Specific D-glucose transport in sarcolemma vesicles

The regulation and importance of glucose uptake in the isolated Atlantic cod heart: rate-limiting steps and effects of hypoxia

This study investigated the regulation of glucose uptake in Atlantic cod(Gadus morhua) hearts. Isolated hearts were perfused with or without glucose in the medium, under either normoxic or severely hypoxic conditions. Working at basal levels, hearts did not require extracellular glucose to maintain power under aerobic conditions. However, cardiac performance was significantly reduced without exogenous glucose under oxygen-limiting conditions. The addition of the glucose transporter inhibitor cytochalasin B caused hypoxic hearts to fail early, and hearts perfused with a glucose analogue, 2-deoxyglucose (2-DG), increased glucose uptake 3-fold under hypoxia. The uptake of 2-DG was only partially inhibited when cytochalasin B was added to the medium. Isolated ventricle strips were also incubated in the presence of 2-DG and the extracellular marker mannitol. Glucose uptake(glucose transport plus intracellular phosphorylation) was assessed by measuring the initial rate of 2-deoxyglucose-6-phosphate (2-DG-6-P)accumulation. At 1 mmol l-1 2-DG, the rate of 2-DG uptake remained linear for 60 min, and 2-DG-6-P, but not free 2-DG, accumulation was increased. The fact that intracellular 2-DG did not increase indicates that glucose transport is the rate-limiting step for glucose utilization in non-stimulated cardiac tissue. Replacement of Na+ by choline in the incubation medium did not affect 2-DG uptake, providing evidence that Na+-coupled glucose transport is absent in cod cardiac tissue. Similar to cytochalasin B, glucose uptake was also inhibited by phloridzin,suggesting that facilitated, carrier-mediated glucose transport occurs in cod hearts. Under the conditions employed in these experiments, it is clear that(1) activation of glucose transport is required to support hypoxic performance, (2) the rate-limiting step for glucose utilization is glucose transport rather than glucose phosphorylation, (3) 2-DG uptake accurately reflects glucose transport activity and (4) glucose uptake in cod hearts does not involve an Na+-dependent mechanism.

Effects of insulin and exercise on amino acid transport in rat skeletal muscle

In the present study, the initial rates of amino acid transport by isolated rat skeletal muscle plasma membrane vesicles were investigated. This approach facilitates the study of the transport of naturally occurring amino acids independent of the effects of cellular metabolism. Alanine and glutamine influxes were measured using a rapid filtration technique. Transport was examined in the presence and absence of Na and the properties of membranes from control, insulin-treated, or acutely exercised rats were studied. Both alanine and glutamine were transported by Na-dependent processes. The values for maximum rate of transport (Vmax) for Na-dependent alanine and glutamine transport were 203 and 224 pmol.mg-1.s-1, respectively. The K1/2 values were 2.9 mM alanine and 1.9 mM glutamine. The Vmax for Na-dependent alanine transport was increased by insulin treatment of the animal and by acute exercise. 2-(Methylamino)-isobutyric acid (MeAIB) partially inhibited the control Na-dependent alanine influx and completely inhibited the increase due to insulin or exercise treatment, indicating the importance of both system A and a non-system A, Na-dependent carrier for alanine transport. The Vmax for Na-dependent MeAIB uptake was also increased by insulin or exercise treatments of the rats. Unlike alanine, Na-dependent glutamine transport was not affected by insulin.

Effect of insulin on D-glucose transport by human placental brush border membranes

We studied the effect of insulin on the uptake of D-glucose by human placental brush border membranes (BBM) in vitro. D-glucose transport through placental BBM is a Na(+)-independent transport, inhibited by 0.5 mM phloretin. Increasing the substrate concentration from 1 to 50 mM resulted in an increase in glucose uptake according to an S-shaped relationship. Hill plot analysis suggests that at least two molecules of D-glucose are transported at the same time by the carrier. Preincubation of the placental tissue with insulin for 45 min at 22 degrees C significantly enhanced the D-glucose influx into the membrane vesicles, without influencing the slope of the Hill plot. A dose-response curve of the effect of insulin revealed that although the effect was already significant at 10(-9) M, the maximal activity was reached at 10(-8) M. The influence of insulin on D-glucose uptake was present only when preincubation of the placental tissue with the hormone was performed in the presence of Mn2+. Incubation of placental tissue with 10(-8) M insulin did not influence D-glucose efflux from the BBM vesicles. Finally, direct incubation of the membranes with insulin had no effect on the glucose influx into these membrane vesicles. We conclude that insulin, at physiological concentrations, enhances glucose uptake by the BBM, and that such a regulation might contribute to the glucose homeostasis in the fetal circulation, independent of the maternal variations in glycemia.

