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Abstract

Studies with the free flow micropuncture technique have shown that the ratio of TF/P_L-glucose to TF/P_Inulin in proximal tubular fluid, in distal tubular fluid, and in more than half of the final urine samples measured was greater than one, which suggests that L-glucose was actively secreted. Studies with the microperfusion technique confirmed this finding and showed that L-glucose was secreted by the proximal tubules. A maximum rate of secretion was reached at a plasma concentration of 4 mM. The tubular secretion ofl-glucose was augmented by the presence of 16.6 mM D-glucose in tubular lumen and inhibited by 10⁻⁴ M phlorizin. Kinetic analysis showed that the V_max values for L-glucose secretion in the absence and in the presence of D-glucose are 5.0×10⁻¹⁰ and 6.3×10⁻¹⁰ mol×cm⁻²×sec⁻¹ respectively which were very close to the value reported for the V_max ford-glucose reabsorption. However, the K_m forl-glucose secretion was 3.1 mM and was reduced to 1.6 mM when D-glucose was present in the perfusion fluid. The K_m for D-glucose reabsorption has been reported to be 0.6 mM (8). The results of this investigation were interpreted as being consistant with the hypothesis that L-glucose secretion and D-glucose reabsorption share the same carrier system.