Effect of uridine on cellular UTP and glycogen synthesis in skeletal muscle: stimulation of UTP formation by insulin.

Abstract

The relation between cellular uracil nucleotides and ability to synthesize glycogen was studied in rat diaphragm incubated in vitro. In the absence of exogenous uridine the tissue content of UTP and rate of glycogen synthesis decreased with time. Uridine added to the medium increased cellular UTP and UDPG and stimulated glycogen synthesis. Insulin significantly increased the synthesis of UTP from extracellular uridine. This action of insulin appeared to be due to a stimulation of phosphorylation of the nucleoside and not to an effect on transport at the concentrations of uridine studied. However, an effect of insulin on transport of uridine at low concentrations cannot be excluded.Studies of the regulation of glycogen synthesis in muscle have been concerned primarily with the properties of glycogen synthase (EC 2.4.1.11; UDP glucose:glycogen 4-a-glucosyltransferase), the rate-limiting enzyme that catalyzes the reaction: UDPG + (glucose),, = UDP + (glucose),,+,. This enzyme exists in two or more forms (1, 2), the best characterized being designated as the independent (I or a) and dependent (D or b) forms differing in their Ka values for activation by glucose-6-P and in their apparent Km values for UDPG. Glycogen synthase b is probably inactive physiologically since it is strongly inhibited by tissue metabolites and its Km for UDPG is in the order of 0.2 mM, whereas glycogen synthase a is less sensitive to inhibition and has a Km of about 0.06 mM (2). Since the actual concentration of UDPG in muscle is approximately 0.03 mM (3), it is clear that any changes in the cellular concentration of this substrate would markedly alter the rate of glycogen synthesis.The formation of UDPG occurs by the reaction: UTP + glucose-1-P = UDPG + P-P and is catalyzed by UDPG pyrophosphorylase (EC 2.7.7.9; UTP:a-D-glucose-l-phosphate uridylyltransferase), an enzyme that has been found not to be rate limiting in the synthesis of glycogen in muscle (4). However, it seemed possible to us that the availability of UTP for the synthesis of UDPG could be a limiting factor in glycogen formation and hence the rate of synthesis of this polysaccharide could be influenced by the intracellular concentration of UTP. If this is true, additional supply of UDPG from exogenous uridine by the pyrimidine nucleotide salvage pathway should lead to an increase in the rate of formation of glycogen. The experiments reported here were designed to study this problem. MATERIALS AND METHODSMaterials. [U-14C]Glucose and [U-14C]uridine were obtained from New England Nuclear, and uridine was from Calbiochem. The insulin preparation used was crystalline porcine insulin (Eli Lilly and Co.; lot 615-DG3-10).Tissue Incubation. Male Wistar rats (125-150 g) fed freely were killed by decapitation and the diaphragms were removed. and collected in chilled 0.15 M NaCl. Wet weights were determined on a torsion balance after gentle blotting on filter paper. The results of metabolic measurements are expressed in units/g of wet weight. The tissues were incubated at 370 for...

Uridine correlates with the concentration of fructosamine and HbA1c in children with type 1 diabetes

“…Haugaard et al. (23,24) showed that insulin stimulates the phosphorylation of uridine in skeletal muscle. The results of those experiments indicate that pyrimidine nucleotide metabolism is regulated by insulin and that insulin activates uridine kinase EC 2.7.1.48, the enzyme transforming uridine to UTP, which is the rate‐limiting component in glycogen biosynthesis and in several metabolic systems leading to the synthesis of essential cell components.…”

Seasonal variation in the effect of dietary RNA on criteria of energy homoeostasis in the rat

“…The recorded effects on blood constituents of administering this base include a brief hyperglycaemia in the rabbit (Hashimoto & Chuman, 1963). A report by Haugaard et al (1977), which appeared during the preparation of this paper, provides the most plausible clue to the hypoglycaemic effect of RNA. They showed that in rat diaphragm incubated in a medium containing uridine, intracellular levels of uridine-5triphosphate and uridine diphosphate glucose were increased, and that this led to an enhancement of glycogen synthesis.…”

Biochemical characterization of the rabbit urinary bladder: II. Intracellular concentration of nucleotides

“…These include 1) utilization of ATP and PC as sources of metabolic energy; 2) utilization of NADH as an electron carrier for mitochondria1 respiration; 3) GTP as an energy source and cofactor for a variety of specific metabolic reactions; 4) UTP as a source of uridine diphosphate-glucose (UDPG) for glycogen synthesis; and 5) CTP as an essential cofactor in the metabolism of phosphatides. Both UTP and CTP exert important roles in the synthesis of membrane phospholipids and glycoproteins [Siegman et al, 1980;Stephens and Wrogemann, 1983;McMurray and Magee, 1972;Larner and Villar-Palasi, 1971;Haugaard et al, 1977;Levine et al, 19741. The rabbit is widely used as an animal model for a variety of urinary bladder dysfunctions [Levin et a]., 1980;Dean and Downie, 1978;Malkowicz et al, 1985;Malkowicz et al, 1986;Gill et al, 1988;Khanna et al, 1983;Gosling et al, 1977;Mayo and Hinman, 1975;Brendt and Stephens, 1975;Ghoniem et al, 19831. In order to evaluate the relationship between cellular biochemistry and bladder contraction in these pathological models, it is first necessary to describe the biochemistry of the normal rabbit bladder. In previous studies we have demonstrated that physiologically and pharmacologically the bladder can be separated between the base and the body.…”