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At least 181 records · Page 10Linked to original sources
PubMed1985

Evidence for the cerebral uptake in vivo from two pools of glucose and the role of glucose-6-phosphatase in removing excess substrate from brain.

We propose the following scheme for cerebral uptake and overall metabolism of glucose in vivo: that brain selects from two pools of glucose anomers in arterial blood, that it takes up excess glucose, that glucose enters the brain tissue as glucose-6-phosphate through the actions of mutarotase and hexokinase, that some glucose-6-phosphate becomes metabolized to CO2 and some becomes incorporated into brain carbon pools, and that excess glucose-6-phosphate leaves brain through glucose-6-phosphatase and mutarotase activities. This results from our observations in arterio-venous studies for the determination of cerebral metabolism in humans in vivo that the cerebral uptake of [14C]glucose often appeared to differ from that of unlabeled glucose. With rapidly falling arterial radioactivity, unlabeled glucose uptake was more than [14C]glucose. With rising arterial radioactivity, [14C]glucose extraction exceeded unlabeled glucose. Studies with [14C]glucose-6-phosphate suggested that glucose-6-phosphatase in brain removes excess substrate by dephosphorylation. However, when arterial [14C]glucose increased slowly, [14C]glucose uptake varied considerably and the data resembled human cerebral metabolism of glucose anomers. An experiment employing [13C]glucose and NMR provided further support for our proposed scheme.

Animals↗
PubMed1987

Compartmentation of glucose 6-phosphate in hepatocytes.

Rat hepatocytes were incubated with 14C-labelled hexoses, and the specific radioactivities of glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate were determined. (1) When suspensions of freshly isolated hepatocytes were incubated with [14C]glucose, the specific radioactivities of glucose 1-phosphate and fructose 6-phosphate were severalfold higher than that of glucose 6-phosphate. The ratios of the specific radioactivities decreased with time of incubation. These relationships were also found when incubations were carried out with primary cultures of rat hepatocytes or with crude homogenates of hepatocytes, but not with isolated nuclei. (2) When cells were incubated with [14C]fructose, the ratios of the specific radioactivities were higher than with [14C]glucose, and also decreased with time. (3) Paired incubations were carried out with a mixture of galactose and fructose, with one or other sugar being labelled with 14C. The specific radioactivity of glucose released into the medium was greater than that of glucose 6-phosphate when fructose was labelled, but not when galactose was labelled. Furthermore, glucose 6-phosphate and glucose in the medium differed with regard to the distribution of 14C between C-1 and C-6. These results are interpreted as evidence that glucose 6-phosphate in hepatocytes does not exist as a homogeneous pool, but that subcompartments exist which are associated with glucose phosphorylation, gluconeogenesis and glycogenolysis.

Animals↗
PubMed1967

Lipid composition and metabolism in testicular and ejaculated ram spermatozoa.

1. Spermatozoa collected directly from the testis of the conscious ram contain 25% more phospholipid than ejaculated spermatozoa. The concentration of lecithin, phosphatidylethanolamine and ethanolamine plasmalogen was greater in testicular spermatozoa; little difference was observed in choline plasmalogen. Both types of spermatozoa had significant amounts of cardiolipin and alkyl ether phospholipid. 2. The fatty acids in the phospholipid extracted from testicular spermatozoa have a very high content of palmitic acid. The phospholipids of ejaculated spermatozoa contained less palmitic acid, but more myristic acid. 3. Ejaculated spermatozoa contained less acyl ester and cholesterol. It is suggested that lipids are a source of substrate for spermatozoa during their passage through the epididymis. 4. Testicular spermatozoa when incubated with [U-(14)C]glucose incorporated more radioactivity into the glycerol part of the phospholipid and neutral lipid fractions than did ejaculated cells. The distribution of radioactivity in the individual phospholipids and neutral lipids was similar for both cell types. No radioactivity was detected in choline plasmalogen, which accounted for approx. 40% of the total phospholipid. 5. Testicular spermatozoa incorporated more radioactivity from glucose into formate than into acetate, whereas a higher proportion of radioactivity was found in acetate in ejaculated cells. 6. The implications of these lipid changes in the process of spermatozoal maturation are discussed.

Animals↗
PubMed1994

A biphasic radiometric assay of glycogenin using the hydrophobic acceptor n-dodecyl-beta-D-maltoside.

