Glucose utilization in rat adipocytes. The interaction of transport and metabolism as affected by insulin

An Updated Organ-Based Multi-Level Model for Glucose Homeostasis: Organ Distributions, Timing, and Impact of Blood Flow

Glucose homeostasis is the tight control of glucose in the blood. This complex control is important, due to its malfunction in serious diseases like diabetes, and not yet sufficiently understood. Due to the involvement of numerous organs and sub-systems, each with their own intra-cellular control, we have developed a multi-level mathematical model, for glucose homeostasis, which integrates a variety of data. Over the last 10 years, this model has been used to insert new insights from the intra-cellular level into the larger whole-body perspective. However, the original cell-organ-body translation has during these years never been updated, despite several critical shortcomings, which also have not been resolved by other modeling efforts. For this reason, we here present an updated multi-level model. This model provides a more accurate sub-division of how much glucose is being taken up by the different organs. Unlike the original model, we now also account for the different dynamics seen in the different organs. The new model also incorporates the central impact of blood flow on insulin-stimulated glucose uptake. Each new improvement is clear upon visual inspection, and they are also supported by statistical tests. The final multi-level model describes >300 data points in >40 time-series and dose-response curves, resulting from a large variety of perturbations, describing both intra-cellular processes, organ fluxes, and whole-body meal responses. We hope that this model will serve as an improved basis for future data integration, useful for research and drug developments within diabetes.

Functional characterization and tissue localization of the facilitative glucose transporter GLUT12

Facilitative glucose transporters (GLUTs) play crucial roles in glucose utilization and homeostasis. GLUT12 was initially isolated as a novel GLUT4-like transporter involved in insulin-dependent glucose transport. However, tissue distribution and biochemical properties of GLUT12 are not well understood. In this study, we investigated the basic kinetic properties and tissue distribution of GLUT12. Human GLUT12 and GLUT1 were overexpressed and purified using Ni-NTA column chromatography. Reconstituted proteoliposomes showed time-dependent d-glucose transport activity, which was inhibited by phloretin and dehydroascorbate. Dose dependence of glucose transport revealed a KM and Vmax values of 6.4 mM and 1.2 μmol/mg/min, respectively, indicating that GLUT12 is a high-affinity type GLUT. Glucose transport by GLUT12 was inhibited by ATP and glucose-1-phosphate, glucose-6-phosphate and disaccharides (properties similar to those of GLUT1). Indirect immunohistochemistry revealed the distribution of mouse GLUT12 in the apical region of distal tubules and collecting ducts in the kidney and epithelial cells of the jejunum. In addition to these cells, GLUT12 was present in chromaffin cells in the adrenal medulla, the anterior pituitary lobe, as well as the thyroid and pyloric glands. These tissue distributions suggest a unique function of GLUT12, besides that of an insulin-dependent glucose transport.

Effect of tyramine, a dietary amine, on glycerol and lactate release by isolated adipocytes from old rats

Amine degradation by adipocyte amine oxidases leads to the production of metabolites that interact with lipid and glucose metabolisms and their hormonal regulations. To further investigate these interactions, we determined the effect of a dietary amine, tyramine (TYR), on glycerol and lactate releases, respectively taken as indices of lipolytic and glycolytic activities of isolated adipocytes. Old male Wistar rats were used to prepare adipocytes by collagenase dissociation of retroperitoneal fat pads. The two tested doses of tyramine (10 microM and 1 mM) had no effect on basal glycerol release. On the other hand, TYR, at the highest dose tested (1 mM), weakly but significantly increased basal lactate release, which was elevated in adipocytes from old rats. Norepinephrine (NE), highly stimulated adipocyte lipolysis with a submaximal effect at 1 microM which was slightly but significantly inhibited by TYR 1 mM. Insulin 1 nM (INS) also poorly inhibited the NE-stimulated lipolysis in adipocytes isolated from old rats. TYR was able to potentiate the poor antilipolytic efficiency of INS. Under similar conditions, a high dose of NE greatly reduced lactate production and TYR (1 mM) reversed this inhibition of lactate release. INS was also able to totally reverse the inhibitory effect of NE on lactate release, but there was no potentiation between insulin and tyramine effects. It can be concluded that high doses of TYR interact with norepinephrine and insulin, at least on the control of glycerol and lactate release, by counteracting catecholamine effects and by mimicking insulin actions.

