Late effects of postnatal administration of monosodium glutamate on insulin action in adult rats

Early postnatal administration of monosodium glutamate (MSG) to rats induces obesity, hyperinsulinemia and hyperglycemia in adulthood, thus suggesting the presence of insulin resistance. We therefore investigated the effects of insulin on glucose transport and lipogenesis in adipocytes as well as insulin binding to specific receptors in the liver, skeletal muscle and fat tissues. An increase of plasma insulin, glucose and leptin levels was found in 3-month-old rats treated with MSG during the postnatal period. The attenuation of insulin stimulatory effect on glucose transport was observed in MSG-treated rats. Despite the lower basal and insulin-stimulated glucose uptake, the incorporation of glucose into lipids was significantly higher in MSG-treated rats, suggesting a shift in glucose metabolism towards lipid synthesis in fat tissue. Insulin binding to plasma membranes from the liver, skeletal muscle and adipocytes was decreased in MSG-treated rats. This is in agreement with the lower insulin effect on glucose transport in these animals. Furthermore, a decreased amount of GLUT4 protein was found in adipocytes from MSG-treated obese rats. The results demonstrated an attenuation of insulin effect on glucose transport due to a lower insulin binding and lower content of GLUT4 protein in MSG-treated rats. However, the effect of insulin on lipogenesis was not changed. Our results indicated that early postnatal administration of MSG exerts an important effect on glucose metabolism and insulin action in adipocytes of adult animals.     

Effects of SH-reagents of different molecular size upon glucose metabolism in isolated rat fat cells.

To study the role of membrane SH-groups in glucose transport of isolated rat fat cells we compared the effects of a small organic mercurial reagent p-CMB with those of a large p--CMB-derivative -- p-CMB-Dextran, MW 10.000 --. It could be shown that both compounds were of almost identical reactivity on fat cell homogenate metabolism. When applied to intact fat cells uncoupled p--CMB showed an (1) insulin like enhancement of 14C incorporation from (U-14C) glucose into CO2 and triglyceride, (2) inhibition of the insulin-stimulatory effect on these parameters and (3) inhibition of basal glucose uptake dependent on the concentrations used. Identical concentrations of p-CMB-Dextran, however, failed to influence basal glucose uptake as well as the insulin mediated increase in glucose metabolism.     

Effect of anti-insulin receptor antibodies on insulin-sensitive phosphodiesterase in rat fat cells

Effect of sera with anti-insulin receptor antibodies (AIRS) on insulin-sensitive phosphodiesterase in rat fat cells was examined. AIRS activated the enzyme when incubated with intact fat cells. AIRS (1:400 dilution) were less potent for activation of the phosphodiesterase than insulin (3 nM), but were more potent for inhibition of 125I-insulin binding to fat cells than insulin. When insulin receptor of fat cells was destroyed with trypsin-treatment, AIRS as well as insulin completely lost the ability to activate the phosphodiesterase. These findings suggest that AIRS bind to or very near the insulin receptor and exhibit insulin-like biological effect of the phosphodiesterase activation.     

Analysis of the effect of a novel therapeutic for type 2 diabetes on the proteome of a muscle cell line

Elevated serum retinol‐binding protein (RBP) concentration has been implicated in the development of insulin resistance and type 2 diabetes. Two series of small molecules have been designed to lower serum levels by reducing secretion of the transthyretin–RBP complex from the liver and enhancing RBP clearance through the kidney. These small molecules were seen to improve glucose and insulin tolerance tests and to reduce body weight gain in mice rendered diabetic through a high fat diet. A proteomics study was conducted to better understand the effects of these compounds in muscle cells, muscle being the primary site for energy expenditure. One lead compound, RTC‐15, is seen to have a significant effect on proteins involved in fat and glucose metabolism. This could indicate that the compound is having a direct effect on muscle tissue to improve energy homeostasis as well as a whole body effect on circulating RBP levels. This newly characterized group of antidiabetic compounds may prove useful in the treatment and prevention of insulin resistance and obesity.     

