Amino-acid uptake compared in motor neurons, spinal cord grey matter, muscle and liver

Abstract

White male rats were injected intravenously with either [ 3H]lysine, [ 3H]tyrosine or [ 14C]glutamine. Uptake of labeled materials into ventral horn motor neurons dissected from the spinal cord grey matter, the spinal cord grey matter as a whole, liver and muscle was determined by liquid scintillation counting methods and the nature of the radioactive material present in the tissues was determined by ascending paper chromatography. The percentage of the injected dose of labeled material present per gram of dry tissue was always found to be highest in the ventral horn motor neurons. Muscle and spinal cord grey matter uptakes were much lower than liver, with muscle uptake being higher in lysine- and glutamine-injected rats and lower when tyrosine was the injected amino-acid. In all instances, some of the injected amino-acid was detected in the neural tissue by ascending paper chromatography. Following lysine injection, only lysine was present, while after glutamine and tyrosine, the various metabolites of these amino-acids were also detected. The high uptake in motor neurons relative to the low uptake in an area of spinal cord grey matter comparable to that from which such neurons were dissected is discussed in relation to the ‘blood-brain barrier’. This comparison indicates the importance of comparing amino-acid uptakes between cell types rather than between organs of heterogenous structure.