Insulin-Induced Accumulation of D-Xylose Against an Apparent Concentration Gradient in Diaphragm Muscle, in Vivo

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Abstract
The effect of insulin administration upon D-xylose-l-C14 penetration into the dia-phragm and gastrocnemius muscles of functionally nephrectomized normal, hypophysectomized, and adrenalectomized rats has been ex-amined. 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 fig/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.