Effects of insulin on adrenoceptor binding and the rate of catecholamine-induced lipolysis in isolated human fat cells

The mechanisms by which insulin inhibits catecholamine-induced lipolysis in fat cells are unknown. In this study the possible role of an interaction between insulin and the adrenoceptors on human fat cells was investigated. Insulin inhibited, in a dose-dependent fashion, the specific binding of hydrophobic as well as hydrophilic nonselective beta-receptor radioligands but had no effect on the binding of alpha 2-selective radioligands. The results of saturation experiments and competition-inhibition experiments under both equilibrium conditions and nonequilibrium conditions revealed that insulin reduced the total number of beta-adrenergic binding sites (maximum effect 25%) without changing the beta-adrenoceptor affinity. This insulin effect was rapid and reversible; one-third of the effect occurred within 1 min of incubation and it was completely reversed within 30 min after withdrawal of insulin. It could be mimicked by a polyclonal rabbit insulin receptor antibody but not by insulin mimickers acting distal to the initial interaction between the hormone and its specific insulin-receptor binding site. The beta-adrenoceptor binding to a plasma membrane-enriched fraction decreased at the same time as it increased to a microsomal enriched fraction after insulin treatment, indicating a redistribution of beta-adrenoceptors in the cell. In lipolysis experiments performed under conditions like those in the binding experiments, insulin inhibited the rate of lipolysis with a lag period of 3 min. Furthermore, the hormone caused a dose-dependent maximum 10-fold shift to the right of the dose-response curve for isoprenaline-induced lipolysis without changing the amplitude of the curve. This effect of insulin was specific for the beta-adrenergic receptors system, since insulin markedly decreased the amplitude of the dose-response curve for parathyroid hormone-induced lipolysis. In addition, the effect of insulin on isoprenaline-induced lipolysis could be mimicked by long-lasting fractional inactivation of the beta-adrenoceptors. The dose-response relationships for the inhibitory effects of insulin on beta-adrenoceptor binding and the lipolytic sensitivity to isoprenaline were almost identical. Half-maximum and maximum effects occurred at about 5 and 100 microunits/ml of insulin, respectively. In conclusion, the exposure of human fat cells to physiological insulin doses is followed by a rapid and dose-dependent translocation of beta-adrenoceptors from the exterior to the interior of the cell and a subsequent dose-dependent decrease in the lipolytic sensitivity to beta-adrenergic agonists, without a change in maximum lipolysis.(ABSTRACT TRUNCATED AT 400 WORDS)     

Biological activity of a fragment of insulin

Insulin controls or alters glucose, protein, and fat metabolism as well as other cellular functions. Insulin binds to a specific receptor on the cell membrane initiating a protein phosphorylation cascade that controls glucose uptake and metabolism and long-term effects such as mitogenesis. This process also initiates insulin uptake and ultimate cellular metabolism in all insulin sensitive cells. The effects of insulin on other cellular metabolic properties have not been clearly related to this mechanism. Here we show that intracellular metabolism of insulin may be related to some aspects of insulin actions, specifically control of fat metabolism. A normal intracellular degradation product of insulin has been synthesized and tested for actions on fat turnover in cultured adipocytes. This 7-peptide, B-chain fragment (HLVEALY) inhibits both basal and stimulated lipolysis as measured by glycerol release, but does not inhibit FFA release because of a lack of effect on FFA reesterification in the adipocyte. HLVEALY also enhances insulin's effects on lipogenesis. This study shows that a fragment of insulin produced by the action of the insulin-degrading enzyme has both independent biological effects and interactions with insulin. This supports a biologically important effect of insulin metabolism and insulin degradation products on insulin action on non-glucose pathways.     

