The regulation of uptake and output of amino acids by rat tissues

Abstract

1. 1)|Uptake and output of amino acids by individual tissues of rats were assessed by measuring differences between concentrations of amino acids in the plasma of various blood vessels. In most tissues of fasted rats, uptake or output of glutamine and alanine remarkably exceed those of other amino acids. So, these amino acids are considered to be “end products” of amino acid metabolism in tissues and to serve as “nitrogen carriers” which convey nitrogen from tissue to tissue. (Glycine and serine also play similar, although minor, roles.) This concept is consonant with the finding that uptake of glutamine and alanine by fetal and tumor tissues exceeds those of other amino acids. In meal-fed rats, movements of most amino acids between the plasma and tissues are very different from those observed in fasted rats, but some features are common to fasted and meal-fed rats. Under both conditions, the nitrogen of amino acids metabolized in tissues is released into the blood plasma in the form of glutamine, alanine, glycine or serine, and glutamine is utilized by the kidney and portal-drained viscera. 2. 2)|In fasted rats, the uptake of alanine by the liver exceeds that of other amino acids and the carbon of alanine is rapidly incorporated into blood glucose. However, the uptake of serine by the liver is relatively small and glutamine is even released from the liver. The carbon of serine and glutamine is only slowly incorporated into blood glucose. So, alanine is an effective precursor for hepatic gluconeogenesis, but glutamine and serine are not. 3. 3)|Portal-drained viscera is a major site for metabolism of glutamine in the plasma. Glutamine is metabolized in these viscera and its nitrogen is released into the plasma in the form of alanine and ammonia. 4. 4)|The concentration of glycine in the arterial plasma is significantly lowered by feeding protein, while concentrations of other amino acids are raised remarkably. In addition, the content of glycine in the liver is also reduced by feeding protein, whereas the uptake of glycine by the liver is increased. It is suggested that the uptake and metabolism of glycine by the liver is stimulated by feeding protein, resulting in the reduction of glycine content in the blood plasma and liver.