Glycerol metabolism in experimental malnutrition during lactation

Regulation of hepatic gluconeogenesis and glycogenolysis by phosphorylated glycerol and glycolytic intermediates in diabetic and control Chinese hamsters

Metabolic regulation of gluconeogenesis and glycogenolysis by two phosphorylated derivatives of glycerol, G3P, and DHAP, and by F2,6BP, was assessed in vitro in liver homogenates obtained from Chinese hamsters (C. griseus) of two types: diabetic animals from sublines with consistent glycosuria and hyperglycemia, and normoglycemic controls. Only FBPase was sensitive to inhibition by the phosphorylated metabolites. G3P was weakly inhibitory of FBPase. Addition of 7 X 10(-3) M DHAP halved FBPase activity in the diabetic hamsters and 4 X 10(-3) M DHAP produced the same effect in the controls. The other gluconeogenic enzymes and phosphorylase a were only negligibly inhibited. In contrast, F2,6BP inhibited FBPase at concentrations in the micromolar range. Liver homogenates from diabetic hamsters appeared significantly more sensitive to F2,6BP inhibition of FBPase than those from controls at concentrations 0.6 X 10(-6) M and higher. These data indicate that in well-fed hamsters phosphorylated glycerol derivatives are unlikely to regulate hepatic gluconeogenesis at physiologic concentrations. However, the effects of F2,6BP on gluconeogenesis and glycolysis may be linked to those mediated by insulin. Thus, the deficiency of insulin, elevated end-organ insulin resistance, the alteration in the glucagon-insulin interaction, or a combination of these possible causes can be involved in an abnormal regulation of glycolysis and gluconeogenesis at the FBPase step, associated with changes in F2,6BP concentration.

Regulation of gluconeogenesis by glycerol and its phosphorylated derivatives

Glycerol, glycerol-3-phosphate (G3P), and dihydroxyacetone phosphate (DHAP) were evaluated as inhibitors of gluconeogenesis on rat liver enzymes in vitro, and for their effects on glucose formation in vivo in well-nourished and malnourished rats. DHAP was more potent as an inhibitor than G3P on fructose-1,6-diphosphatase (FDPase), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6Pase). The I50 for DHAP was 2, 8, and 9 x 10(-3) M, respectively. No effect was observed on rat liver pyruvate carboxylase (PC). Glycerol was a weak inhibitor of FDPase and PEPCK, but did not inhibit PC and G6Pase. In vivo, when G3P was injected before a parenteral L-alanine (Ala) challenge, it produced a hypoglycemic effect in malnourished rats and a lesser, but noticeable, blood glucose level reduction in well-fed animals. Glycerol caused a smaller reduction in glucose formation from Ala. No comparable effects were observed after a fructose pretreatment. These results underscore the potential hypoglycemic effects of phosphorylated glycerol metabolites and identify the steps in gluconeogenesis where this action is exerted. The study also stresses the nutritional component in the glycerol intolerance syndrome, apparent from the far more severe effects observed in malnourished rats given G3P or glycerol prior to Ala.

Intrauterine malnutrition in the rat: alterations of fetal glycerol metabolism

This investigation intended to clarify the effects of malnutrition in utero on enzymes of glycerol metabolism and the stores of phosphorylated glycerol intermediates in liver, striated muscle, and brain in the rat. Pregnant Wistar rats were restricted to an intake of 50% (M) of ad libitum fed controls (C) from the seventh day of gestation onward. Fetuses were removed 2 days (-2), or 1 day (-1), before term, or at the day of birth (DOB) The M fetuses and newborn rats were stunted. Their hepatic alpha-glycerophosphate oxidase (GPO) levels were lower than those of C in utero (mean +/- SEM: M = 23.1 +/- 1.5, 15.8 +/- 1.1, and 31.6 +/- 4.5; C = 34.8 +/- 4.9, 39.8 +/- 7.0, and 23.6 +/- 5.0 nmol/min X cm at -2, -1, and DOB, respectively; F = 7.29 [1,57], P less than .01). In muscle, this enzyme, as well as liver and brain alpha-glycerophosphate dehydrogenase (GPD), alpha-glycerophosphate (GP), and dihydroxyacetone phosphate (DHAP), only varied with the developmental stage. Although the latter was a significant differential factor in all the determinations, maternal diet only affected brain DHAP stores (M = 1.85 +/- 0.36, 1.03 +/- 0.16, 0.74 +/- 0.10; C = 2.33 +/- 0.46, 1.87 +/- 0.21, 1.13 +/- 0.18 mumol/g at -2, -1, and DOB, respectively; F = 9.03 [1,53], P less than .01). These findings support the contention that intrauterine malnutrition can alter normal ontogenesis of glycerol metabolism enzymes in certain organs and become a negative factor disturbing normal gluconeogenesis and glycogenolysis, with potential disruption of energy homeostasis immediately after birth.