Regulation of the d-glucose transport system in isolated fat cells

Summary Recent technical advances have yielded considerable new biochemical insights into the hexose transport systems of both brown and white fat cells. In the present studies a novel filtration method was used to monitor initial rates of 3-O-(³H) methylglucose uptake in isolated white fat cells. Transport of 3-O-methylglucose, a non-metabolizable analogue of glucose, occurred by facilitated diffusion, was inhibited by glucose, phloridzin, cytochalasin B and dipyridamole, and was rapidly stimulated by insulin as well as lectins. Total 3-O-methylglucose uptake in white fat cells could be attributed to two kinetically distinct processes in addition to a certain degree of diffusion.Two important new features of glucose transport in fat cells have been discovered. First, in both brown and white fat cells transport per se does not appear to be necessarily rate-limiting for further glucose metabolism. Thus vitamin K₅, which markedly increases glucose oxidation by brown fat cells, did not affect the glucose transport system activity. Glucose utilization can apparently be significantly enhanced in fat cells by agents which either increase transport system activity or intracellular enzyme activity. Second, the transport system itself, whether in the basal state or after activation by insulin, lectins, or oxidants, is resistant to sulfhydryl reagents such as N-ethylmaleimide, while the increase in transport activity due to these agents is exquisitely sensitive to sulfhydryl blockage. N-ethylmaleimide blocks the stimulatory effect of insulin on transport whereas addition of insulin to fat cells prior to the reagent completely protects against this inhibitory effect. Further, N-ethylmaleimide prevents the elevated rates of transport system activity due to insulin (or other agents) from returning to basal levels once the cells are washed free of hormone. These data are consistent with the concept that activation of the transport system involves oxidation of key membrane sulfhydryls to the disulfide form, but alternative models are also possible. In any case, these findings provide a possible biochemical clue for future studies designed to identify the specific component(s) involved in the regulatory mechanism which modulates transport of glucose in isolated fat cells.Invited ArticleRecipient of the Elliot P. Joslin Research and Development Award of the American Diabetes Association.     

Diacylglycerols modulate insulin action in rat adipocytes

Effect of 1,2-diacylglycerols on the insulin receptor function and insulin action in rat adipocytes was studied. 1,2-dioctanoylglycerol (100 micrograms/ml) did not alter insulin binding but it did stimulate phosphorylation of the beta-subunit of the insulin receptor as well as its tyrosine kinase activity. However, dioctanoylglycerol inhibited insulin-stimulated receptor autophosphorylation. This concentration of dioctanoylglycerol inhibited insulin-stimulated CO2 metabolism, lipogenesis and 3-O-methyl-glucose transport in a dose-dependent manner but did not alter any of these bioeffects in absence of insulin. While there was no direct link between diacylglycerol effect on tyrosine kinase activity of the insulin receptor and insulin action in rat adipocytes, the parallel inhibition of insulin-stimulated receptor autophosphorylation and insulin bioeffects by dioctanoylglycerol suggests its direct or indirect role in insulin signalling in rat fat cells.     

Effects of insulin on the uptake of d-galactose by isolated rat epididymal fat cells

In muscle, insulin stimulates uptake of d-galactose as well as d-glucose and certain other sugar isomers (Kono, T. and Colowick, S.P. (1961) Arch. Biochem. Biophys. 93, 514–519). In fat cells, the hormone also stimulates uptake of d-glucose and certain other monosaccharides. Nonetheless, the hormone does not increase the uptake, as determined by the utilization, of d-galactose by fat cells (Ball, E.G. and Cooper, O. (1960) J. Biol. Chem. 235, 584–588; Kuo, J.F. and Dill, I.K. (1969) Biochim. Biophys. Acta 177, 17–26).As pointed out by Ball and Cooper, this does not necessarily indicate that insulin has no effect on the membrane transport of d-galactose in fat cells. The possible effect of the hormone on transport may not be seen in the utilization data if the intracellular metabolism is considerably slower than the rate of transport and insensitive to insulin.     

