Effects of Bisphosphonates on Glucose Transport in a Conditionally Immortalized Rat Retinal Capillary Endothelial Cell Line (TR-iBRB Cells)

The objective of the present study was to elucidate the effect of bisphosphonates, anti-osteoporosis agents, on glucose uptake in retinal capillary endothelial cells under normal and high glucose conditions. The change of glucose uptake by pre-treatment of bisphosphonates at the inner blood-retinal barrier (iBRB) was determined by measuring cellular uptake of [³H]3-O-methyl glucose (3-OMG) using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB cells) under normal and high glucose conditions. [³H]3-OMG uptake was inhibited by simultaneous treatment of unlabeled D-glucose and 3-OMG as well as glucose transport inhibitor, cytochalasin B. On the other hand, simultaneous treatment of alendronate or pamidronate had no significant inhibitory effect on [³H]3-OMG uptake by TR-iBRB cells. Under high glucose condition of TR-iBRB cells, [³H]3-OMG uptake was increased at 48 h. However, [³H]3-OMG uptake was decreased significantly by pre-treatment of alendronate or pamidronate compared with the values for normal and high glucose conditions. Moreover, geranylgeraniol (GGOH), a mevalonate pathway intermediate, increased the uptake of [³H]3-OMG reduced by bisphosphonates pre-treatment. But, pre-treatment of histamine did not show significant inhibition of [³H]3-OMG uptake. The glucose uptake may be down regulated by inhibiting the mevalonate pathway with pre-treatment of bisphosphonates in TR-iBRB cells at high glucose condition.     

Verapamil Inhibits the Glucose Transport Activity of GLUT1

Calcium channel blocker toxicity has been associated with marked hyperglycemia responsive only to high-dose insulin therapy. The exact mechanism(s) of this induced hyperglycemia has not been clearly delineated. The glucose transporter GLUT1 is expressed in a wide variety of cell types and is largely responsible for a basal level of glucose transport. GLUT1 also is activated by cell stress. The specific purpose of this study was to investigate the effects of the calcium channel blocker verapamil on the glucose uptake activity of GLUT1 in L929 fibroblasts cells. Dose-dependent effects of verapamil on glucose uptake were studied using L929 fibroblast cells with 2-deoxyglucose. Verapamil had a dose-dependent inhibitory effect on both basal and stress-activated transport activity of GLUT1. Basal activity was inhibited 50% by 300 μM verapamil, while 150 μM verapamil completely inhibited the activation induced by the stress of glucose deprivation. These effects were reversible and required verapamil to be present during the stress. Alteration of calcium concentrations by addition of 5 mM CaCl₂ or 4 mM EDTA had no effect on verapamil action. This study reveals the unique finding that verapamil has inhibitory effects on the transport activity of GLUT1 independent of its effects on calcium concentrations. The inhibition of GLUT1 may be one of the contributing factors to the hyperglycemia observed in CCB poisoning.     

盐酸洛美沙星葡萄糖注射液的稳定性

目的:研究盐酸洛美沙星葡萄糖注射液对光、热的稳定性.方法:将样品置于实验条件下,于指定时间取出样品,观察色泽,测定pH、洛美沙星含量,进行盐酸洛美沙星有关物质检查.结果:该品在日光及加热情况下色泽变深,日光下pH、含量明显下降,室温避光条件下稳定.结论:该品应贮于阴凉避光处.     

Role of Microtubules in Glucose Uptake by C6 Glioma Cells

Abstract: Drugs that influence tubulin function were used to investigate the role of microtubules in hexose uptake by C6 glioma cells. In C6 cells, colchicine and vinblastine (which inhibit tubulin polymerization) inhibited radioactive [³H]2-deoxy-D-glucose uptake by about 30%. Paclitaxel (which promotes tubulin polymerization) stimulated hexose uptake by about 25%. To further demonstrate that microtubules play a role in hexose uptake, C6 cells were transfected with GLUT1 cDNA and then challenged with 100 nM paclitaxel. In GLUT1- transfected cells paclitaxel stimulated 2-deoxy-D-glucose uptake by about 35%. To study the role of tubulin in agonist-stimulated hexose uptake, the effect of colchicine on carbach-ol-induced uptake was next examined. Hexose uptake was increased with carbachol in concentration-dependent manner which was abolished by pretreatment with colchicine. To examine the specificity of the inhibitory effect of colchicine on G protein-mediated signal transduction pathway, the influence of colchicine on insulin (which acts via tyrosine kinase pathway) stimulation of 2-deoxy-D-glucose uptake was investigated. Hexose uptake was increased by insulin in a concentration-dependent manner which was unaffected by pretreatment with colchicine. These results suggest that microtubules are involved in basal and carbachol-stimulated glucose uptake by C6 cells.     

