Glucose Metabolism In Rat Adipocytes Research Articles

L-Glutamic Acid γ-Monohydroxamate: A POTENTIATOR OF VANADIUM-EVOKED GLUCOSE METABOLISM IN VITRO AND IN VIVO

We report that the vanadium ligand L-Glu(gamma)HXM potentiates the capacity of free vanadium ions to activate glucose uptake and glucose metabolism in rat adipocytes in vitro (by 4-5-fold) and to lower blood glucose levels in hyperglycemic rats in vivo (by 5-7-fold). A molar ratio of two L-Glu(gamma)HXM molecules to one vanadium ion was most effective. Unlike other vanadium ligands that potentiate the insulinomimetic actions of vanadium, L-Glu(gamma)HXM partially activated lipogenesis in rat adipocytes in the absence of exogenous vanadium. This effect was not manifested by D-Glu(gamma)HXM. At 10-20 microM L-Glu(gamma)HXM, lipogenesis was activated 9-21%. This effect was approximately 9-fold higher (140 +/- 15% of maximal insulin response) in adipocytes derived from rats that had been treated with vanadium for several days. Titration of vanadium(IV) with L-Glu(gamma)HXM led to a rapid decrease in the absorbance of vanadium(IV) at 765 nm, and (51)V NMR spectroscopy revealed that the chemical shift of vanadium(IV) at -490 ppm disappeared with the appearance of a signal characteristic to vanadium(V) (-530 ppm) upon adding one equivalent of L-Glu(gamma)HXM. In summary, L-Glu(gamma)HXM is highly active in potentiating vanadium-activated glucose metabolism in vitro and in vivo and facilitating glucose metabolism in rat adipocytes in the absence of exogenous vanadium probably through conversion of trace intracellular vanadium into an active insulinomimetic compound. We propose that the active species is either a 1:1 or 2:1 L-Glu(gamma)HXM vanadium complex in which the endogenous vanadium(IV) has been altered to vanadium(V). Finally we demonstrate that L-Glu(gamma)HXM- and L-Glu(gamma)HXM.vanadium-evoked lipogenesis is arrested by wortmannin and that activation of glucose uptake in rat adipocytes is because of enhanced translocation of GLUT4 from low density microsomes to the plasma membrane.

Acute effects of progesterone on glucose metabolism in rat adipocytes: Are they modulated by endogenous adenosine?

Progesterone rapidly inhibits glucose oxidation of isolated rat adipocytes. Because this inhibition is triggered by endogenous adenosine, the present study was designed to examine the effect of the steroid on cyclic adenosine monophosphate (cAMP) accumulation, its relation to lipolysis, and the possible participation of adenosine. The results strongly indicate that physiological concentrations of progesterone increase the release of adenosine by isolated adipocytes, with maximal release at the end of a 20-minute incubation. Progesterone decreased both cAMP levels and lipolysis in quiescent adipocytes or in adipocytes stimulated by isoproterenol. The increase of endogenous adenosine may explain the decline of cAMP and glycerol levels observed with progesterone. The effects of the steroid on lipolysis disappeared when adenosine was hydrolyzed by adenosine deaminase (ADA). On the other hand, in the absence of endogenous adenosine, the effect of progesterone on the cAMP level was decreased only in isoproterenol-stimulated cells. The inhibitory effects of progesterone on cAMP and glycerol production seem not to be related directly to the adenosine A 1 receptor, for selective A 1 receptor antagonists (8-cyclopentyl-1,3-dipropylxanthine [DPCPX] and CP 68,247) did not counteract these effects. However, mechanisms mediated by guanyl nucleotide binding proteins cannot be excluded. The decrease of cAMP and of lipolysis may be related to a stimulation of phosphodiesterases (PDEs). When PDEs I (Ca 2+ — calmodulin-regulated PDE family) were blocked by a selective inhibitor (CP 41,757), the progesterone inhibitory effect persisted, suggesting that PDEs I are not regulated by the steroid. On the other hand, the progesterone effect on cAMP accumulation but not on lipolysis disappeared in the presence of a selective inhibitor of the PDE IV family (cAMP-dependent-specific family), Ro 20-1724. When the specific inhibitor of PDE I or PDE IV was combined with ADA, the progesterone effect on cAMP disappeared. Taken together, these results suggest that the progesterone inhibitory action on cAMP levels was not mediated through A 1 receptors or through activation of PDE I, but may be related to PDE IV activities. The progesterone effect on lipolysis seemed not to be directly related to changes in cAMP levels; an effect on PDE III activities in relation with the increase of adenosine release cannot be excluded.

