Biomedical subjects

M E Bizeau

Publications and source records attributed to M E Bizeau.

8 recordsLinked to original sources

PubMed2001

A high-sucrose diet increases gluconeogenic capacity in isolated periportal and perivenous rat hepatocytes.

A high-sucrose (SU) diet increases gluconeogenesis (GNG) in the liver. The present study was conducted to determine the contribution of periportal (PP) and perivenous (PV) cell populations to this SU-induced increase in GNG. Male Sprague-Dawley rats were fed an SU (68% sucrose) or starch (ST, 68% starch) diet for 1 wk, and hepatocytes were isolated from the PP or PV region of the liver acinus. Hepatocytes were incubated for 1 h in the presence of various gluconeogenic substrates, and glucose release into the medium was used to estimate GNG. When incubated in the presence of 5 mM lactate, which enters GNG at the level of pyruvate, glucose release (nmol x h(-1) x mg(-1)) was significantly increased by the SU diet in both PP (84.8 +/- 3.4 vs. 70.4 +/- 2.6) and PV (64.3 +/- 2.5 vs. 38.2 +/- 2.1) cells. Addition of palmitate (0.5 mM) increased glucose release from lactate in PP cells by 11.6 +/- 0.5 and 20.6 +/- 1.5% and in PV cells by 11.0 +/- 4.4 and 51.1 +/- 9.1% in SU and ST, respectively. When cells were incubated with 5 mM dihydroxyacetone (DHA), which enters GNG at the triosephosphate level, glucose release was significantly increased by the SU diet in both cell types. In contrast, glucose release from fructose (0.5 mM) was significantly increased by the SU diet in PV cells only. These changes in glucose release were accompanied by significant increases in the maximal specific activities of glucose-6-phosphatase (G-6-Pase) and phosphoenolpyruvate carboxykinase (PEPCK) in both PP and PV cells. These data suggest that the SU diet influences GNG in both PP and PV cell populations. It appears that SU feeding produces changes in GNG via alterations in at least two critical enzymes, G-6-Pase and PEPCK.

Animals↗

PubMed2001

Increased pyruvate flux capacities account for diet-induced increases in gluconeogenesis in vitro.

High-fat (HF) and high-sucrose (SU) diets increase gluconeogenesis. The present study was designed to determine the contributions of pyruvate dehydrogenase, pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate kinase fluxes to this accelerated gluconeogenesis (GNEO) in the absence and presence of fatty acids. Male Sprague-Dawley rats were fed an HF, SU, or starch (ST) diet for 1 wk, and hepatocytes or mitochondria were isolated. In the absence of palmitate, the tracer estimated rates of GNEO (nmol. min(-1). mg(-1)) were elevated in hepatocytes isolated from SU (32.3 +/- 1.8) and HF (35.4 +/- 1.8) vs. ST (22.8 +/- 1.5). Pyruvate carboxylase and PEPCK flux rates (nmol. min(-1). mg(-1)) were increased in the SU (47.5 +/- 2.2 and 34.8 +/- 1.5) and HF (49.4 +/- 1.8 and 38.2 +/- 1.8) groups compared with the ST group (32.8 +/- 3.2 and 44.3 +/- 2.0). Palmitate (250-1,000 microM) stimulation of these fluxes was not significantly different among groups. Bromopalmitate, an inhibitor of fat oxidation, abolished differences in GNEO, pyruvate carboxylase, and PEPCK fluxes in HF and SU vs. ST. In isolated mitochondria, pyruvate carboxylation and palmitoyl carnitine oxidation were not significantly different among groups. The results of this study suggest that the increased gluconeogenic flux observed with HF and SU diets is associated with an increased pyruvate flux through pyruvate carboxylase and PEPCK. Moreover, the ability of bromopalmitate to normalize gluconeogenic fluxes suggests that endogenous fatty acids contribute to diet-induced increases in GNEO.

Animals↗

PubMed2001

Hyperglycemia compensates for diet-induced insulin resistance in liver and skeletal muscle of rats.