Glucose transport in skeletal muscle membrane vesicles from control and exercised rats

Skeletal muscle responds to exercise by increasing the rate of glucose uptake. Recent studies have indicated that these changes occur via mechanisms modulating the number of transporters in the plasma membrane and/or transporter intrinsic activity. In the present study, a protocol was developed for measuring the initial rate of glucose uptake by rat hindlimb skeletal muscle plasma membrane vesicles. Membranes were isolated from sedentary (control) and acutely exercised rats, and the initial rates of D- and L-glucose influx were assayed under equilibrium exchange conditions to obtain the kinetic constants for carrier-mediated transport. These values were compared with the values for transporter number measured by cytochalasin B binding, and the carrier turnover numbers were calculated. The maximum velocity (Vmax) for carrier-mediated glucose influx was increased 3.7-fold by exercise, from 3.5 nmol.mg protein-1.s-1 for the membranes from control rats to 13 nmol.mg protein-1.s-1 for the membranes from exercised animals. The mean affinity constant (K0.5; approximately 20 mM) was not different between the two groups. The number of transporters in the plasma membrane was increased to a lesser degree, 5.4 to 9.4 pmol/mg protein. As a result, the average carrier turnover number was increased almost twofold by exercise, 719 s-1 in the controls vs. 1,380 s-1 in the exercised rats. These data indicate that the response of glucose transport to exercise involves an increase in the average carrier intrinsic activity as well as a recruitment of transporters to the plasma membrane. Whether the increase in carrier turnover number is due to activation of the transporters or recruitment of a more "active" form of the carrier is unknown.

Reconstitution of the glucose transporter from rat skeletal muscle

As a step in the purification and characterization of the glucose transporter from rat skeletal muscle, we have reconstituted glucose transport activity in liposomes. Plasma membranes were prepared from skeletal muscle which display D-glucose reversible binding of cytochalasin B (10 pmol sites/mg protein; KD = 0.3 microM). Older rats gave a slightly lower specific activity and much lower yield of sites per g muscle than young rats. Glucose transport activity was reconstituted into liposomes by the freeze-thaw procedure using either plasma membranes directly or cholate-extracted membrane proteins; the latter gave a 50% higher specific activity. The reconstituted transport activity was stereospecific, saturable, and inhibited by cytochalasin B, phloretin, and mercuric chloride. The optimum cholate concentration for extraction and reconstitution of transport activity was about 1.5%, and the highest specific activity of reconstituted transport was seen only at low ratios of protein to lipid in the reconstitution. Chromatography on agarose lentil lectin and agarose ethanethiol doubled both the specific activity of reconstituted transport and the fraction of glucose uptake which was stereospecific. In all of these respects the results were similar to our results with the bovine heart transporter (T. J. Wheeler and M. A. Hauck, Biochim. Biophys. Acta 818, 171-182 (1985)). Our findings suggest that further purification procedures developed for the heart transporter may be applicable to the skeletal muscle transporter as well.

Chelation of intracellular calcium blocks insulin action in the adipocyte

The hypothesis that intracellular Ca2+ is an essential component of the intracellular mechanism of insulin action in the adipocyte was evaluated. Cells were loaded with the Ca2+ chelator quin-2, by preincubating them with quin-2 AM, the tetrakis(acetoxymethyl) ester of quin-2. Quin-2 loading inhibited insulin-stimulated glucose transport (IC50, 26 microM quin-2 AM) without affecting basal activity. The ability of insulin to stimulate glucose uptake in quin-2-loaded cells could be partially restored by preincubating cells with buffer supplemented with 1.2 mM CaCl2 and the Ca2+ ionophore A23187. These conditions had no effect on basal activity and omission of CaCl2 from the buffer prevented the restoration of insulin-stimulated glucose uptake by A23187. Quin-2 loading also inhibited insulin-stimulated glucose oxidation (IC50, 11 microM quin-2 AM) and the ability of insulin to inhibit cAMP-stimulated lipolysis (IC50, 78 microM quin-2 AM), without affecting their basal activities. Incubation of cells with 100 microM quin-2 or quin-2 AM had no effect on intracellular ATP concentration or the specific binding of 125I-labeled insulin to adipocytes. These findings suggest that intracellular Ca2+ is an essential component in the coupling of the insulin-activated receptor complex to cellular physiological/metabolic machinery. Furthermore, differing quin-2 AM dose-response profiles suggest the presence of dual Ca2+-dependent pathways in the adipocyte. One involves insulin stimulation of glucose transport and oxidation, whereas the other involves the antilipolytic action of insulin.