Magnesium deficiency and glucose metabolism in rat adipocytes

We examined the effect of reducing ambient and intracellular free Mg ion ([Mg]i) concentrations on insulin action in epididymal adipocytes from male Sprague-Dawley rats in terms of (1) cellular transport of nonmetabolizable 2-deoxyglucose, (2) [U- 14C]glucose oxidation to CO 2, and (3) d-[ 3H]glucose incorporation into triglycerides. There were no significant differences in basal or insulin-stimulated transport of 2-deoxyglucose between adipocytes cultured in physiologic (1.24 mmol) or low (0.16 mmol) Mg for up to 24 hours. In contrast, insulin-stimulated but not basal [U- 14C]glucose oxidation to CO 2 was significantly reduced in adipocytes cultured in low versus physiologic Mg ( P < .05 to .01). Similarly, there were no differences in basal glucose incorporation into triglycerides between cells cultured in low or physiologic Mg media for up to 24 hours. However, long-term (24-hour) but not short-term (2-hour) exposure of cells to low Mg was associated with a significant 30% reduction in insulin-stimulated d-[ 3H]glucose incorporation into triglycerides. When adipocytes incubated in low Mg were reincubated in high Mg (1.24 or 5 mmol) for 30 minutes, normal insulin-stimulated d-[ 3H]glucose incorporation into triglycerides was restored. Incubation of adipocytes in low Mg (0.16 mmol) for 24 hours resulted in a significant decrease in [Mg]i (264 ± 89 v 437 ± 125 μmol/cell [mean ± SEM]) as compared with cells incubated in physiologic Mg (1.24 mmol; P < .01). These data support a role for intracellular Mg deficiency in the development of insulin resistance and suggest that the effect occurs at a site(s) distal to glucose entry into the cell. The effect of Mg deficiency on insulin action appears to be reversible.

Involvement of Adenosine in the Acute Action of Progesterone on Glucose Metabolism In Rat Adipocytes

Abstract Progesterone and synthetic steroids (RU-5020, progestomimetic; RU-38486, anti-progesterone and anti-glucocorticoid) inhibit glucose netabolism in female rat adipocytes. The inhibitory effect, which is dose-related, occured after 20-min of incubation and is sustained during 2-hours of incubation. This effect of progesterone is similar to that observed during pregnancy and after treatment of ovariectomized rats with progesterone (5 mg/rat/day-15 days).

18F-labeled insulin: a prosthetic group methodology for incorporation of a positron emitter into peptides and proteins.

In the present study we synthesize 18F-labeled insulin of high specific radioactivity. A new prosthetic group methodology, in which [18F]fluoride displaces a bromide group of 4-(bromomethyl)-benzoylamine intermediates, was used. The 4-(fluoromethyl)benzoyl product was chemically stable. 18F-Labeled insulin retains the essential biological properties of native insulin, as measured in vitro by binding to insulin receptors on human cells and stimulation of glucose metabolism in rat adipocytes. The overall process can be carried out speedily to yield a product of sufficient purity to permit in vivo studies. The method appears to be applicable to a wide variety of peptides.

Cadmium stimulates glucose metabolism in rat adipocytes

Cd 2+ caused an increase in CO 2 formation from glucose in rat adipocytes. The apparent K m value for glucose was 2.02 mM for control condition, with Cd 2+, and with insulin. Cd 2+ stimulates glucose metabolism even though specific diffusion of glucose is blocked. A possible site effected by Cd 2+ is discussed.

Insulin stimulation of glucose metabolism in rat adipocytes: possible implication of protein kinase C.