High-fat and high-sucrose diets increase the contribution of gluconeogenesis to glucose appearance (glc R(a)) under basal conditions. They also reduce insulin suppression of glc R(a) and insulin-stimulated muscle glycogen synthesis under euglycemic, hyperinsulinemic conditions. The purpose of the present study was to determine whether these impairments influence liver and muscle glycogen synthesis under hyperglycemic, hyperinsulinemic conditions. Male rats were fed a high-sucrose, high-fat, or low-fat, starch control diet for either 1 (n = 5-7/group) or 5 wk (n = 5-6/group). Studies involved two 90-min periods. During the first, a basal period (BP), [6-3H]glucose was infused. In the second, a hyperglycemic period (HP), [6-3H]glucose, [6-14C]glucose, and unlabeled glucose were infused. Plasma glucose (BP: 111.2 +/- 1.5 mg/dl; HP: 172.3 +/- 1.5 mg/dl), insulin (BP: 2.5 +/- 0.2 ng/ml; HP: 4.9 +/- 0.3 ng/ml), and glucagon (BP: 81.8 +/- 1.6 ng/l; HP: 74.0 +/- 1.3 ng/l) concentrations were not significantly different among diet groups or with respect to time on diet. There were no significant differences among groups in the glucose infusion rate (mg x kg(-1) x min(-1)) necessary to maintain arterial glucose concentrations at approximately 170 mg/dl (pooled average: 6.4 +/- 0.8 at 1 wk; 6.4 +/- 0.7 at 5 wk), percent suppression of glc R(a) (44.4 +/- 7.8% at 1 wk; 63.2 +/- 4.3% at 5 wk), tracer-estimated net liver glycogen synthesis (7.8 +/- 1.3 microg x g liver(-1) x min(-1) at 1 wk; 10.5 +/- 2.2 microg x g liver(-1) x min(-1) at 5 wk), indirect pathway glycogen synthesis (3.7 +/- 0.9 microg x g liver(-1) x min(-1) at 1 wk; 3.4 +/- 0.9 microg x g liver(-1) x min(-1) at 5 wk), or tracer-estimated net muscle glycogenesis (1.0 +/- 0.3 microg x g muscle(-1) x min(-1) at 1 wk; 1.6 +/- 0.3 microg x g muscle(-1) x min(-1) at 5 wk). These data suggest that hyperglycemia compensates for diet-induced insulin resistance in both liver and skeletal muscle.

Analysis of Variance↗

PubMed2001

Sucrose diets increase glucose-6-phosphatase and glucose release and decrease glucokinase in hepatocytes.

A high-sucrose diet (SU) decreases insulin action in the liver (Pagliassotti MJ, Shahrokhi KA, and Moscarello M. Am J Physiol Regulatory Integrative Comp Physiol 266: R1637-R1644, 1994). The present study was conducted to characterize the effect of SU on glucagon action in isolated periportal (PP) and perivenous (PV) hepatocytes by measuring glucagon-stimulated glycogenolysis and glucose release. Male rats were fed a SU (68% sucrose) or starch diet (ST, 68% starch) for 1 wk, and hepatocytes were isolated from PP or PV regions (n = 4/diet/cell population). Hepatocytes were incubated for 1 h in the presence of varying concentrations of glucagon (0-100 nM). In PP and PV cells, glucagon stimulation of glucose release and glycogenolysis (sum of glucose release and lactate accumulation) was not significantly different between SU and ST cells. However, in the SU PP cells, glucose release was increased compared with ST PP cells, both in the absence of glucagon (76.1 +/- 4 vs. 54.8 +/- 3 nmol x h(-1) x mg cell wet x wt(-1)) and at all glucagon concentrations. In SU-fed PV cells, glucose release was increased compared with ST PV cells in the absence of glucagon (79.3 +/- 5 vs. 56.4 +/- 5 nmol x h(-1) x mg cell wet x wt(-1)) and at low glucagon concentrations. Maximal glucose-6-phosphatase activity (in nmol x min(-1) x mg protein(-1)) was elevated in SU compared with ST cells (61.4 +/- 3 vs. 37.5 +/- 4 in PP and 37.5 +/- 4 vs. 29.5 +/- 3 in PV cells). In contrast, maximal glucokinase activity (in nmol x min(-1) x mg protein(-1)) was elevated in ST compared with SU cells (15.9 +/- 2 vs. 12.1 +/- 1 in PP and 19.4 +/- 2 vs. 14.2 +/- 1 in PV cells). These data demonstrate that SU increases the capacity for glucose release in both PP and PV hepatocytes, in part because of reciprocal changes in glucose-6-phosphatase and glucokinase.

