Effect of inhibitors on D-xylose permeability in rat diaphragm muscle

The influence of metabolic inhibitors and low temperatures upon D-xylose transfer has been studied in rat diaphragm muscle preparations in vitro. Using intact fiber preparations, it has been confirmed that at body temperature metabolic inhibitors like DNP have an insulin-like action in that they permit D-xylose to distribute into previously unavailable intracellular aqueous regions; inhibitors, unlike insulin, disturb cation distribution in association with increased sugar penetration. Although the studies with inhibitors suggest an energy requirement for maintenance of D-xylose exclusion, the D-xylose exclusion mechanism is effectively maintained at 0° for many hours, and under these conditions, inhibitors have little or no effect on D-xylose distribution, though they do produce potassium loss. In cut muscle fiber preparations, in which insulin significantly increases the rate at which D-xylose equilibrates between cell water and external medium, DNP does not increase the rate of D-xylose entry, but does abolish the effect of insulin in this preparation. The results suggest that insulin action upon sugar permeability in muscle involves two barrier systems; some of the characteristics of these systems have been defined.

Insulin-induced accumulation of D-xylose against an apparent concentration gradient in diaphragm muscle. in vivo

The effect of insulin administration upon D-xylose-1-C(14) penetration into the diaphragm and gastrocnemius muscles of functionally nephrectomized normal, hypophysectomized, and adrenalectomized rats has been examined. It was found in all groups that after the administration of tracer amounts of D-xylose, this sugar enters the cell water of diaphragm to a greater extent than in gastrocnemius muscle, both in the presence and absence of exogenous insulin. Insulin increases the apparent intracellular distribution of D-xylose in both muscles in all three types of rats. After insulin administration, the intracellular concentration of D-xylose in diaphragm muscle was estimated to be about two times greater than D-xylose concentration in plasma; D-xylose accumulation was not observed in gastrocnemius muscle of insulin-treated rats. Intracellular accumulation of D-xylose occurs in diaphragm of insulin-treated rats at plasma concentrations of D-xylose ranging from 4 to 2200 microg/ml; however, a "saturation" phenomenon appears to be operative, since intracellular distribution declines as plasma D-xylose concentration is increased within this range. A decline in intracellular D-xylose distribution also occurs in gastrocnemius as plasma D-xylose is increased, suggesting that entry into this muscle as well does not exhibit the characteristics of a simple diffusion process. The significance of these in vivo observations is briefly discussed in relation to widely accepted assumptions concerning sugar permeability in muscle.

Anaerobic conversion of d-xylose to triose phosphate and hexose phosphate by extracts of Pseudomonas hydrophila

By analysis of reaction mixtures containing D-xylose, adenosinetriphosphate, and cell-free sonic extracts of Pseudomonas hydrophila, evidence has been obtained for the formation of triose and hexose phosphate is under anaerobic conditions. The hexose phosphate fraction was shown to be a mixture of fructose-6-phosphate and glucose-6-phosphate by chromatographic and by enzymatic methods. The time course of the formation and disappearance of pentose, heptulose, triose, and hexose phosphate intermediates was also determined. These data, together with the results of previous work from this laboratory, have been combined to present an over-all scheme of enzymatic reactions for the anaerobic conversion of D-xylose to triose and hexose phosphates.