Tm glucose in a case of congenital intestinal and renal malabsorption of monosaccharides.

Abstract

Extract: Maximal rate of renal tubular reabsorption of glucose (TmG) was determined in a 9-month-old girl with congenital intestinal malabsorption of glucose and galactose associated with constant glucosuria. At the time of the procedure, the patient had attained normal growth and development and was enjoying good health following a 5-month dietary regimen which excluded virtually all carbohydrates other than fructose. The standard glucose titration of renal tubules was carried out, using 4 arterial plasma glucose concentrations (PG), each maintained over 3 consecutive 20-minute periods while the glomerular filtration rate was kept relatively constant. The following observations were made: 1. The rate of filtered glucose (FG) was found to exceed that of reabsorption (TG) at all PG levels ranging from 31 to 243 mg%, revealing a very low renal plasma threshold. 2. A TmG of 76.7 mg/min/1.73 m2 was achieved at a PG level of 174 mg%, when FG/TG = 1.35. This amounts to 25% of the normal adult value, taken as 303 mg/min/1.73 m2. Compared with data on premature and full-term newborns, it reflects a profound deficiency of tubular reabsorption of glucose in our patient. 3. A marked drop of TGwas recorded during the final hour of titration with prolonged elevation of FG; and the observation was felt to be valid. A search of the literature revealed that others have observed the same phenomenon in subjects with suboptimal tubular reabsorption of glucose relative to the filtered load. Speculation: The finding of a low TmG in our patient with documented intestinal malabsorption of glucose and galactose strengthens Crane's original concept that a common mechanism mediates active transport of glucose at the intestinal and renal tubular epithelia. The marked drop of TG after prolonged elevation of FG in subjects with suboptimal tubular reabsorption of glucose seems to reflect a sudden ‘fatigue’ or inhibition of an already strained transport mechanism. It is postulated that this propensity for ‘fatigue’ or inhibition, by whatever processes, may be part of the profile of defective epithelial transport of glucose in these patients.