Metabolic adaptation of the renal carbohydrate metabolism. III

Abstract

The adaptive response of renal metabolism of glucose was studied in isolated rat proximal and distal renal tubules after a high protein-low carbohydrate diet administration. This nutritional situation significantly stimulated the gluconeogenic activity in the renal proximal tubules (about 1.5 fold at 48 hours) due, in part, to a marked increase in the fructose 1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK) activities. In this tubular fragment, FBPase activity increased only at subsaturating fructose 1,6-bisphosphate concentration (30% at 48 hours) which involved a significant decrease in the Km (31%) for its substrate without changes in the Vmax. This enzymatic behaviour is probably related to modifications in the activity of the enzyme already present in the renal cells. Proximal PEPCK activity progressively increased at all substrate concentrations (almost 2 fold at 48h of high protein diet) which brought about changes in Vmax without changes in Kim. These changes are in agreement with variations in the cellular concentration of the enzyme. Neither gluconeogenesis nor the gluconeogenic enzymes changed in the distal fractions of the renal tubules. On the other hand, a high protein diet did not apparently modify the glycolytic ability in any fragment of the nephron, although a significant increase in the phosphofructokinase (PFK) and pyruvate kinase (PK) activities was found in the distal renal tubules. This short term regulation involved a significant decrease from 24 hours in the Km value of distal PFK (almost 40%) without changes in Vmax. The kinetic behaviour of distal PK was mixed. In the first 24h after high protein diet a significant decrease in the Km for phosphoenolpyruvate was found (30%) without variation in the Vmax, however during the second 24 hours the activity of this glycolytic enzyme increased significantly (almost 1.3 fold) without modifications in its Km value. On the contrary, this nutritional state did not modify the kinetic behaviour of any glycolytic enzyme in the proximal regions of the renal tubules.