Metabolic changes during early starvation in rats fed a low-protein diet in the postweaning growth period

Abstract

Metabolic changes during the first 24 hours of starvation were studied in rats previously adapted for 3 weeks during the postweaning growth period to a low-protein diet using lactalbumin as a dietary protein source. Previous adaptation to a high-quality, low-protein diet reduced the effects of early starvation on the loss of body and liver weight. In rats fed a low-protein diet (6% lactalbumin, LP rats), free triiodothyronine (T 3) concentration remained higher than in control rats (13% lactalbumin, C rats) throughout the experiment (+38%, 24 hours), and the plasma insulin concentration, which was lower than in C rats during the first 6 hours (−56%), was not different thereafter. Plasma insulin to glucagon molar ratio was lower (−54%) and liver cyclic adenosine monophosphate (cAMP) concentration was higher (+28%) in LP than in C rats in the fed state, but these were not different at 24 hours of starvation. Plasma glucose concentration was slightly lower in LP than in C rats (−15%) in the fed state, but it was not different in both groups during starvation. Whereas they were unchanged in the fed state, plasma lactate concentration was lower (−57%) and free fatty acid and total ketone body concentrations were higher (+38% and + 183%, respectively) in LP than in C rats at 24 hours of starvation. Liver glycogen, fructose-2,6-bisphosphate (F-2,6-P 2), and fructose-1,6-bisphosphate (F-1,6-P 2) concentrations were higher in LP than in C rats in the fed state (+63%, +105%, and +74%, respectively); liver glycogen, F-2,6-P 2, and F-1,6-P 2 concentrations decreased during starvation in both groups and were not different at 24 hours, except for F-2,6-P 2 concentration, which was lower in LP than in C rats (−50%). Liver pyruvate kinase activity measured at 0.4 mmol/L phosphoenolpyruvate (PK v) and pyruvate kinase activity measured at 4 mmol/L phosphoenolpyruvate (PK V) were higher in LP than in C rats over the first 6 hours, and PK v and the PK v PK V ratio were lower at 24 hours of starvation. Liver phosphoenolpyruvate carboxykinase (PEPCK) activity was higher in LP than in C rats in the fed state; during starvation, enzyme activities increased but were not different in both groups. Thus, previous adaptation to a low-protein diet delayed the down-regulation of the glycolytic pathway during the first period of starvation; then, the observed increase in plasma lipids might be consistent with their use as metabolic fuel.