Effects of age and cell size on rat adipose tissue metabolism.

In order to analyze separately the effects of cell size and age on the metabolism of rat adipose tissue, fat cells of different sizes were obtained from the same animals. The rats were 4 or 15 wk old. The results show that age as well as cell size influences the metabolic rates. At a given cell size, the basal lipolysis, the lipolytic effects of glucagon and noradrenaline, the rate of glucose incorporation into the triglycerides, and the effect of insulin on glucose metabolism were considerably increased in the young animals. Furthermore, irrespective of fat cell size the lipolytic action of glucagon was reduced in old animals. The data thus show that experiments with large fat cells from old rats and with small cells from young animals cannot be directly compared because both variables may influence metabolic reactions.     

Histochemical evidence for the differential surface labeling, uptake, and intracellular transport of a colloidal gold-labeled insulin complex by normal human blood cells

A colloidal gold-labeled insulin-bovine serum albumin (GIA) reagent has been developed for the ultrastructural visualization of insulin binding sites on the cell surface and for tracing the pathway of intracellular insulin translocation. When applied to normal human blood cells, it was demonstrated by both visual inspection and quantitative analysis that the extent of surface labeling, as well as the rate and degree of internalization of the insulin complex, was directly related to cell type. Further, the pathway of insulin (GIA) transport via round vesicles and by tubulo-vesicles and saccules and its subsequent fate in the hemic cells was also related to cell variety. Monocytes followed by neutrophils bound the greatest amount of labeled insulin. The majority of lymphocytes bound and internalized little GIA, however, between 5-10% of the lymphocytes were found to bind considerable quantities of GIA. Erythrocytes rarely bound the labeled insulin complex, while platelets were noted to sequester large quantities of the GIA within their extracellular canalicular system. GIA uptake by the various types of leukocytic cells appeared to occur primarily by micropinocytosis and by the direct opening of cytoplasmic tubulo-vesicles and saccules onto the cell surface in regions directly underlying surface-bound GIA. Control procedures, viz., competitive inhibition of GIA labeling using an excess of unlabeled insulin in the incubation medium, preincubation of the GIA reagent with an antibody directed toward porcine insulin, and the incorporation of 125I-insulin into the GIA reagent, indicated the specificity and selectivity of the GIA histochemical procedure for the localization of insulin binding sites.     

Proposed mechanism of insulin-resistant glucose transport in the isolated guinea pig adipocyte. Small intracellular pool of glucose transporters

A marked resistance to the stimulatory action of insulin on glucose metabolism has previously been shown in guinea pig, compared to rat, adipose tissue and isolated adipocytes. The mechanism of insulin resistance in isolated guinea pig adipocytes has, therefore, been examined by measuring 125I-insulin binding, the stimulatory effect of insulin on 3-0-methylglucose transport and on lipogenesis from [3-3H]glucose, the inhibitory effect of insulin on glucagon-stimulated glycerol release, and the translocation of glucose transporters in response to insulin. The translocation of glucose transporters was assessed by measuring the distribution of specific D-glucose-inhibitable [3H]cytochalasin B binding sites among the plasma, and high and low density microsomal membrane fractions prepared by differential centrifugation from basal and insulin-stimulated cells. At a glucose concentration (0.5 mM) where transport is thought to be rate-limiting for metabolism, insulin stimulates lipogenesis from 30 to 80 fmol/cell/90 min in guinea pig cells and from 25 to 380 fmol/cell/90 min in rat cells with half-maximal effects at approximately 100 pM in both cell types. Insulin similarly stimulates 3-O-methylglucose transport from 0.40 to 0.70 fmol/cell/min and from 0.24 to 3.60 fmol/cell/min in guinea pig and rat fat cells, respectively. Nevertheless, guinea pig cells bind more insulin per cell than rat cells, and insulin fully inhibits glucagon-stimulated glycerol release. In addition, the differences between guinea pig and rat cells in the stimulatory effect of insulin on lipogenesis and 3-O-methylglucose transport cannot be explained by the greater cell size of the former compared to the latter (0.18 and 0.09 micrograms of lipid/cell, respectively). However, the number of glucose transporters in the low density microsomal membrane fraction prepared from basal guinea pig cells is markedly reduced compared to that from rat fat cells (12 and 70 pmol/mg of membrane protein, respectively) and the translocation of intracellular glucose transporters to the plasma membrane fraction in response to insulin is correspondingly reduced. These results suggest that guinea pig adipocytes are markedly resistant to the stimulatory action of insulin on glucose transport and that this resistance is the consequence of a relative depletion in the number of intracellular glucose transporters.     

