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Nutrient transport systems in dog brain.

The cerebral circulation of the dog was isolated and an extracorporeal system was used to perfuse the brain with diluted blood. Transport of blood-borne substances was investigated by measurement of arteriovenous (a-v) differences (net transport) and by the indicator dilution method (unidirectional influx). The net movement of amino acids between blood and brain was qualitatively and quantitatively different when measured by whole blood a-v differences than when measured by plasma a-v differences. This result suggests that erythrocytes are involved in the movement of amino acids between the blood and brain, a finding similar to that observed in other vascular beds (Elwyn et al. Am. J. Physiol. 222: 13338 1972). The transport of 13 different blood-borne substances was investigated at normal and elevated solute concentrations using the indicator dilution method. Saturable processes were observed for the unidirectional transport of tyrosine, tryptophan, L-dopa, free palmitic acid, adenine, thymine, adenosine, and guanosine. However, the blood-brain interface did not appear to contain a carrier for dopamine, folic acid, cyanocobalamin, guanine, hypoxanthine, and free oleic acid.