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Free keywords:
Hexose Transport; Sodium Cotransport; Kidney Tubules; Sugar Specificity; Kidney Micropuncture
Abstract:
With the technique of stop flow microperfusion with simultaneous capillary perfusion, the zero net flux transtubular concentration difference (Δc) of labelled sugars was measured.
The following sequence of Δc values, which are a measure for the active transtubular transport rate, were evaluated:d-glucose ≅β methyl-d-glycoside >α-methyl-d-glycoside >d-galactose >3-O-methyl-glucose >d-allose. When 10−4 M phlorrhizin was given in the luminal perfusate the Δc's dropped to zero (±8%). Δc-values in the same range i.e. indicating no active transport, were found for:l-glucose,d-mannose, 2-deoxy-d-glucose,d-fructose,d-glucosamine, 6-deoxy-d-galactose (=d-fucose),d-ribose and the reference polyalcohold-mannitol. Inhibition of thed-galactose δc was achieved by 15 mmol/l of the following sugars: α-methyl-d-glycoside ≅d-glucose ≅ 6-deoxy-d-glucose >3-O-methyl-d-glucose an no significant inhibition byd-xylose andd-mannose. Against Δc of α-methyl-d-glucose the following inhibitory potency was observed:d-glucose >6-deoxy-d-glucose >3-O-methyl-d-glucose ≅d-galactose >d-xylose and no inhibition byd-mannose.
When the ambient sodium was replaced by choline, the Δc values of all actively transported sugars dropped toward zero. An analysis of the Na+ dependence of the α-methyl-d-glycoside transport revealed that the sodium dependence is of the affinity type i.e. that only Km increased with increasing Na+ concentration while Vmax remained almost constant.
From these data one can conclude: 1. The Crane specificity, i.e. that only the α-position of the OH-group on carbon atom 2 is essential, which was found for the intestinal hexose transport holds for the rat proximal kidney tubule, too. 2. The hexose transport system in the rat works only when Na+-ions are present. The sodium ions augment the affinity of the hexose transport system for the hexoses.
