MEASUREMENT OF TISSUE PROTEIN-SYNTHESIS RATES IN-VIVO - A CRITICAL ANALYSIS OF CONTRASTING METHODS

The extension of the flooding method for measuring the rate of protein synthesis, from animal to human tissues, has led to criticism. This i s based on the observation that in human muscle, unlike animal tissues , the rate of synthesis in the fasting state measured with constant in fusion is lower than that obtained with the flooding technique. Moreov er, incorporation of infused tracer can be enhanced with a simultaneou s flood, although an inhibition of incorporation has also been reporte d. Explanations for these observed discrepancies are explored. Evidenc e from studies in human muscle both with flooding and with a nonisotop ic technique have given no indication of a stimulation of protein synt hesis during flooding. It is therefore concluded that the most likely explanation for the discrepancy between methods is that changes in the isotopic enrichment of the precursor amino acid, which are minimized by the flooding procedure, are not adequately accounted for with the c onstant infusion method.