Evidence for gluconeogenesis in the amphibian retina

Gluconeogenesis in frogs during cooling and dehydration exposure: new insights into tissue plasticity of the gluconeogenic pathway dependent on abiotic factors

Anurans undergo significant physiological changes when exposed to environmental stressors such as low temperatures and humidity. Energy metabolism and substrate management play a crucial role in their survival success. Therefore, understanding the role of the gluconeogenic pathway and demonstrating its existence in amphibians is essential. In this study, we exposed the subtropical frog Boana pulchella to cooling (-2.5°C for 24 h) and dehydration conditions (40% of body water loss), followed by recovery (24 h), and assessed gluconeogenesis activity from alanine, lactate, glycerol and glutamine in the liver, muscle and kidney. We report for the first time that gluconeogenesis activity by 14C-alanine and 14C-lactate conversion to glucose occurs in the muscle tissue of frogs, and this tissue activity is influenced by environmental conditions. Against the control group, liver gluconeogenesis from 14C-lactate and 14C-glycerol was lower during cooling and recovery (P<0.01), and gluconeogenesis from 14C-glutamine in the kidneys was also lower during cooling (P<0.05). In dehydration exposure, gluconeogenesis from 14C-lactate in the liver was lower during recovery, and that from 14C-alanine in the muscle was lower during dehydration (P<0.05). Moreover, we observed that gluconeogenesis activity and substrate preference respond differently to cold and dehydration. These findings highlight tissue-specific plasticity dependent on the nature of the encountered stressor, offering valuable insights for future studies exploring this plasticity, elucidating the importance of the gluconeogenic pathway and characterizing it in anuran physiology.

Ubiquitous presence of gluconeogenic regulatory enzyme, fructose-1,6-bisphosphatase, within layers of rat retina

To shed some light on gluconeogenesis in mammalian retina, we have focused on fructose-1,6-bisphosphatase (FBPase), a regulatory enzyme of the process. The abundance of the enzyme within the layers of the rat retina suggests that, in mammals in contrast to amphibia, gluconeogenesis is not restricted to one specific cell of the retina. We propose that FBPase, in addition to its gluconeogenic role, participates in the protection of the retina against reactive oxygen species. Additionally, the nuclear localization of FBPase and of its binding partner, aldolase, in the retinal cells expressing the proliferation marker Ki-67 indicates that these two gluconeogenic enzymes are involved in non-enzymatic nuclear processes.

Glycogen metabolism in the rat retina

It has been reported that glycogen levels in retina vary with retinal vascularization. However, the electrical activity of isolated retina depends on glucose supply, suggesting that it does not contain energetic reserves. We determined glycogen levels and pyruvate and lactate production under various conditions in isolated retina. Ex vivo retinas from light- and dark-adapted rats showed values of 44 +/- 0.3 and 19.5 +/- 0.4 nmol glucosyl residues/mg protein, respectively. The glycogen content of retinas from light-adapted animals was reduced by 50% when they were transferred to darkness. Glycogen levels were low in retinas incubated in glucose-free media and increased in the presence of glucose. The highest glycogen values were found in media containing 20 mm of glucose. A rapid increase in lactate production was observed in the presence of glucose. Surprisingly, glycogen levels were the lowest and lactate production was also very low in the presence of 30 mm glucose. Our results suggest that glycogen can be used as an immediate accessible energy reserve in retina. We speculate on the possibility that gluconeogenesis may play a protective role by removal of lactic acid.

Changes in the redox state in the retina and brain during the onset of diabetes in rats

Diabetic retinopathy is thought to result from chronic changes in the metabolic pathways of the retina. Hyperglycemia leads to increased intracellular glucose concentrations, alterations in glucose degradation and an increase in lactate/pyruvate ratio. We measured lactate content in retina and other ocular and non-ocular tissues from normal and diabetic rats in the early stages of streptozotocin-induced diabetes. The intracellular redox state was calculated from the cytoplasmic [lactate]/[pyruvate] ratio. Elevated lactate concentration were found in retina and cerebral cortex from diabetic rats. These concentrations led to a significant and progressive decrease in the NAD+/NADH ratio, suggesting that altered glucose metabolism is an initial step of retinopathy. It is thus possible that tissues such as cerebral cortex have mechanisms that prevent the damaging effect of lactate produced by hyperglycemia and/or alterations of the intracellular redox state.