Glycogenin is a self-glycosylating protein that catalyzes early glucosyl transfer steps in the biosynthesis of glycogen. In currently used assays of glycogenin activity, the enzyme is incubated with radioactive UDP-glucose, and the labeled reaction product is then isolated by precipitation with trichloroacetic acid. A new assay is reported here which is based on the observation that glycogenin is not only self-glycosylating but may also use exogenous alkyl maltosides as substrates. After incubation of the enzyme with n-dodecyl-beta-D-maltoside and UDP-[3H]glucose, the radioactivity in the resultant n-dodecyl-beta-D-[3H]maltotrioside is determined by any one of the following three procedures, which all rely on the hydrophobic properties conferred on the reaction product by the alkyl aglycone: (i) adsorption of the product to a Sep-Pak C18 cartridge and elution with 70% ethanol; (ii) biphasic liquid scintillation counting in ScintiLene/25% isoamyl alcohol, without isolation of the product, and (iii) precipitation with trichloroacetic acid in the presence of carrier protein. The Sep-Pak C18 procedure has the advantage that it allows essentially quantitative isolation of the reaction product, while, under the conditions chosen, only about 50% of the product is precipitated by trichloroacetic acid. For most applications, however, biphasic liquid scintillation counting is the method of choice, since close to 90% of the labeled product is extracted into the organic phase and can be counted directly without interference from the labeled nucleotide sugar which remains in the aqueous phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1975

Uptake of radioactive D-glucose anomers by pancreatic islets.

Isolated rat islets were incubated in media containing either the alpha or beta anomer of D-[1-3H]glucose for 5 min at 37 degrees. The amounts of the two anomers incorporated were determined using L-[1-14C]glucose as an extracellular space marker. The incorporation of beta-D-glucose was about twice that of alpha-D-glucose. Our previous and present results suggest that the two anomers of D-glucose each have a preferential function in pancreatic beta cells; alpha-D-glucose stimulates insulin secretion, and beta-D-glucose is transported into the cells.

Animals↗
PubMed1983

The measurement of glucose turnover and oxidation using radioactive and stable isotopes.

Isotopes have become the best means for investigating glucose kinetics in vivo. With the recent greater availability of stable isotopes there has developed a need to understand how data may be obtained from the use of both radioactive and stable glucose tracers. Described for the nonexpert is the calculation of glucose appearance and disappearance, clearance and oxidation using both stable and radioactive glucose isotopes, administered both by bolus and primed constant infusion and under both steady and nonsteady state conditions. Other substrates may be studied using similar methodology. The use of stable substrate isotopes will be an expanding field of metabolic research in man.

Glucose↗
PubMed1974

Determination of synthesis, recycling and body mass of glucose in rats and rabbits in vivo 3H-and 14C-labelled glucose.

1. Glucose labelled with (3)H in position 2 and uniformly with (14)C was administered simultaneously to rabbits and rats either as a single injection or by continuous infusion. Plasma glucose specific radioactivity and the yield of (3)H in the plasma water were monitored. 2. The rates of synthesis, recycling of carbon and total body mass of glucose were calculated, without assuming a multicompartmental model and without fitting data by exponential expressions. 3. The rate of synthesis of glucose in starved-overnight rabbits was 4mg/min per kg (range 3-4.5mg/min per kg) and 25-35% of the glucose carbon was recycled. The mass of total body glucose in starved rabbits was 290mg/kg (range 220-390mg/kg). About one-third of the total body glucose equilibrates nearly instantaneously with plasma glucose. 4. In rats starved overnight, glucose synthesis was about 10mg/min per kg and recycling of carbon ranged from 30-40%. Total body mass (per kg body weight) is similar to that in rabbits. 5. The activity in plasma water after injection of [2-(3)H]glucose was determined. The initial rate of (3)H(2)O formation is rapid, indicating that the major site of glucose catabolism is in the rapidly mixing pool. The curve of total body glucose radioactivity was obtained from the (3)H(2)O yield, and total mass of glucose was calculated. This agrees with that obtained from the (3)H specific-radioactivity curve.

Animals↗
PubMed1994

Glucose diffusion coefficients determined from concentration profiles in EMT6 tumor spheroids incubated in radioactively labeled L-glucose.

A method for performing and evaluating autoradiography of diffusible 14C labeled substances in multicellular tumor spheroids is presented that allows one to obtain a diffusion coefficient of the substance investigated from each individual spheroid. Application of the method with 14C labeled L-glucose resulted in a glucose diffusion coefficient of 5 x 10(-6) cm2/s. It also revealed problems of the method at very short incubation times of about 10 s or less. These problems are most likely caused by the large penetration depth of beta particles irradiated by 14C labels (as compared to 3H labels) which tends to transform steep 14C concentration gradients into much more shallow optical density gradients during exposure. This transformation can be corrected for by deconvolution of the recorded optical density distributions. Basic data and mathematical tools necessary for the process of deconvolution are presently being developed. It is planned to use this method for determining diffusion coefficients of other substances of interest. One such group of substances are the metabolic waste products, most importantly lactate. Another group consists of larger molecules, e.g. peptides and comprises the various growth factors important in tumor biology. Since for members of this latter group little is known about their velocity of penetration into tissue, model calculations may be applied to predict a range of incubation times suitable for determining diffusion coefficients. Moreover, the algorithms for data analysis will have to be modified to allow for receptor binding of the substance under study.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