Regulation of glucose uptake by muscle. 4. The specificity of monosaccharide-transport systems in rat-diaphragm muscle

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Abstract
The specificity of monosaccharide transport in isolated diaphragm has been investigated by measuring the effect of other sugars on the intracellular accumulations of D-xylose, D-arabinose, D-lyxose, D-galactose and L-arabinose. Competition for transport could be demonstrated between a group of sugars comprising D-glucose, D-mannose, D-3-O-methylglucose, D-xylose, D-arabinose, D-lyxose and L-arabinose, but not between members of this group and D-galactose, D-fructose, maltose, [alpha]-methyl-D-glucoside, [beta]-methyl-D-glucoside or D-sorbitol. Transport of D-xylose was inhibited by phlorrhizin and the -SH poisons p-chloromer-curibenzoate and N-ethylmaleimide. We conclude that there is more . than one monosaccharide-transport system in muscle and that a -SH compound, possibly a -SH protein, is involved in monosaccharide transport. The factors of importance to the specificity of monosaccharide transport in diaphragm are discussed. Insulin did not increase the incorporation of C14 from [C14] glucose into glycogen of perfused isolated rat heart under anaerobic conditions although it increases glucose uptake under these conditions. We conclude that an effect of the hormone on transport of glucose does not depend upon increased formation of glycogen. [C12] Glucose was found to decrease the incorporation of C14 from [C14] mannose into maltose and glycogen in isolated rat diaphragm. C14 from [C14] mannose was present only in glucose residues in glycogen; no [C14] mannose could be detected in the polysaccharide. We conclude that the formation of glycogen from mannose follows transport and phosphorylation of the sugar and that sugars do not pass directly from the transport form into polysaccharide.