Glucose infused through the portal vein enhances liver gluconeogenesis and glycogenesis from [3-14C]pyruvate in the starved rat

After a pulse of [3-14C]pyruvate, 24 hr starved rats were infused through the portal vein with two different doses of glucose (7.8 or 20.8 mg/min) or the medium, and blood was collected from the inferior cava vein at the level of the suprahepatic veins. The highest dose of glucose enhanced the appearance of [14C]glucose in blood from the 2nd to the 20th min after tracer delivery. It also enhanced production of [14C]glycogen and concentration of glycogen in the liver after 5 and 20 min. At 20 min of glucose infusion the appearance of [14C]glyceride glycerol in liver as well as liver lactate concentration and lactate/pyruvate ratio were increased. The low dose of glucose used enhanced liver values of [14C]glycogen, [14C]glycogen specific activity and glycogen concentration. Our results support the hypothesis that in the starved rat glucose is converted into C3 units prior to being deposited as liver glycogen and based on the liver zonation model (Jungermann et al., 1983) it is proposed that glucose stimulated gluconeogenesis by shifting the liver to the cytosolic redox state as a secondary consequence of increased glycolytic activity.     

Short-term insulin infused through the portal vein enhances liver gluconeogenesis and glycogenesis from [3-14C]pyruvate in the starved rat

Insulin infusion through the portal vein immediately after a pulse of [3-14C]pyruvate in 24 hr starved rats enhanced the appearance of [14C]glucose at 2, 5 and 10 min and glucose specific activity at 1, 2 and 20 min in blood collected from the cava vein at the level of the suprahepatic veins. Insulin infusion for 5 min decreased liver pyruvate concentration and enhanced both liver and plasma lactate/pyruvate ratio, and it decreased the plasma concentration of all amino acids. When insulin was infused together with glucose, [14C]glucose levels and glucose specific activity decreased in blood but there was a marked increase in liver [14C]glycogen, glycogen specific activity and glycogen concentration, and an increase in liver lactate/pyruvate ratio. The effect of insulin plus glucose infusion on plasma amino acids concentration was smaller than that found with insulin alone. It is proposed that insulin effect enhancing liver gluconeogenesis is secondary to its effect either enhancing liver glycolysis which modifies the liver's cytoplasmic oxidoreduction state to its more reduced form, increasing liver amino acids consumption or both. In the presence of glucose, products of gluconeogenesis enhanced by insulin are diverted into glycogen synthesis rather than circulating glucose. This together with results of the preceding paper (Soley et al., 1985), indicates that glucose enhances liver glycogen synthesis from C3 units in the starved rat, the process being further enhanced in the presence of insulin.     

Comparative utilization in vivo of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose and [1-C14] palmitate in the rat

Female rats were injected i.v. with comparable trace amounts of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose, or [1-14C] palmitate, and killed 30 min afterwards. The radioactivity remaining in plasma at that time was maximal in animals receiving [U-14C] glucose while the appearance of radioactive lipids was higher in the [U-14C] glycerol animals than in other groups receiving hydrosoluble substrates. The carcass, more than the liver, was the tissue where the greatest proportion of radioactivity was recovered, while the greatest percentage of radioactivity appeared in the liver in the form of lipids. The values of total radioactivity found in different tissues were very similar when using either labelled glucose or glycerol but the amount recovered as lipids was much greater in the latter. The maximal proportion of radioactive lipids appeared in the fatty-acid form in the liver, carcass, and lumbar fat pads when using [U-14C] glycerol as a hydrosoluble substrate, and the highest lipidic fraction appeared in adipose tissue as labelled, esterified fatty acids. In the spleen, heart, and kidney, most of the lipidic radioactivity from any of the hydrosoluble substrates appeared as glyceride glycerol. The highest proportion of radioactivity from [1-14C] palmitate appeared in the esterified fatty acid in adipose tissue, being followed in decreasing proportion by the heart, carcass, liver, kidney, and spleen. Thus at least in part, both labelled glucose and glycerol are used throughout different routes for their conversion in vivo to lipids. A certain proportion of glycerol is directly utilized by adipose tissue. The fatty acids esterification ability differs among the tissues and does not correspond directly with the reported activities of glycerokinase, suggesting that the alpha-glycerophosphate for esterification comes mainly from glucose and not from glycerol.     

Fate of [G-3H]glutamate and [U-14C]glucose in the rat liver in vivo

• 1.1. After injection of a mixture of [G-³H]glutamate and [U-¹⁴C]glucose to rats, the highest amount of ¹⁴C was found in an unidentified compound (glycopeptide?) of the acid soluble extract of the liver at 2 min.
• 2.2. With increasing time after the injection the specific radioactivity of [³H]glutamate decreased and that of [³H]glutamine increased in the liver.
• 3.3. The labelling of the liver protein with ¹⁴C was due to [¹⁴C]glutamate and [¹⁴C]aspartate, and that with ³H was exclusively due to [³H]glutamate.

     

Evaluation of radiation dose resulting from the ingestion of [3H]- and [14C]thymidine in the rat

Average doses to rat tissues from the ingestion of 2-[14C]thymidine were compared with those from methyl-[3H]thymidine or 6-[3H]thymidine. Among the three precursors, [14C]thymidine gave the highest dose to spleen and small intestine. The doses to other tissues from [14C]thymidine were almost the same or lower as compared with those from [3H]thymidine, irrespective of the 9 times higher beta-ray energy of 14C than that of 3H. In the case of [14C]thymidine, most of the dose was given by radioactivity incorporated into the organic tissue constituents (non-volatile radioactivity). In the case of [3H]thymidine, however, the dose contributions by non-volatile radioactivity were very small and the major contributions were rather from volatile radioactivity (3HHO), formed by degradation of [3H]thymidine. No significant difference in their total doses was found between the two [3H]precursors, but the dose from non-volatile radioactivity alone was 2-3 times higher with methyl-[3H]thymidine than with 6-[3H]thymidine. Estimates of the dose to cell nuclei in various tissues after the ingestion of [3H]thymidine were also made in order to predict more precisely possible radiation hazards.     

