Long term control of renal carbohydrate metabolism--I. Effect of starve-feed cycles on renal tubules gluconeogenesis.

1. Adaptive responses of renal gluconeogenesis to alternative starve-feed cycles in isolated kidney tubules are reported. 2. An increase of renal gluconeogenesis during the starve state of the cycles took place, reaching values between 1.7 and 3.2-fold in the starve-feed and feed-starve cycles respectively. 3. Conversely, a decrease in this metabolic pathway took place during the feed state of the cycles. During the feed-starve cycle the decrease reached 70% whereas in the opposite cycle it was almost 60%. 4. The activities of renal gluconeogenic enzymes, phosphoenolpyruvate carboxykinase and fructose 1,6-bisphosphatase are parallel to the gluconeogenic capacity throughout the different nutritional conditions although different regulating mechanisms appear in both enzymes. 5. Phosphoenolpyruvate carboxykinase changed its activity at all substrate concentrations without significant changes in Km values during the development of the nutritional cycles, whereas fructose 1,6-bisphosphatase activity only varied at subsaturating substrate concentrations with modifications in the Km values for fructose 1,6-bisphosphate in these nutritional conditions.