Rana esculenta L. liver Fru-1,6-P2ase and G-6-Pase activity and Fru-2,6-P2 concentration after acclimation at 5 and 25 degrees C

The activities of Fru-1,6-P2ase and G-6-Pase in liver and kidney of frogs acclimated at 5 and 25 degrees C and Fru-2,6-P2 level in liver were investigated. The aim of this study was to examine the effect of thermal acclimation on regulatory enzymes of gluconeogenesis and on concentration of gluconeogenesis regulator. Fru-1,6-P2ase activity in liver of frogs acclimated at 5 degrees C was 6.16 +/- 0.77 and 4.46 +/- 0.46 U/g wt in those acclimated at 25 degrees C; the respective values for G-6-Pase were 0.46 +/- 0.04 and 0.25 +/- 0.02 U/g wt. Fru-1,6-P2ase activity in kidney was 3.2 +/- 0.48 U/g wt at 5 degrees C and 2.64 +/- 0.23 U/g wt at 25 degrees C; the respective values for G-6-Pase were 0.2 +/- 0.05 and 0.17 +/- 0.05 U/g wt. K(m) of frog liver Fru-1,6-P2ase determined after acclimation at 5 degrees C and to 25 degrees C was 1.36 and 1.41 microM, respectively. Frog liver Fru-1,6-P2ase was allosterically inhibited by AMP. I0.5 determined after acclimation at 5 degrees C was 10.55 microM and after acclimation at 25 degrees C was 10.88 microM. Liver Fru-2,6-P2 concentration after acclimation at 5 degrees C was 0.44 +/- 0.13 nmol/g wt in comparison with 0.58 +/- 0.19 nmol/g wt after acclimation at 25 degrees C. In conclusion, cold exposure increased hepatic gluconeogenic capacity of Rana esculenta.

Incorporation of radioactivity from [14C]lactate into the glycogen of cultured mouse astroglial cells. Evidence for gluconeogenesis in brain cells

A pure population of astroglial cells was selected from heterogeneous astroglia-rich primary cultures in a medium containing sorbitol instead of glucose. It was shown that astroglial cells synthesize glycogen when they are returned to a glucose-containing medium, and that when [14C]lactate is also present the synthesized glycogen is radioactively labelled. Compared with the degree of incorporation of radioactivity in the presence of tritiated glucose, the incorporation of radioactivity from lactate was small but significant. After incubation of astroglial cells with radioactively labelled lactate, the glycogen was isolated and enzymatically hydrolysed to glucose, which was found to be radioactively labelled. Astrocytes are therefore able to convert lactate to glucosyl residues, a metabolic pathway known as gluconeogenesis. It is proposed that astrocytic gluconeogenesis may consume lactic acid formed in neighboring cells such as neurons, during anaerobic glycolysis at times of high energy demand.

Glycogenesis in the amphibian retina: in vitro conversion of [2-3H]mannose to [3H]glucose and subsequent incorporation into glycogen