Animals↗

PubMed2000

Membrane physical properties do not explain increased cyclic AMP production in hepatocytes from rats fed menhaden oil.

To study the effect of altering plasma membrane fatty acid composition on the glucagon signal transduction pathway, cAMP accumulation was measured in hepatocytes from rats fed diets containing either menhaden oil (MO) or coconut oil (CO). Hepatocytes from MO-fed animals produced significantly more cAMP in response to glucagon and forskolin compared to CO-fed animals. Glucagon receptor number and affinity were similar in MO- and CO-fed rats. Liver plasma membranes from MO-fed animals were enriched in long-chain n-3 fatty acids and contained significantly lower amounts of saturated C10-C16 and 18:1n-9 than CO-fed animals. Membrane physical properties were examined using both Fourier transform infrared spectroscopy (FTIR) and the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). FTIR analysis revealed that below 34 degrees C, CO membranes were more ordered than MO membranes. However, as assay temperature approached 37 degrees C, MO and CO membranes became similarly ordered. DPH polarization values indicated no differences in membrane order at 37 degrees C, whereas membrane order was decreased in CO-fed animals at 25 degrees C. These data indicate the importance of assay temperature in assessing the influence of membrane physical properties on the activity of signal transduction pathways. Whereas increased signal transduction activity has been correlated to reduced membrane order in MO-fed animals, these data indicate that at physiological temperatures membrane order did not vary between groups. Enhanced cAMP accumulation in response to forskolin indicates that adenylate cyclase activity or content may be elevated in MO- vs. CO-fed rats. Enhanced adenylate cyclase activity may result, in part, from changes in specific fatty acids in hepatocyte plasma membranes without demonstrable changes in membrane physical properties.

Animals↗

PubMed2000

Is fluid-phase endocytosis conserved in hepatocytes of species acclimated and adapted to different temperatures?

Our primary objective was to determine if rates of fluid-phase endocytosis (FPE) were conserved in hepatocytes from organisms acclimated and adapted to different temperatures. To this aim, the fluorescent dye Lucifer yellow was employed to measure FPE at different assay temperatures (AT) in hepatocytes from 5 degrees C- and 20 degrees C-acclimated trout, Oncorhynchus mykiss (at 5 and 20 degrees C AT), 22 degrees C- and 35 degrees C-acclimated tilapia, Oreochromis nilotica (at 22 and 35 degrees C AT), and the Sprague-Dawley rat (at 10, 20, and 37 degrees C AT). FPE was also studied in rats fed a long-chain polyunsaturated fatty acid (PUFA)-enriched diet (at 10 degrees C AT). Despite being temperature dependent, endocytic rates (values in pl. cell(-1). h(-1)) in both species of fish were compensated after a period of acclimation. For example, in 20 degrees C-acclimated trout, the rate of endocytosis declined from 1.84 to 1.07 when the AT was reduced from 20 to 5 degrees C; however, after a period of acclimation at 5 degrees C, the rate (at 5 degrees C AT) was largely restored (1.80) and almost perfectly compensated (95%). In tilapia, endocytic rates were also temperature compensated, although only partially (36%). Relatively similar rates obtained at 5 degrees C in 5 degrees C-acclimated trout (1.8), at 20 degrees C in 20 degrees C-acclimated trout (1.84), and at 22 degrees C in 22 degrees C-acclimated tilapia (2.2) suggest that endocytic rates are somewhat conserved in these two species of fish. In contrast, the rate in rat measured at 37 degrees C (16.83) was severalfold greater than in fish at their respective body temperatures. A role for lipids in determining rates of endocytosis was supported by data obtained at 10 degrees C in hepatocytes isolated from rats fed a long-chain PUFA-enriched diet: endocytic rates were higher (5.35 pl. cell(-1). h(-1)) than those of rats fed a standard chow diet (2.33 pl. cell(-1). h(-1)). The conservation of endocytic rates in fish may be related to their ability to conserve other membrane characteristics (i.e., order or phase behavior) by restructuring their membrane lipid composition or by modulating the activities of proteins that regulate endocytosis and membrane traffic, whereas the lack of conservation between fish and rat may be due to differences in metabolic rate.

