Glucose-Stimulated Protein Synthesis in Rat Testis Slices: Substrate Specificity and Effects of Insulin and Substrate Analogs1

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Abstract
The effects of substrate, substrate concentration, insulin and substrate analogs on the incorporation of lysine into rat testis slices were studied. Incorporation was stimulated by 10 mM glucose and 10 mM mannose, to a lesser extent by 10 mM fructose, but not by galactose, glucitol or xylitol. Half-maximal stimulation occurred at 2 mM glucose or 2 mM mannose and was maximal at concentrations above 4 mM. Fructose at a concentration of 20 mM was about as effective as 2 mM glucose. The glucose-stimulated incorporation was not affected by insulin. Several analogs were tested at a final concentration of 10 mM as inhibitors of glucose- and fructose-stimulated lysine incorporation. At a glucose concentration of 4 mM only 2-deoxyglucose and 2-deoxygalactose were inhibitory, reducing incorporation to levels not significantly different from basal. When the glucose concentration was reduced to 2 mM, other analogs were also inhibitory; the most effective of these were 5-thioglucose, 3-0-methylglucose, 4,6-ethylideneglucose and galactosamine. When 10 mM fructose was used as a substrate, the same analogs plus the fructose analogs mannoheptulose, 1-deoxyfructose, 2,5-anhydromannose and 2,5-anhydromannitol were inhibitory. Differences in analog inhibitions against fructose and glucose suggest different rate-limiting steps for the stimulation by these 2 sugars. The response to glucose and mannose, but not fructose, apparently is mediated by an uptake system which is limiting at low substrate concentrations. Inhibition by various sugar analogs generally results from uptake system which is limiting at low substrate concentrations. For most of the analogs this occurs under conditions in which a step subsequent to the hexose uptake system, possibly a phosphorylation step, is not saturated.

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