Calcium as modulator of the hormonal-receptors-biological-response coupling system. Effects of Ca2+ ions on the insulin activated 2-deoxyglucose transport in rat fat cells.

Activation of 2-deoxyglucose transport in isolated rat fat cells by insulin is dependent upon the presence of Ca2+ in the external medium. When calcium concentration is kept below 100 micron, insulin acts like a partial agonist, giving only half of the maximal activation obtained normally with a millimolar concentration of this ion. Oxytocin, whose insulin-like action on adipocytes activates glucose oxidation by these cells, was found to be unable to affect the rate of 2-deoxyglucose transport. This, together with previous observations, suggests that calcium ions play a role in the mechanism of insulin action possibly by binding selectively to membrane sites involved in the transmission of the hormonal message to the glucose carrier. Oxytocin seems to trigger only intracellular glucose metabolism and it appears that there is an absolute requirement for calcium ions in the activation of a still unknown membrane signal.     

The effect of phenformin and other adenosine triphosphate (ATP)-lowering agents on insulin binding to IM-9 human cultured lymphocytes

In the present study, we investigated the mechanism by which the antidiabetic drug phenformin increases insulin binding to its receptors in IM-9 human cultured lymphocytes. After a 24-hr preincubation, phenformin induced a twofold increase in specific 125I-insulin binding, and removal of phenformin was followed 6 hr later by a return in binding to control levels. This effect of phenformin on insulin binding was not a consequence of either inhibition of cell growth, changes in cellular cyclic adenosine monophosphate (AMP) levels, or changes in guanosine triphosphate (GTP) content. Since phenformin is known to inhibit various aspects of cellular energy metabolism, the relationship between 125I-insulin binding and energy metabolism in IM-9 cells was investigated. The phenformin-induced increase in insulin binding to IM-9 cells was related to a time- and dose-dependent decrease in ATP levels. Other agents that lowered ATP levels, including antimycin, dinitrophenol, and 2-deoxyglucose, also raised insulin binding. These studies indicated, therefore, that phenformin enhances insulin binding to receptors on IM-9 cells and that this effect on insulin receptors may be related to alterations in metabolic functions that are reflected by a lowering of ATP levels.     

Glucose metabolism in isolated fat cells: enhanced response of larger adipocytes from older rats to epinephrine and adrenocorticotropin.

The effects of insulin and of two lipolytic hormones (epinephrine and ACTH1) on the rate and pattern of glucose metabolism were compared during incubation of isolated fat cells, obtained from epididymal fat pads of rats of varying age and degrees of adiposity. Glucose metabolism and the intracellular free fatty acid levels were expressed on a per cell basis and in relation to adipocyte size. The data for total glucose metabolism show that, in contrast to the declining insulin effect observed with adipocyte enlargement, the stimulation of glucose uptake and metabolism by these lipolytic hormones was significantly greater in the larger fat cells from the older fatter rats than in the smaller ones from the younger leaner rats. Lipolytic hormones suppressed, whereas insulin enhanced, fatty acid synthesis; moreover the lipolytic hormones stiumlated glucose ce effect of epinephrine on the intracellular free fatty acid levels was greater in the small fat cells than in the large ones; this effect of epinephrine was markedly curtained by the presence of glucose in the incubation medium, making it unlikely that acceleration of glucose metabolism by the lipolytic stimulus was mediated by an elevation of the intracellular free fatty acid level. The present results show a markedly enhanced capacity of the large adipocytes to accelerate glucose metabolism in response to these liplytic hormones. Thus, in contrast to prevailing notions of declining hormonal responsiveness with expanding fat cell size in older and more obese animals, this study documents an instance of increased hormonal response in enlarged adipocytes and points to the need for a more comprehensive reevaluation of the various hormonal effects in adipocytes of different size.     

