L-system of amino acid transport, presumably through glutamine synthesis within the endothelial cells.