In vivo and in vitro regulation of hepatic glucagon receptor mRNA concentration by glucose metabolism

We have recently cloned the murine glucagon receptor (GR) gene and shown that it is expressed mainly in liver, In this organ, the glucagon-GR system is involved in the control of glucose metabolism as it initiates a cascade of events leading to release of glucose into the blood stream, which is a main feature in several physiological and pathological conditions, To better define the metabolic regulators of Gn expression in liver we analyzed GR mRNA concentration in physiological conditions associating various glucose metabolic pathways in vivo and in vitro in the rat and in the mouse, First, we report that the concentration of the GR mRNA progressively increased from the first day of life to the adult stage. This effect was abolished when newborn rodents were fasted, Second, under conditions where intrahepatic glucose metabolism was active such as during fasting, diabetes, and hyperglycemic clamp, the concentration of GR mRNA increased independent of the origin of the pathway that generated the glucose flux. These effects were blunted when hyperglycemia was corrected by phlorizin treatment of diabetic rats or not sustained during euglycemic clamp, In accordance with these observations, we demonstrated that the glycolytic substrates glucose, mannose, and fructose, as well as the gluconeognic substrates glycerol and dihydroxyacetone, increased the concentration of GR mRNA in primary cultures of hepatocytes from fed rats, Glucagon blunted the effect of glucose without being dominant, The stimulatory effect of those substrates was not mimicked by the nonmetabolizable carbohydrate L-glucose or the glucokinase inhibitor glucosamine or when hepatocytes were isolated from starved rats, In addition, inhibitors of gluconeogenesis and lipolysis could decrease the concentration of GR mRNA from hepatocytes of starved rats, Combined, these data strongly suggest that glucose flux in the glycolytic and gluconeogenic pathways at the level of triose intermediates could control expression of GR mRNA and participate in controlling its own metabolism.