Effect of physical training on glucose transporters in fat cell fractions

Physical training increases maximally insulin-stimulated glucose assimilation and 3-O-methylglucose transport in epididymal fat cells. In the present report, glucose-inhibitable cytochalasin B binding in subcellular fractions of epididymal adipocytes was measured to assess changes in number of glucose transporters induced by training. Groups of rats trained by swimming were compared to control groups of the same age, matched with respect to body weight by restricted feeding. It was found that in trained rats the number of glucose transporters in the low density microsome fractions from non-insulin-stimulated fat cells was larger than in untrained rats. In both groups of rats, insulin stimulation of adipocytes decreased the number of glucose transporters in low-density microsomes by about 60% and increased the number of glucose transporters in the plasma membrane fractions. The number of glucose transporters in the plasma membrane fractions from maximally insulin-stimulated fat cells was larger in trained rats than in control rats. [U-¹⁴C]Glucose incorporation into lipids varied in proportion to plasma membrane cytochalasin B binding per cell under all conditions tested. The results explain the enhancing effect of training on insulin responsiveness transport of hexose in fat cells.     

Swainsonine对胰岛素受体功能影响的研究

以Ｓｗａｉｓｏｎｉｎｅ（Ｓｗ）作为高尔基体付糖链加工酶系中α－甘露糖苷酶Ⅱ的特异抑制剂,研究Ｎ－糖链结构和胰岛素受体（Ｉｎｓ－Ｒ）功能的关系．发现Ｓｗ不影响细胞生长和３Ｈ－亮氨酸参入ＳＭＭＣ７７２１细胞,但明显促进３Ｈ－甘露糖参入细胞总糖蛋白和表面糖蛋白,并使后者的ＣｏｎＡ强结合组分显著增加,提示Ｓｗ使Ｉｎｓ－Ｒ的Ｎ－糖链变成杂合型及高甘露糖型。胰岛素结合试验后作Ｓｃａｔｃｈａｒｄ分析：发现Ｓｗ不改变Ｉｎｓ－Ｒ的结合容量和每个细胞表面的结合位点数,也不改变结合动力学。再用部分纯化的Ｉｎｓ－Ｒ研究自身磷酸化和对外源底物的酪氨酸蛋白激酶活力,也未发现Ｓｗ处理和对照细胞间的明显区别,表示Ｓｗ也不影响Ｉｓｎ－Ｒ的跨膜信息传递,结合已报道的衣霉素使细胞表面Ｉｎｓ－Ｒ减少的结果,提示Ｉｎｓ－Ｒ运送至细胞膜需要Ｎ－糖链存在,但糖链的类型对ＩＮＳ－Ｒ的代谢和结合动力学并不重要     

Insulin stimulates cellular iron uptake and causes the redistribution of intracellular transferrin receptors to the plasma membrane

