Alanine and glutamine synthesis and release from skeletal muscle. I. Glycolysis and amino acid release.

A J Garber,I E Karl,D M Kipnis

doi:10.1016/s0021-9258(17)33859-0

Abstract

The synthesis and release of alanine and glutamine were investigated with an intact rat epitrochlaris muscle preparation. This preparation will maintain on incubation for up to 6 hours, tissue levels of phosphocreatine, ATP, ADP, lactate, and pyruvate closely approximating those values observed in gastrocnemius muscles freeze-clamped in vivo. The epitrochlaris preparation releases amino acids in the same relative proportions and amounts as a perfused rat hindquarter preparation and human skeletal muscle. Since amino acids were released during incubation without observable changes in tissue amino acids levels, rates of alanine and glutamine release closely approximate net amino acid synthesis. Large increases in either glucose uptake or glycolysis in muscle were not accompanied by changes in either alanine or glutamine synthesis. Insulin increased muscle glucose uptake 4-fold, but was without effect on alanine and glutamine release. Inhibition of glycolysis by iodacetate did not decrease the rate of alanine synthesis. The rates of alanine and glutamine synthesis and release from muscle decreased significantly during prolonged incubation despite a constant rate of glucose uptake and pyruvate production. Alanine synthesis and release were decreased by aminooxyacetic acid, an inhibitor of alanine aminotransferase. This inhibition was accompanied by a compensatory increase in the release of other amino acids, such as aspartate, an amino acid which was not otherwise released in appreciable quantities by muscle. The release of alanine, pyruvate, glutamate, and glutamine were observed to be interrelated events, reflecting a probable near-equilibrium state of alanine aminotransferase in skeletal muscle. It is concluded that glucose metabolism and amino acid release are functionally independent processes in skeletal muscle. Alanine release reflects the de novo synthesis of the amino acid and does not arise from the selective proteolysis of an alanine-rich storage protein. It appears that the rate of alanine and glutamine synthesis in skeletal muscle is dependent upon the transformation and metabolism of amino acid precursors.

Highlights

The synthesis and release of alanine and glutamine were investigated with an intact rat epitrochlaris muscle preparation
Protein is not substantially different from that of most other amino acids [15]. These data imply that the comparative enrichment of alanine release from skeletal muscle results either from the selective proteolysis of an alanine-rich storage protein [11] or that de nouo synthesis of alanine occurs in skeletal muscle
An in vitro rat skeletal muscle preparation has been described which on prolonged incubation demonstrafes (a) constant rates ofglucose uptake and lactate and pJ.ruvate production, [6] levels of phosphocreatine, ATP, and ADP which are comparable to levels found in skeletal muscle freeze-clamped in uiuo, (c) uncut muscle fibers with minimal tendonous tissue and grossly no observable contamination by adherent adipose or connective tissue, and (d) the release of’ amino acids in a profile similar to that seen with the perfused rat hindquarter preparation or by the human forearm muscle

Summary

Introduction

The synthesis and release of alanine and glutamine were investigated with an intact rat epitrochlaris muscle preparation. These data imply that the comparative enrichment of alanine release from skeletal muscle results either from the selective proteolysis of an alanine-rich storage protein [11] or that de nouo synthesis of alanine occurs in skeletal muscle Based on these observations, a glucose-alanine cycle has been proposed [9, 16]. If glucose formation from lactate and alanine represents recycling of already existing glucose carbon, there is no known mechanism to account for the net appearance of significant amounts of new carbon from extra hepatic sources into glucose In light of this inconsistency, the interrelationships in skeletal muscle between glucose metabolism and amino acid release were investigated using an isolated intact rat skeletal muscle preparation. Portions of this study have been presented previously [19]

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Conclusion