Hormonal regulation of liver phosphoenolpyruvate carboxykinase and glucokinase gene expression at weaning in the rat

During the suckling period, the rats are fed continuously with milk, which is a high-fat low-carbohydrate diet (HF). At weaning, milk is progressively replaced by the rat's laboratory chow which is a high-carbohydrate low-fat diet (HCHO), and this is accompanied by large hormonal modifications: an increase in plasma insulin and a decrease in plasma glucagon concentrations, and by marked changes in metabolic pathways in liver: decrease in hepatic gluconeogenesis and increase in glycolysis and lipogenesis. Most of the data concerning these changes are related to maximal activity of enzymes. The recent availability of specific cDNA probes for phosphoenolpyruvate carboxykinase (PEPCK), and glucokinase (GK) has allowed the study of the role of pancreatic hormones and nutrition in the changes of the expression of these genes at weaning in the rat. Regarding phosphoenolpyruvate carboxykinase gene transcription, the concentration of mRNA as well as the activity of PEPCK are elevated in the liver of suckling rat until the onset of weaning, 21 d after delivery. After weaning to a HCHO diet, both mRNA and activity of PEPCK rapidly decrease to a very low level. In contrast, weaning on an HF diet, which maintains high plasma glucagon and low plasma insulin levels, does not decrease in plasma glucagon concentration and a 90% decrease in PEPCK gene transcription and PEPCK mRNA concentration in 1 h. Regarding glucokinase gene transcription, the concentration of mRNA as well as the activity of GK are not detectable before 15 d after birth in the liver of the rat. They markedly increase when the newborn are weaned on an HCHO diet but not when they are weaned on an HF diet.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo regulation of glycolytic and gluconeogenic enzyme gene expression in newborn rat liver

Glucagon and its second messenger, cAMP, are known to rapidly block expression of the L-type pyruvate kinase gene and to stimulate expression of phosphoenolpyruvate (PEP) carboxykinase gene in the liver in vivo. The respective roles, however, of hyperglucagonemia, insulinopenia, and carbohydrate deprivation in the inhibition of L-type pyruvate kinase gene expression during fasting are poorly understood. In addition, the long-term effects of physiological hyperglucagonemia on expression of the two genes are not known. In this study, we investigate the effects of long-term physiological hyperglucagonemia and insulinopenia induced by suckling (which provides a high-fat, low-carbohydrate diet) on expression of the two genes in the liver of normal newborn rats. We show that transcription of the L-type pyruvate kinase gene is inhibited at birth and remains low during the whole suckling period, whereas transcription of the PEP carboxykinase gene is maximal in the neonate, and then decreases despite very high levels of plasma glucagon during suckling. In contrast to the adult, however, in which L-type pyruvate kinase gene expression in the liver is blocked by cAMP and stimulated by carbohydrates, the regulation of L-type pyruvate kinase gene expression in the newborn undergoes a developmental maturation: the inhibitory effect of glucagon is never complete in developing rat liver and the stimulatory effect of glucose could not be detected during suckling, due to either hyperglucagonemia, immaturity of the gene regulatory system, or both.

Development of insulin-sensitivity at weaning in the rat. Role of the nutritional transition

This study was undertaken to determine the factors involved in the development of insulin-sensitivity at weaning. Glucose kinetics were studied in suckling rats and in rats weaned on to a high-carbohydrate (HC) or a high-fat (HF) diet, in the basal state and during euglycaemic-hyperinsulinaemic-clamp studies. These studies were coupled with the 2-deoxyglucose technique, allowing a measure of glucose utilization by individual tissues. In the basal state, the glycaemia was higher in HF-weaned rats (124 +/- 4 mg/dl) than in suckling (109 +/- 1 mg/dl) and HC-weaned rats (101 +/- 3 mg/dl). Glucose turnover rates were similar in the three groups of animals (14 mg/min per kg). Nevertheless, basal metabolic glucose clearance rate was 20% lower in HF-weaned rats than in the other groups. During the euglycaemic-hyperinsulinaemic experiments, hepatic glucose production was suppressed by 90% in HC-weaned rats, whereas it remained at 40% of basal value in suckling and HF-weaned rats, indicating an insulin resistance of liver of these animals. Glucose clearance rate during the clamp was 18.3 +/- 0.9 ml/min per kg in suckling rats, whereas it was 35.3 +/- 1.2 ml/min per kg in HC-weaned rats and 27.8 +/- 1.1 ml/min per kg in HF-weaned rats, indicating an insulin resistance of glucose utilization in suckling, and to a lower extent, in HF-weaned rats. The deoxyglucose technique showed that peripheral insulin resistance was localized in muscles and white adipose tissue of suckling and HF-weaned rats. These results indicate that the switch from milk to a HC diet is an important determinant of the development of insulin-sensitivity at weaning in the rat.