Acclimatization↗

PubMed2000

Hyperthermia impairs liver mitochondrial function in vitro.

The effects of temperature on the relationships among the rates of pyruvate carboxylation, O(2) uptake (J(o)), oxidative phosphorylation (J(p)), and the free energy of ATP hydrolysis (G(p)) were studied in liver mitochondria isolated from 250-g female rats. Pyruvate carboxylation was evaluated at 37, 40, and 43 degrees C. In disrupted mitochondria, pyruvate carboxylase maximal reaction velocity increased from 37 to 43 degrees C with an apparent Q(10) of 2.25. A reduction in ATP/ADP ratio decreased enzyme activity at all three temperatures. In contrast, in intact mitochondria, increasing temperature failed to increase pyruvate carboxylation (malate + citrate accumulation) but did result in increased J(o) and decreased extramitochondrial G(p). J(p) was studied in respiring mitochondria at 37 and 43 degrees C at various fractions of state 3 respiration, elicited with a glucose + hexokinase ADP-regenerating system. The relationship between J(o) and G(p) was similar at both temperatures. However, hyperthermia (43 degrees C) reduced the J(p)/J(o) ratio, resulting in lower G(p) for a given J(p). Fluorescent measurements of membrane phospholipid polarization revealed a transition in membrane order between 40 and 43 degrees C, a finding consistent with increased membrane proton conductance. It is concluded that hyperthermia augments nonspecific proton leaking across the inner mitochondrial membrane, and the resultant degraded energy state offsets temperature stimulation of pyruvate carboxylase. As a consequence, at high temperatures approaching 43 degrees C, the pyruvate carboxylation rate of intact liver mitochondria may fail to exhibit a Q(10) effect.

Adenosine Triphosphate↗

PubMed1998

Differential responses to endurance training in subsarcolemmal and intermyofibrillar mitochondria.

To examine the effect of endurance training (6 wk of treadmill running) on regional mitochondrial adaptations within skeletal muscle, subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria were isolated from trained and control rat hindlimb muscles. Mitochondrial oxygen consumption (VO2) was measured polarographically by using the following substrates: 1 mM pyruvate + 1 mM malate (P+M), 10 mM 2-oxoglutarate, 45 microM palmitoyl-DL-carnitine + 1 mM malate, and 10 mM glutamate. Spectrophotometric assays of cytochrome-c reductase and NAD-specific isocitrate dehydrogenase (IDH) activity were also performed. Maximal (state III) and resting (state IV) VO2 were lower in SS than in IMF mitochondria in both trained and control groups. In SS mitochondria, training elicited significant 36 and 20% increases in state III VO2 with P+M and glutamate, respectively. In IMF mitochondria, training resulted in a smaller (20%), yet significant, increase in state III VO2 with P+M as a substrate, whereas state III VO2 increased 33 and 27% with 2-oxoglutarate and palmitoyl-DL-carnitine + malate, respectively. Within groups, cytochrome-c reductase and IDH activities were lower in SS when compared with IMF mitochondria. Training increased succinate-cytochrome-c reductase in both SS (30%) and IMF mitochondria (28%). IDH activity increased 32% in the trained IMF but remained unchanged in SS mitochondria. We conclude that endurance training promotes substantial changes in protein stoichiometry and composition of both SS and IMF mitochondria.

Animals↗