The Specificity of Amino Acid and Sugar Carriers in the Capillaries of the Dog Brain Studied in Vivo by Rapid Indicator Dilution

Abstract

It is well established that some metabolites are transported from blood to brain by carrier systems which presumably are located in the capillary endothelium. In the dog the original evidence of carrier mechanisms was derived from Crone’s and our own experiments on intact animals in which the unidirectional flux of sugar^{6, 25} and amino acids²⁶ was studied by means of the first passage multiple tracer dilution technique⁴. With David Gilboe, when he came to my laboratory in Chile some years ago, we applied these techniques to his isolated dog brain preparation in order to measure kinetic constants for the glucose carrier³. He reports elsewhere in this Symposium the results he and his collaborators obtained when extending these methods to other molecules and experimental conditions. I shall concentrate here on demonstrating that the glucose carrier of the brain is of a type similar to that of the erythrocyte and in characterising the neutral amino acid carrier demonstrated by Yudilevich, de Rose and Sépulveda²⁶. I also wish to give what I think are convincing arguments for the localisation of these carriers in the capillary endothelium. It is important, I believe, to emphasize that similar indicator dilution experiments could be made in humans¹⁰, particularly since recently glucose and other metabolites have been labelled with C-11, a short-life radioactive isotope of carbon¹⁹.