In vivo oxidation of [9-14C] cyclic fatty acids derived from linolenic acid in the rat

Heating oils and fats may lead to cyclization of polyunsaturated fatty acids, as for example linolenic acid. Cyclohexenyl and cyclopentenyl fatty acids are subsequently present in some edible oils and these are suspected to induce metabolic disorders. In a previous experiment using [1-14C] labeled molecules, we published that these cyclic fatty acids are beta oxidized to the same extent as linolenic acid, at least for the first cycle of beta oxidation. However, it is possible that the presence of a ring could alter the ability of the organism to fully oxidize the molecule. In order to test this hypothesis, we assessed the oxidative metabolism of cyclic fatty acids carrying a 14C atom at the vicinity of the ring. For this purpose, rats were force-fed from 1.1 to 1.3 MBq of a representative fraction of dietary cyclohexenyl cyclic fatty acid monomers of [9-14C] 9-(6-propyl-cyclohex-3-enyl)-non-8-enoic acids and 14CO2 production was monitored for 24h. The animals were then necropsied and the radioactivity was determined in different tissues. No consistent radioactivity was recovered as 14CO2 24h after administration of the molecules. Sixty percent of the radioactivity was recovered in the urine and 30% in the gastrointestinal tract. By combining our previous data on the oxidation of [1-14C] cyclic fatty acids and the present results, we suggest that cyclohexenyl fatty acids are first beta oxidized in a similar way as linolenic acid and that the remaining molecule carrying the ring is detoxified and eliminated in the urine and feces.     

Utlization of D-3-hydroxy[3-14C]butyrate for lipogenesis in vivo in lactating rat mammary gland.

Incorporation of D-3-hydroxy[3-14C]butyrate into lipid in vivo suggests that lactating mammary gland is a major site of ketone-body utilization. The incorporation decreases in short-term insulin deficiency (2h) and on starvation (24h), but increases again on refeeding (2h). The activity of cytosolic acetoacetyl-CoA synthetase parallels the changes in nutritional state, but is not affected by short-term insulin deficiency.     

Oxidation of [1-14C]palmitoyl-CoA, [1-14C]acetyl-CoA and [2-14C]pyruvate in rabbit adrenal, liver and heart mitochondria under normal conditions and in acute stress

The acute immobilized stress was studied for its effect on oxidation rate of [1-14C]palmitoyl-CoA, [1-14C]acetyl-CoA and [2-14C]pyruvate in mitochondria of the adrenals, liver and heart of rabbits. The stress effect on the energy metabolism of adrenals is associated with an increase of the rate of CO2 formation from pyruvate and with a decrease of the rate of CO2 formation from palmitoyl-CoA. Intensified oxidation of all substrates is observed in the heart mitochondria. The processes of beta-oxidation are more active in the liver. The data obtained evidence for differences in the mechanisms of energy metabolism reconstruction under acute stress in tissues with different functional specialization.     

The metabolism of [2-14C]indole in the rat

1. [2-¹⁴C]Indole has been synthesized from [¹⁴C]formate and o-toluidine via N[¹⁴C]-formyltoluidine. 2. When fed to rats, the ¹⁴C of [¹⁴C]indole (dose 70–80mg./kg. body wt.) is fairly rapidly excreted, and in 2 days an average of 81% appears in the urine, 11% in the faeces and 2·4% as carbon dioxide in the expired air. 3. Radioactivity is excreted in the urine as indoxyl sulphate (50% of the dose), indoxyl glucuronide (11%), oxindole (1·4%), isatin (5·8%), 5-hydroxyoxindole conjugates (3·1%), N-formylanthranilic acid (0·5%) and unchanged indole (0·07%). The faeces contain indoxyl sulphate (0·4% of the dose) and indole (0·2%), but the major metabolites have not been identified. 4. Fed to rats with biliary cannulae an average of 5·6% of a dose of [¹⁴C]indole (20–60mg./kg. body wt.) is excreted in the bile in 2 days. Radioactivity is present as indoxyl sulphate (0·8% dose) and 5-hydroxyoxindole conjugates (0·6%). 5. Rats further metabolize indoxyl into N-formylanthranilic acid and anthranilic acid, and oxindole into 5-hydroxyoxindole. 6. With rat-liver microsomes plus supernatant under aerobic conditions, indole gives indoxyl, oxindole, possibly isatin, N-formylanthranilic acid and anthranilic acid, but under anaerobic conditions gives only oxindole. Similarly, under aerobic conditions, oxindole gives 5-hydroxyoxindole, anthranilic acid and o-aminophenylacetic acid. 7. Indole is metabolized by two pathways, one via indoxyl to isatin, N-formylanthranilic acid and anthranilic acid, and the other via oxindole to 5-hydroxyoxindole and possibly to o-aminophenylacetic and anthranilic acid. 8. The following new compounds are described: 4-hydroxy-2-nitrophenylacetic acid, 3-, 4- and 5-benzyloxy-2-nitrophenylacetic acid, 5- and 7-hydroxyoxindole and 5-aminoacridine indoxyl sulphate.