Insulin stimulates the accumulation of iron by isolated fat cells by increasing the uptake of diferric transferrin. Analysis of the cell-surface binding of diferric 125I-transferrin indicated that insulin caused a 3-fold increase in the cell surface number of transferrin receptors. This result was confirmed by the demonstration that insulin increases the binding of an anti-rat transferrin receptor monoclonal antibody (OX-26) to the surface of fat cells. The basis of this effect of insulin was examined by investigating the number of transferrin receptors in membrane fractions isolated from disrupted fat cells. Two methods were employed. First the binding isotherm of diferric 125I-transferrin to the isolated membranes was studied. Second, the membranes were solubilized with detergent, and the number of transferrin receptors was measured by immunoblotting using the monoclonal antibody OX-26. It was observed that insulin treatment of intact fat cells resulted in an increase in the number of transferrin receptors located in the isolated plasma membrane fraction of the disrupted fat cells. Furthermore, the increase in the number of plasma membrane transferrin receptors was associated with a concomitant decrease in the transferrin receptor number in a low density microsome fraction previously shown to consist of intracellular membranes. This redistribution of transferrin receptors between cellular membrane fractions in response to insulin is remarkably similar to the regulation by insulin of glucose transporters and type II insulin-like growth factor receptors. We conclude that insulin stimulates fat cell iron uptake by a mechanism that may involve the redistribution of transferrin receptors from an internal membrane compartment (low density microsomes) to the cell surface (plasma membrane).     

Current status of the thiol redox model for the regulation of hexose transport by insulin.

Data obtained over the last two years pertinent to the thiol redox model for the modulation of hexose transport activity by insulin is summarized. The model proposes that activation of hexose transport in fat cells involves sulfhydryl oxidation to the disulfide form in a key protein component of the fat cell surface membrane. Theoretically, the rapid activation of transport by insulin may involve either the conversion of inactive membrane carriers to the active form as originally proposed, or the conversion of a low Vmax transport system to a high Vmax form. The present experiments showed that the percent inhibition of insulin-activated transport rates by submaximal levels of cytochalasin B was decreased compared to its effects on basal transport. Treatment of fat cells with N-ethylmaleimide inhibited cytochalasin B action but not transport activity. When insulin or the oxidant vitamin K5 was added to cells 5 minutes before the N-ethylmaleimide, the elevated transport activity was also resistant to the sulfhydryl reagent, but cytochalasin B retained its potent inhibitory effect on transport. The data demonstrate that unique properties characterize basal versus insulin-activated transport activity with respect to the sensitivity of cytochalasin B action to sulfhydryl blockade in isolated fat cells. The data are consistent with the concept that activation of transport activity reflects the conversion of a reduced (sulfhydryl) system characterized by a low Vmax to an oxidized (disulfide), high Vmax transport system.     

Stimulation of glucose metabolism by lectins in isolated white fat cells

Addition of 5 μg/ml concanavalin A to isolated white fat cells in the presence of 1 % albumin maximally stimulated the conversion of d-[1-¹⁴C]glucose to CO₂, glyceride-glycerol and fatty acids over a 1 h incubation period; as little as 1 μg/ml agglutinin increased fat cell glucose oxidation more than 2-fold. Labelled CO₂ production in the presence of concanavalin A was linear for at least 90 min and was inhibited by 40 mM α-methyl-d-glucoside which had little effect on basal or insulin-stimulated glucose oxidation. The effect of a submaximal concentration of the agglutinin was additive to that of submaximal but not maximal concentrations of insulin.Concanavalin A caused agglutination of fat cells which could be readily detected by light microscopy. Digestion of fat cells with 0.5 mg/ml trypsin for 15 min did not affect subsequent agglutination and inhibited the increased glucose oxidation due to concanavalin A by less than 30%. Thus the action of concanavalin A was much less sensitive to trypsinization of fat cells than insulin since trypsin under the above conditions completely abolished the effect of insulin. An anti-blood group A agglutinin from Phaseolus lunatus and Lens culanaris agglutinin also markedly stimulatedfat cell glucose conversion to CO₂. Agglutinin-stimulated glucose metabolism was inhibited by phloretin. This binding of several types of specific plant lectins to fat cell membrane glycoprotein(s) and/or glycolipid(s) apparently initiates events which results in increased glucose transport.