Actions of Fenfluramine on Glucose Uptake in Vitro and in Vivo

Abstract Fenfluramine stimulates the glucose uptake of the isolated hemi-diaphragm from normal and streptozotocin diabetic rats in the presence of insulin. The drug does not cause an increased storage of glycogen. During hind leg perfusion of the dog fenfluramine stimulated peripheral glucose uptake. No increase in lactate or in free fatty acids release was observed during prolonged infusions of the drug. Fenfluramine caused an improvement of glucose tolerance in normal glucose-primed rats. A single dose of fenfluramine significantly lowered the blood glucose levels in streptozotocin diabetic rats and in diabetic dogs treated with insulin. Prolonged treatment with fenfluramine of streptozotocin diabetic rats had no significant effect on blood glucose levels. Additional treatment for one week of insulin dependent diabetic dogs with small non-anorexic doses of fenfluramine resulted in slightly decreased blood glucose levels. The dogs could not be maintained on fenfluramine alone (without insulin).     

The Distribution of a Glucose Load in Lithium Treated Rats

Abstract: The distribution of an intravenous glucose load was investigated in rats without and with previous lithium administration. Lithium caused an increased rate of glycogen formation in muscle tissue but not in liver tissue. Uptake of a ¹⁴C-labelled glucose load in skin, liver, muscle, fat and brain was measured. Lithium increased the uptake of labelled glucose in skin and muscle with a concomitant decrease of the amount in blood. The findings are in agreement with an increased glucose tolerance after lithium administration as the uptake was increased in the tissues of quantitative importance for the disposal of a glucose load.     

Glucose phosphorylation as a barrier to muscle glucose uptake

1. Glucose phosphorylation is the first irreversible step of the muscle glucose uptake pathway and is catalysed by a hexokinase isozyme. 2. While glucose transport is the primary barrier to muscle glucose uptake during basal conditions, glucose phosphorylation becomes an important barrier to muscle glucose uptake during stimulated conditions such as hyperinsulinaemia or exercise. 3. High fat feeding markedly impairs insulin- and exercise-stimulated muscle glucose uptake. As hexokinase II overexpression corrects this dietary-induced deficit during exercise, glucose phosphorylation is a site of impairment following high fat feeding. 4. Exercise is an important tool for diagnosing deficits in glucose phosphorylation.     

转化生长因子β及大黄酸对肾小球系膜细胞葡萄糖转运蛋白功能的影响(英文)

目的:对人肾小球系膜细胞葡萄糖转运蛋白-1(GLUT1)进行鉴定。研究转化生长因子(TDF-β_1)对GLUT1表达和功能的影响及大黄酸的干预作用。方法:分别用RT-PCR、免疫荧光染色、流式细胞仪、Northern杂交和[~3H]-2脱氧葡萄糖摄入率对系膜细胞GLUT1 mRNA表达、蛋白质分布和功能进行了研究。观察不同浓度TGF-β_1在加或不加大黄酸的情况下对系膜细胞葡萄糖摄入以及GLUT1 mRNA表达的影响。结果:研究发现人类肾小球系膜细胞上存在功能性的GLUT1。TGF-β_1能上调系膜细胞GLUT1 mRNA的表达和增加系膜细胞葡萄糖摄入率。大黄酸对正常糖浓度培养条件下系膜细胞葡萄糖摄入无影响,但能明显抑制TGF-β_1增加系膜细胞GLUT1 mRNA表达和葡萄糖摄入的作用。结论:TGF-β_1增加人肾小球系膜细胞GLUT1 mRNA的表达和细胞葡萄糖的摄入,该作用能够明显地被大黄酸所拮抗。     

Effect of cholinesterase inhibitors on acetylcholine and insulin induced glucose uptake and certain hepatic enzymes in pigeon liver: an in vitro study

Insulin and acetylcholine (ACh) are both known to promote glucose uptake by liver of birds. Acetylcholine induced glucose uptake can be predictably potentiated by inhibiting acetylcholinesterase activity. Monocrotophos, acothione (organophosphorus compound) and prostigmine are known inhibitors of acetylcholinesterase (AChE). In the present study the action of these three inhibitors of AChE alone as well as in combination with insulin and acetylcholine on in vitro glucose uptake by pigeon liver slices was investigated. Both organophorus compounds potentiated the action of insulin as well as acetycholine mediated glucose uptake by liver slices while prostigmine had inhibitory influence. The three compounds also induced alterations in enzyme activities in the liver slices. These results are discussed in detail in the text.     