Insulin binding and glucose transport in rat skeletal muscle sarcolemmal vesicles

A new method is described for isolation of sarcolemma (SL) from skeletal muscle of rats that produces vesicles of high purity and yield. There was a mean 59-fold purification (n = 22) of the SL marker enzyme K+-p-nitrophenylphosphatase. Specific activities of marker enzymes for sarcoplasmic reticulum and mitochondria were low, indicating minimal contamination. Despite the high purity and low contamination, a relatively high protein yield was achieved (0.43 +/- 0.03 mg/g wet wt, n = 25). Electron microscopy showed that the membranes were primarily vesicles. Specific 125I-insulin binding association constants derived from the high- and low-affinity portion of the Scatchard plots were 0.764 +/- 0.154 and 0.0096 +/- 0.0012 X 10(9) M-1, whereas the apparent number of receptors were 15.0 +/- 4.1 and 925 +/- 80 X 10(9) per mg of SL protein. Equilibrium exchange glucose transport studies at 37 degrees C indicated that the SL vesicles exhibited specific D-glucose transport which was responsive to in vivo insulin stimulation. We conclude that this isolation procedure, especially in light of the high purity and yield, provides a good and practical experimental model for studying insulin binding and glucose transport in skeletal muscle.

The glucose transport system of muscle plasma membranes: characterization by means of [3H]cytochalasin B binding

A membrane-rich preparation was isolated from adult rat skeletal muscle in low salt media and further fractionated in sucrose gradients. Fraction F2, with a relative density of 1.092-1.119, consisted of sealed membrane vesicles which were enriched in plasma membrane markers. These vesicles were capable of stereospecific D-glucose uptake which was sensitive to cytochalasin B (CB). The membranes were also enriched in high affinity [3H]CB binding activity (Kd of 0.28 microM). [3H]CB binding to the glucose carrier of these plasma membranes, estimated as the fraction of binding protectable by D-glucose, ranged between 2.5 and 7.4 pmol/mg protein in several membrane preparations. The amount of [3H]CB binding to muscle membranes from newborn and adult rats was not markedly different. Trypsin, at low concentrations, altered the molecular weight of several membrane components, without affecting [3H]CB binding. Higher concentrations of trypsin abolished [3H]CB binding. Both 2,4-dinitrofluorobenzene (0.1 mM) and N-ethylmaleimide (15 mM) inhibited [3H]CB binding; inhibition by these reagents was prevented by inclusion of micromolar concentrations of CB in the reaction mixture. Several procedures that extracted specific proteins enriched the D-glucose-sensitive [3H]CB binding to the protein-depleted membranes. Antibody raised against the glucose carrier of human red cell membranes cross-reacted with a polypeptide of Mr about 45K of muscle membranes which might represent the glucose carrier.

Solubilization and reconstitution of an amiloride-inhibited sodium transporter from rabbit kidney medulla

An octyl glucoside extract has been formed from rabbit kidney medulla microsomes from which reconstituted proteoliposomes can be formed by lipid addition and dialysis to remove detergent. These proteoliposomes are capable of amiloride-inhibited 22Na+ transport. The amiloride-inhibited Na+ transport process is complete within 10 min and directly proportional to the vesicle protein concentration. Sodium accumulation by the proteoliposomes has been proven to represent transport by the demonstration that all Na+ taken up by the vesicles can be removed by the ionophore nigericin. The process has been shown to be specific for amiloride by the demonstration that the effect of amiloride on Na+ transport could not be reproduced by the similar compound sulfaguanidine nor by pyrazine, 2-pyrazinecarboxamide, 2-pyrazinecarboxylate, or 3-amino-2-pyrazinecarboxylate. The relationship between Na+ uptake into proteoliposomes and Na+ concentration was similar to the relationship between Na+ uptake and concentration observed with medulla microsomes. The concentration of amiloride required for half-maximal inhibition of Na+ uptake into either proteoliposomes or medulla microsomes was also the same. The evidence seems clear that the protein responsible for amiloride-inhibited Na+ transport into rabbit kidney medulla microsomes has been extracted from the membranes and incorporated into purified lipid vesicles.