The hypothesis that insulin stimulation of glucose and lipid metabolism in rat adipocytes may involve the activation of protein kinase C was evaluated. 4 beta-Phorbol 12 beta-myristate, 13 alpha-acetate (PMA, 0.1-1000 ng/ml), a potent tumor promoter acting as a substitute for diacylglycerol to activate protein kinase C, stimulated 2-deoxyglucose transport in a time- and dose-dependent manner, without affecting passive glucose diffusion. PMA (0.1-1000 ng/ml) also elicited a dose-dependent activation of lipogenesis from [3-3H] glucose. Maximal PMA effects (100 ng/ml) on both processes were 60% of insulin maximal effects. In contrast, PMA (1-1000 ng/ml) failed to mimic the ability of insulin to stimulate lipogenesis from [3H]acetate. 4 beta-Phorbol 12,13 dibutyrate, mezerein, 1-oleyl-2-acetyl-glycerol, 1,2 diolein, known as protein kinase C activators, also markedly stimulated glucose metabolism whereas 4 alpha-phorbol 12,13 didecanoate and 4 beta-phorbol 13-monoacetate, shown not to activate protein kinase C, were ineffective. Mellitin, a cytotoxin-inhibiting protein kinase C, markedly decreased both PMA and insulin stimulation of glucose metabolism. PMA decreased insulin submaximal effects on 2-deoxyglucose transport without inhibiting insulin binding. Maximal PMA and insulin effects on 2-deoxyglucose transport and lipogenesis from [3-3H]glucose were not additive. Both PMA and insulin activated each metabolic process in a calcium-dependent manner. PMA, like insulin, no longer stimulated 2-deoxyglucose transport in fat cells treated with 2,4-dinitrophenol. These data show that PMA exhibited specific insulin-like properties on glucose metabolism in fat cells, without any effect on lipid synthesis from acetate. They indicate that PMA and insulin bioeffects may share a common step implicating a calcium- and energy-dependent process, distal to the initial insulin binding event. Our results suggest that protein kinase C may play a role in insulin regulation of glucose metabolism.

Effect of lactation on insulin sensitivity of glucose metabolism in rat adipocytes

During lactation glucose metabolism in paraovarian adipocytes is characterized by a 40 and 80% decrease of glucose incorporation into CO 2 and fatty acids in the presence of insulin. In contrast with the stimulation by insulin of glucose incorporation into lactate, glycerol remains unchanged. As a result, insulin sensitivity of total glucose metabolism (oxidation and lipid synthesis) is not altered in adipocytes from lactating rats.

Changes in growth hormone binding and metabolic effects of growth hormone in rat adipocytes following hypophysectomy.

Growth hormone (GH) binding and the effect of GH and insulin on glucose metabolism in rat adipocytes were studied at various time periods following hypophysectomy. Male rats were hypophysectomized at 33-34 days of age. After 6 h, 20 h or 3, 7 and 14 days adipocytes were prepared from epididymal fat pads by mild collagenase digestion (0.5 mg X ml-1, 60 min, 37 degrees C). Glucose metabolism was studied by determining the production of CO2 from [14C]glucose and the incorporation of [14C]glucose into lipids. GH binding was measured in cell aliquots using [125I]hGH. No difference in GH binding to adipocytes was observed between control rats and rats hypophysectomized or sham-operated 6 h earlier. GH binding was significantly decreased 20 h after hypophysectomy and declined further with time after hypophysectomy. Adipose tissue from normal rats is usually refractory to the insulin-like effect of GH. Adipocytes isolated from normal rats were, however, usually responsive to GH immediately after cell isolation, suggesting that refractoriness to the insulin-like effect of GH was lost during the time required for the preparation of adipocytes. The magnitude of the response to GH in adipocytes progressively declined with time after hypophysectomy. The decreased responsiveness to GH with time after hypophysectomy parallelled the decrease in GH binding. The results suggest that the pituitary, directly or indirectly, is necessary for the maintenance of GH binding sites in adipose tissue and that these binding sites are related to the insulin-like effect of GH.