Fiber type-specific determinants of Vmax for insulin-stimulated muscle glucose uptake in vivo

The aim of this study was to determine barriers limiting muscle glucose uptake (MGU) during increased glucose flux created by raising blood glucose in the presence of fixed insulin. The determinants of the maximal velocity (V(max)) of MGU in muscles of different fiber types were defined. Conscious rats were studied during a 4 mU x kg(-1) x min(-1) insulin clamp with plasma glucose at 2.5, 5.5, and 8.5 mM. [U-(14)C]mannitol and 3-O-methyl-[(3)H]glucose ([(3)H]MG) were infused to steady-state levels (t = -180 to 0 min). These isotope infusions were continued from 0 to 40 min with the addition of a 2-deoxy-[(3)H]glucose ([(3)H]DG) infusion. Muscles were excised at t = 40 min. Glucose metabolic index (R(g)) was calculated from muscle-phosphorylated [(3)H]DG. [U-(14)C]mannitol was used to determine extracellular (EC) H(2)O. Glucose at the outer ([G](om)) and inner ([G](im)) sarcolemmal surfaces was determined by the ratio of [(3)H]MG in intracellular to EC H(2)O and muscle glucose. R(g) was comparable at the two higher glucose concentrations, suggesting that rates of uptake near V(max) were reached. In summary, by defining the relationship of arterial glucose to [G](om) and [G](im) in the presence of fixed hyperinsulinemia, it is concluded that 1) V(max) for MGU is limited by extracellular and intracellular barriers in type I fibers, as the sarcolemma is freely permeable to glucose; 2) V(max) is limited in muscles with predominantly type IIb fibers by extracellular resistance and transport resistance; and 3) limits to R(g) are determined by resistance at multiple steps and are better defined by distributed control rather than by a single rate-limiting step.

Coordinate regulation of L-arginine uptake and nitric oxide synthase activity in cultured endothelial cells

Despite intracellular L-arginine concentrations that should saturate endothelial nitric oxide synthase (eNOS), nitric oxide production depends on extracellular L-arginine. We addressed this 'arginine paradox' in bovine aortic endothelial cells by simultaneously comparing the substrate dependence of L-arginine uptake and intracellular eNOS activity, the latter measured as L-[3H]arginine conversion to L-[3H]citrulline. Whereas the Km of eNOS for L-arginine was 2 microM in cell extracts, the L-arginine concentration of half-maximal eNOS stimulation was increased to 29 microM in intact cells. This increase likely reflects limitation by L-arginine uptake, which had a Km of 108 microM. The effects of inhibitors of endothelial nitric oxide synthesis also suggested that extracellular L-arginine availability limits intracellular eNOS activity. Treatment of intact cells with the calcium ionophore A23187 reduced the L-arginine concentration of half-maximal eNOS activity, which is consistent with a measured increase in L-arginine uptake. Increases in eNOS activity induced by several agents were closely correlated with enhanced L-arginine uptake into cells (r = 0.89). The 'arginine paradox' may be explained in part by regulated L-arginine uptake into a compartment, probably represented by caveolae, that contains eNOS and that is distinct from the bulk cytosolic L-arginine.

Network thermodynamics and complexity: a transition to relational systems theory

Most systems of interest in today's world are highly structured and highly interactive. They cannot be reduced to simple components without losing a great deal of their system identity. Network thermodynamics is a marriage of classical and non-equilibrium thermodynamics along with network theory and kinetics to provide a practical framework for handling these systems. The ultimate result of any network thermodynamic model is still a set of state vector equations. But these equations are built in a new informative way so that information about the organization of the system is identifiable in the structure of the equations. The domain of network thermodynamics is all of physical systems theory. By using the powerful circuit simulator, the Simulation Program with Integrated Circuit Emphasis (SPICE), as a general systems simulator, any highly non-linear stiff system can be simulated. Furthermore, the theoretical findings of network thermodynamics are important new contributions. The contribution of a metric structure to thermodynamics compliments and goes beyond other recent work in this area. The application of topological reasoning through Tellegen's theorem shows that a mathematical structure exists into which all physical systems can be represented canonically. The old results in non-equilibrium thermodynamics due to Onsager can be reinterpreted and extended using these new, more holistic concepts about systems. Some examples are given. These are but a few of the many applications of network thermodynamics that have been proven to extend our capacity for handling the highly interactive, non-linear systems that populate both biology and chemistry. The presentation is carried out in the context of the recent growth of the field of complexity science. In particular, the context used for this discussion derives from the work of the mathematical biologist, Robert Rosen.