Inhibitory effect of steviol, a metabolite of stevioside, on glucose absorption in everted hamster intestine in vitro

The effects of stevioside and steviol (a product of enzymatic hydrolysis of stevioside) on intestinal glucose absorption were examined in hamster jejunum. By using the everted sac technique, we found that stevioside (1 and 5 mM) had no inhibitory effect on glucose absorption. In contrast, glucose absorption was inhibited 29% by 1 mM steviol. The inhibition of glucose absorption by steviol was related to steviol concentration and incubation time. The possible mechanism of steviol inhibitory action of glucose absorption was also investigated. Reductions in the intestinal mucosal ATP content and absorptive surface area were responsible for the inhibition of glucose absorption by steviol. The decrease in the intestinal mucosal ATP content was accompanied by a decrease in the activities of mitochondrial NADH cytochrome c reductase and cytochrome oxidase. Morever, no inhibitory effects of steviol on the activity of intestinal Na⁺,K⁺-ATPase and glucose uptake in the intestinal brush-border membrane vesicles were seen. These results suggest that inhibition of intestinal glucose absorption by steviol in hamsters is due to the reduction in mucosal ATP content and an alteration of the morphology of the intestinal absorptive cells.     

Inhibition of aconitase by alloxan and the differential modes of protection of glucose, 3-O-methylglucose,and mannoheptulose

Summary Alloxan inhibited aconitase with a half maximal inhibitory concentration of 0.5 mM in sonically disrupted and 2.3 mM in intact isolated liver mitochondria. For dialuric acid the half maximal inhibitory concentrations were 1.1 mM and 2.5 mM, respectively. Ninhydrin and N-ethylmaleimide (NEM) also inhibited aconitase with half maximal inhibitory concentrations in the submillimolar range and t-butylhydroperoxide (BuOOH) in the millimolar range, which, however, were not different for disrupted and intact mitochondria. Only the aconitase substrate citrate, but not glucose provided protection of the enzyme against inhibition. In intact liver cells the half maximal inhibitory concentration for alloxan was 6.8 mM. Again, dialuric acid and BuOOH were less potent inhibitors while ninhydrin and NEM were more potent inhibitors of aconitase in intact liver cells. In intact liver cells, glucose and 3-O-methylglucose, but not mannoheptulose and citrate provided protection against alloxan inhibition. The results show that aconitase is not an enzyme particularly sensitive towards alloxan inhibition and thus apparently not a primary site for mediation of alloxan toxicity as it is the glucokinase. This makes a primary site of alloxan action in the mitochondria extremely unlikely. On the other hand the results demonstrate that both the intact mitochondrial and plasma membrane as uptake barriers provide protection against alloxan toxicity. In addition the results clearly show, that 3-O-methylglucose provides protection against alloxan action only at the level of the plasma membrane through inhibition of alloxan uptake into the cell, while the site of protection of mannoheptulose is only the sugar binding site of the glucokinase. In contrast, glucose is shown here to be the only sugar with a dual protective effect both through inhibition of alloxan uptake through the plasma membrane like 3-0methylglucose and through protection of the glucokinase sugar binding site against alloxan inhibition of the en zyme like mannoheptulose. In the light of these results the unique protective potency of glucose as compared to that of other sugars is not surprising.     

Effects of sulfhydryl inhibition on the regulation of basal lipolysis and glucose uptake in human adipose tissue.

Stimulation of basal lipolysis and inhibition of glucose uptake by N-ethylmaleimide (NEM) was demonstrated in vitro in human omental adipose tissue. NEM, at concentrations ranging from 4 to 5 × 10^?4 M, produced a maximal stimulation of basal lipolysis as well as a 50 per cent inhibition of the rate of glucose uptake, whereas concentrations above 1 × 10^-3M reduced the basal rate of glycerol release. In contrast, under the conditions in which it stimulated basal lipolysis, NEM strongly inhibited theophylline induced lipolysis. NEM-stimulated lipolysis was not directly related to decreased glucose uptake to α- or β-adrenergic mechanisms or to inhibition of phosphodiesterase. The lipolytic response of human adipose tissue to NEM was, however, markedly reduced by insulin and nicotinate, suggesting that adenylcyclase activation could be responsible for the lipolytic effect of NEM. The inhibitory effect exerted by NEM on glucose uptake was similarly reversed by the addition of insulin. The rate of basal lipolysis and glucose uptake by fat pads removed from diabetic patients was unaffected by NEM. A possible involvement of insulin in the effects induced by NEM on both basal lipolysis and glucose uptake in normal human adipose tissue is discussed.     