Acute growth hormone deficiency rapidly alters glucose metabolism in rat adipocytes. Relation to insulin responses and binding.

To investigate cellular aspects of the antagonism between endogenous GH and insulin in adipose tissue, we examined glucose metabolism, insulin responses, and insulin binding in adipocytes isolated from rats made GH deficient by treatment with antibodies to rat GH (ArGH), which neutralize the biological actions of GH. Adipocytes were incubated in the presence or absence of insulin, and their ability to convert [14C] glucose to CO2 and lipid was measured. Basal glucose utilization was significantly elevated in adipocytes from ArGH-treated rats after 6, 3, or even 1 h of serum treatment compared to that in controls treated with nonimmune serum. This suggests that endogenous GH suppresses glucose metabolism in adipocytes from normal rats. In the presence of insulin, the absolute values of glucose utilization were also higher in cells from ArGH-treated rats than in controls. However, because basal glucose oxidation was higher with GH deficiency, the percent stimulation by insulin was not different in adipocytes from ArGH-treated rats and controls. In keeping with this, the binding of [125I]iodoinsulin was not altered by GH deficiency. These findings indicate that endogenous GH suppresses basal glucose metabolism in adipocytes, and that this effect is rapidly reversed by treatment of normal rats with ArGH. Furthermore, this effect of GH on adipocyte metabolism is independent of changes in insulin binding and appears to account for the enhanced responses to insulin observed with GH deficiency.

Immediate, short- and long-term opposite effects of oestradiol-17 beta on glucose metabolism in rat adipocytes: relationship with the biphasic changes in body weight and food intake.

The changes in the effects of oestradiol-17 beta on body weight, food intake and [1-14C]glucose oxidation in adipocytes were followed in sham-operated, ovariectomized and adrenalectomized-ovariectomized rats to eliminate effects of endogenous progesterone and corticosterone. During the first 5 days oestradiol induced a dramatic fall in food intake and body weight concomitant with a decrease in glucose oxidation by adipocytes, when tested 12 h and 3 days after the beginning of treatment. In-vitro incubations with oestradiol showed that this was a direct effect of this hormone. On the other hand, from days 5 to 14 of treatment, body weight and food intake increased, though they were still lower than in sham-operated controls. On day 14, as values of treated rats tended to reach those of controls, glucose oxidation in adipocytes was stimulated by oestradiol treatment. An insulin effect was still observable and none of these effects was dependent on the adrenal gland. These biphasic changes in the parameters studied could be closely related; moreover, a relationship with other oestradiol actions on metabolism that are known to be corticosterone-dependent could be eliminated.

The effects of chronic hyperinsulinaemia on insulin binding and glucose metabolism in rat adipocytes.

The effects of chronic hyperinsulinaemia on insulin binding and glucose metabolism in rat adipocytes.

Effects of fasting and refeeding of insulin receptors and glucose metabolism in rat adipocytes.

To investigate the mechanism of insulin insensitivity during fasting and hyperresponsiveness to insulin after refeeding, insulin binding, 2-deoxyglucose uptake and glucose oxidation were measured in adipocytes from three groups of rats: control, 48 h fasted, and 48 h refed after 48 h fasting. In 48 h fasted rats, fat cell diameters were decreased to five-sixths of that of the controls and plasma insulin levels were decrease to less than half of the controls; however, after 48 h refeeding, these parameters were restored to normal. In adipocytes from fasted rats, insulin binding was increased but insulin-stimulated 2-deoxyglucose uptake glucose oxidation were significantly decreased as compared to that of the controls. In adipocytes of refed rats, insulin bindings was slightly increased as compared to the controls, accompanied by a slightly decreased 2-deoxyglucose uptake and completely restored glucose oxidation. Conclusions from these studies are that both disorders in coupling of insulin-receptor complexes to glucose transport systems and intracellular glucose metabolism play major roles in the insulin insensitivity of adipocytes from fasted rats, whereas, in adipocytes from refed rats, intracellular glucose metabolism, rather than glucose transport, is mainly involved in restoring insulin sensitivity.