Functional limitations to glucose uptake in muscles comprised of different fiber types

Skeletal muscle glucose uptake requires delivery of glucose to the sarcolemma, transport across the sarcolemma, and the irreversible phosphorylation of glucose by hexokinase (HK) inside the cell. Here, a novel method was used in the conscious rat to address the roles of these three steps in controlling the rate of glucose uptake in soleus, a muscle comprised of type I fibers, and two muscles comprised of type II fibers. Experiments were performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed, constant infusions of 3-O-methyl-[3H]glucose (3-O-MG) and [1-14C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration, which distributes based on the transsarcolemmal glucose gradient (TSGG), were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in muscle. Muscle glucose uptake was much lower in muscle comprised of type II fibers than in soleus under both basal and insulin-stimulated conditions. Under all conditions, the TSGG in type II muscle exceeded that in soleus, indicating that glucose transport plays a more important role to limit glucose uptake in type II muscle. Although hyperinsulinemia increased [G](im) in soleus, indicating that phosphorylation was a limiting factor, type II muscle was limited primarily by glucose delivery and glucose transport. In conclusion, the relative importance of glucose delivery, transport, and phosphorylation in controlling the rate of insulin-stimulated muscle glucose uptake varies between muscle fiber types, with glucose delivery and transport being the primary limiting factors in type II muscle.

Analysis of insulin-stimulated skeletal muscle glucose uptake in conscious rat using isotopic glucose analogs

An isotopic method was used in conscious rats to determine the roles of glucose transport and the transsarcolemmal glucose gradient (TSGG) in control of basal and insulin-stimulated muscle glucose uptake. Rats received an intravenous 3-O-[3H]methylglucose (3-O-[3H]MG) infusion from -100 to 40 min and a 2-deoxy-[3H]glucose infusion from 0 to 40 min to calculate a glucose metabolic index (Rg). Insulin was infused from -100 to 40 min at rates of 0.0, 0.6, 1.0, and 4.0 mU.kg-1.min-1, and glucose was clamped at basal concentrations. The ratios of soleus intracellular to extracellular 3-O-[3H]MG concentration and soleus glucose concentrations were used to estimate the TSGG using principles of glucose counter-transport. Tissue glucose concentrations were compared in well-perfused, slow-twitch muscle (soleus) and poorly perfused, fast-twitch muscle (vastus lateralis, gastrocnemius). Data show that 1) small increases in insulin increase soleus Rg without decreasing TSGG, suggesting that muscle glucose delivery and phosphorylation can accommodate the increased flux; 2) due to a limitation in soleus glucose phosphorylation and possibly delivery, insulin at high physiological levels decreases TSGG, and at supraphysiological insulin levels the TSGG is not significantly different from 0; 3) maximum Rg is maintained even though TSGG decreases with increasing insulin levels, indicating that glucose transport continues to increase and is not rate limiting for maximal insulin-stimulated glucose uptake; and 4) muscle consisting of fast-twitch fibers that are poorly perfused exhibits a 35-45% fall in tissue glucose with insulin, suggesting that glucose delivery is a major limitation in sustaining the TSGG. In conclusion, control of glucose uptake is distributed between glucose transport and factors that determine the TSGG. Insulin stimulation of glucose transport increases the demands on the factors that maintain glucose delivery to the muscle membrane and glucose phosphorylation inside the muscle.