We previously demonstrated by light and electron microscopic autoradiography that Xenopus retinas incubated with [3H]mannose exhibit tunicamycin-insensitive radiolabeling of glycogen storage compartments, especially cone parabaloids. In the present study, we utilized biochemical methods to evaluate the identity of the material presumed to be [3H]glycogen in Xenopus retinas obtained from eyecups incubated under similar conditions. A crude glycogen-containing fraction was isolated, solubilized with 8 M urea, and purified by Sepharose CL-4B column chromatography. The retinal glycogen was hydrolyzed either chemically or with specific amylolytic enzymes, followed by Sephacryl S-200 column chromatography and HPLC of the hydrolysis products. Under the conditions employed, [3H]glycogen represented at least 10% of the total radiolabeled macromolecules. Hydrolysis of the [3H]glycogen released all of the radiolabel in the form of [3H]glucose, not [3H]mannose, which indicated that direct incorporation of [3H]mannose into glycogen had not occurred. [3H]Glucose was distributed throughout the glycogen molecule, not just in the outer tiers, which indicated that de novo glycogenesis had occurred. Furthermore, enzymatic isomerization of the glycogen-derived [3H]glucose with glucose isomerase yielded fructose with retention of tritium. This demonstrated that positions other than the C-2 carbon of glucose were radiolabeled. Analysis of the medium after several hours of incubation revealed the presence of 3H2O as the major radiolabeled compound. These results support the conclusion that the in vitro incorporation of [2-3H]mannose into retinal glycogen involves initial catabolism of the radiolabeled substrate and subsequent reincorporation of the label via gluconeogenesis into precursors utilized for de novo glycogenesis.

Gluconeogenesis in the amphibian retina. Lactate is preferred to glutamate as the gluconeogenic precursor

The capacity for gluconeogenesis in the isolated amphibian retina was found to be approx. 70-fold greater with lactate than with glutamate as the gluconeogenic precursor, 1426 versus 21 pmol of glucose incorporated into glycogen/h per mg of protein. It was also found that 11-15% of the glucosyl units in glycogen are derived from C3 metabolites of the glycolytic pathway, suggesting that lactate is recycled within the retina. In concert with these metabolic observations, a full complement of the gluconeogenic enzymes was detected in retinal homogenates. These included: glucose-6-phosphatase, fructose-1,6-bisphosphatase, acetyl-CoA-dependent pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Agents that regulate the rate of gluconeogenesis in hepatic tissue were tested on the retina. At concentrations of glutamate and lactate that are presumed to be relevant physiologically, it was found that vasoactive intestinal peptide, ionophore A23187 and elevated [K+] each enhanced the rate of gluconeogenesis in Ringer containing 50 microM-glutamate, whereas in Ringer containing 8.5 mM-lactate these agents inhibited the rate of gluconeogenesis. Further, it was found that the classic gluconeogenic hormone glucagon inhibited gluconeogenesis in both glutamate- and lactate-containing Ringer. Retinal energy metabolism was found to be altered in lactate-containing Ringer, in that lactate production was suppressed completely. In addition, glycogen metabolism appeared to be dependent on increased cytosolic Ca2+ and was insensitive to increased retinal cyclic AMP.

Stratified distribution of synapses in the inner plexiform layer of primate retina

Distributions of bipolar (B) and amacrine (A) synapses and postsynaptic ganglion cell (G) dendritic profiles in the inner plexiform layer (IPL) were analyzed in EM montages of monkey central and human foveal and peripheral retinae. Synapses and profiles were counted and plotted for each 5% interval of IPL, with 0% at the inner edge of the inner nuclear layer and 100% at the outer edge of the ganglion cell layer. In monkey and human retinae, both A and B synapses occur throughout the IPL, but the ratio of A to B synapses varies from 2:1 to more than 6:1. In the monkey central retina, four bands of A conventional synapses are concentrated at 15, 35, 60, and 80% depth. In the human foveal slope, there are two main A bands at 45 and 85%, whereas in the human periphery, there are five bands at 15, 35, 60, 75, and 90%. In both species, A processes containing large dense-core vesicles are concentrated in three bands at 10-20, 50, and 80-90% depth, corresponding to previously described levels of peptides, dopamine, and GABA. B ribbon synapses are distributed fairly evenly throughout the IPL, with a suggestion of four broadly overlapping bands. Most B ribbons are presynaptic to one A and one G (B----A/G). In the human, there are significantly more B dyads with postsynaptic G's (B----A/G, B----G/G) in the fovea (91%) than in the periphery (66%), implying greater A cell processing peripherally. Also in the human, B terminals containing glycogenlike granules are concentrated in the outer half of the IPL, with agranular terminals in the inner half. Our results demonstrate multiple strata containing different types of synaptic contacts in primate IPL.