Rottlerin inhibits multiple steps involved in insulin-induced glucose uptake in 3T3-L1 adipocytes

Recently, it was shown that rottlerin inhibits insulin-stimulated glucose uptake and reduces intracellular adenosine triphosphate (ATP) levels in 3T3-L1 adipocytes, suggesting that these two events are causally linked. However, several other reports show that ATP-depletion induces glucose uptake in both muscle cells and adipocytes. In the present study, the mechanism of inhibition by rottlerin was studied in detail, in order to resolve this apparent discrepancy. It was found that rottlerin strongly reduces insulin-stimulated 2-deoxyglucose (2-DOG) uptake in 3T3-L1 adipocytes by a partial inhibition of the translocation of the insulin-responsive GLUT4 glucose transporter towards the plasma membrane (PM). Whereas the insulin-induced phosphatidyl-inositol-3' (PI-3') kinase signaling pathway is unaffected by rottlerin, Cbl tyrosine phosphorylation, which provides an essential, PI-3' kinase-independent signal towards GLUT4 translocation, is markedly attenuated. Furthermore, we also observed a direct inhibitory effect of rottlerin on insulin-induced glucose uptake in 3T3-L1 adipocytes. The direct inhibition of insulin-stimulated 2-DOG uptake by rottlerin displayed characteristics of uncompetitive inhibition: with the K(m(app)) of glucose uptake reduced from 1.6 to 0.9 mM and the V(max(app)) reduced from 5.2 to 1.0 nmol/minmg in the presence of rottlerin. In conclusion, rottlerin inhibits multiple steps involved in insulin-stimulated 2-DOG uptake in 3T3-L1 adipocytes. The observed reduction in GLUT4 translocation towards the PM and the uncompetitive inhibition of the glucose transport process provide alternative explanations for the inhibitory effects of rottlerin aside from the effects of rottlerin on intracellular levels of ATP.     

Tissue- and time-dependent effects of endothelin-1 on insulin-stimulated glucose uptake.

Hyperendothelinaemia is associated with various insulin-resistant states, e.g., diabetes, obesity and heart failure, but whether endothelin-1 (ET-1) has a direct effect on insulin-mediated glucose uptake is unclear because the interpretation of in vivo metabolic studies is complicated by ET-1 effects on muscle blood flow and insulin secretion. This study investigated the effects of ET-1 (1-10 nM) on basal and insulin-stimulated 2-deoxy-D-[3H]glucose (2-DOG) uptake in cultured L6 myoblasts and 3T3-adipocytes. RT-PCR analysis showed that both cell types express ET(A) but not ET(B) receptors. ET-1 had no effect on basal (non-insulin-mediated) glucose transport, but there was evidence of a tissue- and time-dependent inhibitory effect of ET-1 on insulin-stimulated glucose uptake. Specifically, ET-1 10 nM had a transient (0.5 h) inhibitory effect on glucose uptake in 3T3 cells (C(I-150) [dose of insulin required to increase glucose uptake by 50%, relative to control 100%] increased from 89 +/- 14 nM to 270 +/- 12 nM at 30 mins, P < 0.05) but no effect on insulin sensitivity in L6 myoblasts (C(I-150) was 56 +/- 14 nM [control], 43 +/- 14 [30 mins] and 26 +/- 16 [2 h]). In conclusion, the inhibitory effect of ET-1 on insulin-stimulated glucose uptake is transient and occurs in 3T3-L1 adipocytes but not skeletal muscle-derived cells, perhaps reflecting tissue differences in ET(A)-receptor signaling. It is therefore unlikely that chronic hyperendothelinaemia has a direct insulin-antagonist effect contributing to peripheral (ie muscle/fat) insulin resistance in vivo.     

Inhibitory effect of valproate on weak organic acid uptake in rat renal proximal tubules.