Preliminary characterization of calcium binding in islet-cell plasma membranes

Preliminary characterization of calcium binding was determined in a highly-enriched islet-cell plasma membrane fraction using a membrane filtration technique. Equilbrium calcium binding was specific, concentration dependent and saturable. Scatchard analysis indicated the existence of more than one class of calcium binding sites. The affinity constants and maximum binding capacities were 1.14 X 10(5) M-1 and 1.2 picomol/microgram protein and 1.17 X 10(3) M-1 and 64.8 picomol/microgram for the high and low affinity sites, respectively. Bound 45Ca2+ was dissociate from the plasma membranes in a biphasic manner in the presence of excess unlabelled calcium.

Amino acid and glucose transport in sarcolemmal vesicles from chick embryo heart

Purified plasma membranes from chick embryo heart are shown to retain several functional properties of intact cardiac cells. (1) Muscarinic cholinergic receptors copurify with previously used cell-surface markers, i.e., (K+-dependent) p-nitrophenylphosphatase and insulin receptors (Paris, S., Fosset, M., Samuel, D. and Ailhaud, G. (1977) J. Mol. Cell. Cardiol. 9, 161-174). (2) Neutral amino acids (L-alanine and alpha-aminoisobutyric acid) are actively transported into these osmotically active vesicles when an Na+ electrochemical gradient is imposed. The affinity and specificity for amino acids are similar to those described for intact cardiac cells. (3) D-Glucose is taken up more rapidly than L-glucose. The sterospecific transport system is saturable and Na+-independent. The Km value is close to that observed in intact cells with glucose analogues.

Stereospecific D-glucose transport in mixed membrane and plasma membrane vesicles derived from cultured chick embryo fibroblasts

Mixed membrane vesicles prepared from cultured chick embryo fibroblasts possess a stereospecific D-glucose transport system, the properties of which are identical to those of the system in intact cells. Uptake of D-glucose proceeds without chemical alteration. The rate of stereospecific uptake of D-glucose into the mixed vesicles is 70% greater than that of the homogenate and uptake is directly proportional to membrane protein concentration. Stereospecific D-glucose uptake appears linear for 0.3 min, reaches a maximum at 2--5 min, and declines to zero by 5 h as L-glucose enters the vesicles. Uptake is osmotically sensitive and inhibited by cytochalasin B (Ki = 0.13 microM) and the structural analogues of D-glucose : D-mannose, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, D-galactose and maltose, but not by sucrose of L-glucose. Uphill counterflow can be demonstrated and the apparent activation energy displays a transition from 47.7 kcal/mol below 11 degrees C to 18.1 kcal/mol above 11 degrees C. Stereospecific uptake rates of mixed vesicles prepared from Rous sarcoma virus-transformed cells are increased 30% over control values, and are increased 66% in vesicles derived from cells incubated for 24 h in glucose-free medium. Plasma membrane vesicles prepared from these cells by a dextran cushion centrifugation procedure display a 9-fold increase in the specific activity of stereospecific D-glucose uptake relative to the homogenate. Extraction of these membranes with dimethylmaleic anhydride (5 mg/mg protein) results in substantial or complete removal of major polypeptides of molecular weight 40 000, 55 000, 75 000, 78 000 and 200 000 with no loss in total uptake activity. Following extraction, major polypeptides of molecular weight 28 000, 33 000 and 68 000 remain in the membrane residue.

Direct addition of insulin inhibits a high affinity Ca2+-ATPase in isolated adipocyte plasma membranes

The mechanism by which insulin regulates cellular metabolism remains unknown although indirect evidence suggests that alterations in intracellular calcium are important. More specifically, it has been proposed that insulin triggers an increase in intracellular calcium which is responsible for the subsequent modification of metabolic activities. The cell maintains a large electrochemical gradient for ionised calcium between the cytoplasm (less than 10(-6) M, as determined for muscle and nerve) and the extracellular environment (less than 10(-3) M). The plasma membrane may, therefore, be important in the regulation of calcium homeostasis, as a slight alteration in the processes maintaining this gradient could result in marked changes in cytoplasmic calcium. One such process is the active extrusion of calcium from the cell by a high affinity calcium-stimulated ATPase (Ca2+-ATPase). Such a mechanism has been well established in red cells and is postulated in nerve, liver and muscle. We have identified a high affinity Ca2+-ATPase in a plasma membrane-enriched subcellular fraction isolated from rat adipocytes which may provide the enzymatic basis for a calcium extrusion pump. We demonstrate here that the Ca2+-ATPase is specifically inhibited by the direct addition of physiological concentrations of insulin to the direct addition of physiological concentrations of insulin to the isolated plasma membranes. This effect suggests that direct regulation of calcium homeostasis may represent an important event in the mechanism of action of insulin.