Effect of Aspartame-Derived Phenylalanine on Neutral Amino Acid Uptake in Human Brain: A Positron Emission Tomography Study

The possible effects of elevation of the plasma phe-nylalanine level secondary to the ingestion of aspartame on brain amino acid uptake in human subjects have been investigated by means of positron emission tomography (PET). 1-[ 11 C]Aminocyclohexanecarboxylate ([ 11 C]ACHC) is a poorly metabolized synthetic amino acid that crosses the blood-brain barrier by the same carrier that transports naturally occurring large neutral amino acids. Quantitative test-retest PET studies were performed on 15 individuals. Seven received two identical baseline scans, whereas eight received a baseline scan followed by a scan performed ∼40–45 min following ingestion of an orange-flavored beverage containing 34 mg/kg of body weight of the low-calorie sweetener aspartame, a dose equivalent to the amount in 5 L of diet soft drink consumed all at once by the study subjects, weighing an average of 76 kg. The 40–45-min interval was selected to maximize the detection of possible decreases in ACHC uptake resulting from increased competition for the carrier, because the plasma phenylalanine level is known to peak at this time. We observed an 11.5% decrease in the amino acid transport rate constant Kt and a smaller decrease in the tissue distribution volume of ACHC (6%). Under conditions of normal dietary use, aspartame is thus unlikely to cause changes in brain amino acid uptake that are measurable by PET.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65668/1/j.1471-4159.1991.tb02047.x.pd

Metabolic fate of the heart agent [18F]6-fluorometaraminol

Studies were performed to determine whether [18F]6-fluorometaraminol (18F-FMR), a new neuronal heart radiopharmaceutical, is metabolized in vivo and if the metabolites are taken up in heart. Rat, dog, baboon and guinea pig were injected with 18F-FMR and tissue samples were analyzed for metabolites by HPLC. Liver contained the most metabolites of the tissues studied with 25-90% of the radioactivity present as metabolites at 1 h in all the species studied. While metabolites of 18F-FMR are found in blood, no significant accumulation of these metabolites is found in heart ([les]0.3%) 1 h after i.v. administration in any species except rat. These studies suggest that 18F-FMR is a suitable agent for quantitative imaging of the heart by positron emission tomography.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28226/1/0000679.pd