The effects of an antiepileptic drug, valproic acid (VPA), on transport mechanisms involved in renal excretion of anionic xenobiotics were investigated on rat renal proximal tubules in vitro. It was found that VPA (0.1-1 mM) dose dependently inhibited the baseline uptake of a marker organic anion, fluorescein, in the tubules. The inhibition could not be exclusively accounted for by competition between VPA and fluorescein. Taking into account a proposed relationship between the weak organic anion uptake and ammoniagenesis, the influence of VPA (0.5 mM) on the effects of glutamine and glutamate (both at 5 mM) on fluorescein uptake and ammonia production were examined. Glutamine stimulated ammonia production by the tubules, with the glutamine-induced ammoniagenesis being further augmented by VPA, while glutamate failed to affect the basal ammoniagenesis. Both glutamine (5 mM) and glutamate (5 mM) slightly inhibited fluorescein uptake, with the inhibitory effects not modified by VPA. Thus, there was no coincidence in the effects of VPA on organic anion uptake and renal ammoniagenesis. At the same time, the inhibitory effect of VPA (0.5 mM) on fluorescein uptake was largely overcome by addition of pyruvate (5 mM) to the incubation medium. In addition, VPA strongly inhibited glucose production from pyruvate. A known modulator of pyruvate metabolism, dichloroacetic acid (DCA, 1 mM), also inhibited fluorescein uptake, although its inhibitory effect was less pronounced than that of VPA. Both inhibitors failed to alter the tissue content of alpha-ketoglutarate or lactate but did slightly augment the pyruvate level. The inhibitory effects of VPA and DCA on the baseline fluorescein uptake were not additive, suggesting their similar intracellular targeting. It is assumed that the inhibitory effect of VPA on baseline fluorescein uptake in rat renal proximal tubules in vitro may be associated with its action on pyruvate metabolism.     

Glucose uptake stimulator YM‐138552 activates gene expression and translocation of glucose transporter isotype 4

In this study, we examined the effect of YM‐138552 on the glucose uptake, gene expression, and transport activities of the insulin‐regulatable glucose transporter isotype 4 (Glut4) in skeletal muscle cells. YM‐138552 stimulated medium glucose consumption in a dose‐dependent manner (EC₅₀ = 0.07 μM) in myoblast muscle C2C12 cells under differentiation conditions with 2% horse serum supplement. The stimulatory effect of glucose consumption was verified by radiolabeled 2‐DG uptake assay. The compound showed dose‐dependent stimulation of 2‐DG uptake in G8 myoblast muscle cells up to a 1 μM concentration (EC₅₀ = 0.19 μM). To investigate the mechanism of glucose uptake stimulation by YM‐138552, the mRNA level of Glut4 was determined using real‐time quantitative RT‐PCR. The Glut4 mRNA level expressed in C2C12 cells treated with YM‐138552 increased up to at least 24 h (227% vs. control), after which it gradually decreased to the initial level at 36 h. In addition, we established that C2C12 cells stably expressed the myc‐tagged Glut4 protein (C2C12‐Glut4myc) and measured the Glut4 translocation activity. The Glut4 translocation activity was stimulated by treatment of YM‐138552 without insulin in a dose‐dependent manner (EC₅₀ = 0.62 μM), and no insulin effect (100 nM) was observed. This suggests that YM‐138552 has an insulin‐like effect. These results suggest that the stimulation of glucose uptake by YM‐138552 in muscle cells was partly due to upregulation of the Glut4 gene expression and its translocation activation. Our findings on the in vitro effects of YM‐138552 glucose uptake stimulation through the Glut4 transporter may suggest a direction for the development of new drugs for the treatment of NIDDM. Drug Dev. Res. 51:43–48, 2000. © 2000 Wiley‐Liss, Inc.     

Effects of sodium valproate and acetazolamide on cerebral respiration

The effects of sodium valproate and acetazolamide on the oxygen uptake of guinea pig brain cortex slices were investigated. In calcium-free medium, sodium valproate inhibited the oxygen uptake appreciably in the presence of glucose and glutamic acid. Acetazolamide, on the other hand, was more effective in inhibiting oxygen uptake in the presence of glucose than in the presence of glutamic acid. Addition of 0.2 mM CaCl₂ in the medium containing 5 mM KCl could appreciably reverse the inhibition of oxygen uptake by acetazolamide in the presence of glucose. The inhibition of oxygen uptake by these drugs in the presence of glucose, however, could be completely reversed by increasing the dose of K⁺ ions (100 mM) in the medium which had no effect on the inhibition of oxygen uptake in the presence of glutamic acid. When the concentration of Ca²⁺ ions in the medium was elevated to 0.75 mM, the inhibitory effects of these drugs on the oxygen uptake in the presence of both glucose and glutamic acid could be completely abolished. Sodium valproate also inhibited the endogenous respiration of guinea pig brain cortex slices, whereas acetazolamide was almost without any effect. Increase in the concentration of Ca²⁺ ions in the medium failed to counteract the inhibition of endogenous respiraton of guinea pig brain cortex slices by sodium valproate.     

Glucose metabolism and insulin sensitivity in inactive inflammatory bowel disease

BACKGROUND: Inflammatory mediator concentration was found to be increased in active inflammatory bowel disease, and this could be related to an insulin-resistant state. Moreover, glucocorticoids, which are widely used in the treatment of inflammatory bowel disease, are notoriously related to insulin resistance. AIM: To measure body composition, whole body glucose uptake and oxidation in Crohn's disease and ulcerative colitis patients with inactive disease. METHODS: All patients had clinical, ultrasound and biochemical assessment. Body composition was determined by isotopic dilution technique; basal metabolic rate and substrate oxidation were measured by indirect calorimetry. Insulin sensitivity was assessed by the euglycaemic hyperinsulinaemic clamp. Ten patients with inactive Crohn's disease (five males, aged 31.1 +/- 7.0 years) and 10 patients with inactive ulcerative colitis (five males, aged 33.4 +/- 8.8 years) participated in the study. Forty healthy subjects, matched for age and height were used as a control group. RESULTS: Crohn's disease patients showed lower BMI (P < 0.001), fat mass (P < 0.05) and respiratory quotient (P < 0.001) values compared to both ulcerative colitis and control subjects. No difference in peripheral glucose uptake (micromol/kg/min) was found between groups (respectively 42.5 +/- 6.78 in Crohn's disease, 40.2 +/- 8.00 in ulcerative colitis and 41.4 +/- 10.8 in control subjects). Glucose storage and oxidation did not differ between groups. CONCLUSION: Our data showed that inflammatory bowel disease patients in a remission phase of the disease activity had a whole body glucose uptake and oxidation similar to those of control subjects, probably due to fat-free mass preservation and low blood and tissue cytokine concentration.     

Modeling Glucose Transport From Systemic Circulation to Sweat

Sweat sensing may provide a noninvasive means of estimating blood biomarker levels if a number of technological hurdles can be overcome. This report describes progress on a physiologically based transport model relating sweat glucose and key electrolyte concentrations to those in blood. Iontophoretically stimulated sweat glucose and fasted blood glucose were simultaneously measured in 2 healthy human subjects. Sweat glucose was measured with a novel, prototype skin sweat collection/analysis system and blood glucose with a commercial fingerstick glucometer. These data, in combination with data from 3 published studies, were used to calibrate a dynamic mathematical model for glucose transport and uptake in human skin, followed by extraction into sweat. Model simulations revealed that experimental and literature sweat glucose values were well represented under varying physiologic conditions. The glucose model, calibrated under a variety of experimental conditions including electrical enhancement, revealed a 10 min blood-to-sweat lag time and a sweat/blood glucose level ranging from 0.001 to 0.02, depending on the sweat rate. These values are consistent with those reported in the literature. The developed model satisfactorily described the sweat-to-blood relationship for glucose concentrations measured under different conditions in 4 human studies including the present pilot study. The algorithm may be used to facilitate sweat biosensor development.     

Dual effect of isoprenaline on glucose transport and response to insulin in isolated adipocytes

Glucose transport as assessed by the uptake rate of 3-O-methylglucose was stimulated in isolated rat fat cells by preincubation with isoprenaline or orciprenaline. The effect was apparently mediated by β₁-receptors, since (1) it was abolished by propranolol, (2) it closely paralleled the stimulation of lipolysis, and (3) isoprenaline was 10² times more potent that orciprenaline. Isoprenaline enhanced the effect of submaximal insulin concentrations as well as the basal transport rate but failed to increase the maximal effect of insulin. The stimulatory effect of isoprenaline was antagonized by adenosine deaminase which removes adenosine spontaneously released from the cells, and by bordetella toxin (IAP) which blocks the inhibitory coupling component of adenylate cyclase. Moreover, bordetella toxin uncovered an inhibitory effect of isoprenaline on insulin stimulated glucose transport. There was no apparent correlation between the effects on glucose transport and the response of cellular cyclic AMP levels to the agents investigated. It is suggested that a step in the coupling of β-receptors and adenylate cyclase, but not total cellular cyclic AMP levels, may mediate stimulatory as well as inhibitory effects of catecholamines on glucose transport in the adipocyte.