Glycogen: a carbohydrate source for GLUT-1 glycosylation during glucose deprivation of 3T3-L1 adipocytes

In 3T3-L1 adipocytes, the glycosylation of the GLUT-1 transporter is altered beyond 12 h of glucose deprivation. To determine whether glycogen degradation provides substrate for normal protein glycosylation during this delay, we measured the glycogen content of 3T3-L1 adipocytes. From an initial value of 0.537 +/- 0.097 mumol glucose/10(6) cells, glycogen was depleted in a time-dependent manner in response to glucose deprivation, exhibiting a half-time of 6 h. Surprisingly, fructose did not prevent glycogen depletion. However, in such glycogen-depleted adipocytes, the alteration of GLUT-1 glycosylation in response to glucose deprivation was more rapid than in normal adipocytes. Chinese hamster ovary (CHO) cells, which synthesize abbreviated dolichol-linked oligosaccharides within minutes of glucose deprivation (J. I. Rearick, A. Chapman, and S. Kornfeld. J. Biol. Chem. 256: 6255-6261, 1981), contained only 1% of the level of glycogen found in 3T3-L1 adipocytes. Glycosylation of GLUT-1 was altered in CHO cells within 3 h of glucose deprivation. These data demonstrate that, during glucose stress, glycogen may serve as a buffer for oligosaccharide biosynthesis and provide a potential explanation for varying sensitivities of different cell types to glucose deprivation.

Coupled glucose transport and metabolism in cultured neuronal cells: determination of the rate-limiting step

In brain and nerves the phosphorylation of glucose, rather than its transport, is generally considered the major rate-limiting step in metabolism. Since little is known regarding the kinetic coupling between these processes in neuronal tissues, we investigated the transport and phosphorylation of [2-3H]glucose in two neuronal cell models: a stable neuroblastoma cell line (NCB20), and a primary culture of isolated rat dorsal root ganglia cells. When transport and phosphorylation were measured in series, phosphorylation was the limiting step, because intracellular glucose concentrations were the same as those outside of cells, and because the apparent Km for glucose utilization was lower than expected for the transport step. However, the apparent Km was still severalfold higher than the Km of hexokinase I. When [2-3H]glucose efflux and phosphorylation were measured from the same intracellular glucose pool in a parallel assay, rates of glucose efflux were three- to-fivefold greater than rates of phosphorylation. With the parallel assay, we observed that activation of glucose utilization by the sodium channel blocker veratridine caused a selective increase in glucose phosphorylation and was without effect on glucose transport. In contrast to results with glucose, both cell types accumulated 2-deoxy-D-[14C]glucose to concentrations severalfold greater than extracellular concentrations. We conclude from these studies that glucose utilization in neuronal cells is phosphorylation-limited, and that the coupling between transport and phosphorylation depends on the type of hexose used.

Persistence of disturbed adipocyte metabolism in streptozocin-induced diabetic rats despite near-euglycemia with phlorizin

It is widely accepted that hyperglycemia per se incites and perpetuates the diabetic state by adverse effects on beta cell insulin secretion and peripheral insulin action. Examination of the latter locus has revealed glucose-related abnormalities in facilitated glucose transport. Beyond the plasma membrane, however, there is scant data examining whether hyperglycemia influences important intracellular metabolic events. We recently described a sizable reduction in post-transport, in situ metabolism in permeabilized fat cells from streptozocin-induced diabetic rats. Of importance, the diabetes-related deficit was entirely ameliorated by insulin therapy. In this study we examined whether hyperglycemia per se contributes to this altered intracellular metabolic effect. By infusing phlorizin, near euglycemia was achieved for at least four days in streptozocin-induced diabetic rats. The phlorizin-treated diabetic rats had improved (intact cell) rates of insulin-stimulated 2-deoxyglucose uptake. Despite this, permeabilized fat cell studies revealed no improvement or deterioration in diabetic intracellular metabolism as measured by both the oxidation of [6-14C]glucose-6-phosphate via the citric acid cycle or its incorporation into triglyceride. We conclude that hypoinsulinemia, and not hyperglycemia, mediates the disturbance in porous diabetic adipocyte cellular metabolism.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

The one-site model of human erythrocyte glucose transport: testing its predictions using network thermodynamic computer simulations

Network thermodynamic computer simulations were carried out using parameters experimentally derived by Lowe and Walmsley ((1987) Biochim. Biophys. Acta 903, 547-550) for two tests of the one-site model of human erythrocyte glucose transport. In the temperature-jump experiment, the simulations predicted the amplitude and relaxation time of accelerated uptake, but underestimated the net uptake due to an unexpectedly low measured basal rate. In the maltose-acceleration experiment, the dissociation constant of maltose was assessed at 0 degrees C by measuring the inhibitory effects of maltose on both cytochalasin B binding and on 3-O-methylglucose uptake, and using this value (52 mM) to calculate the dissociation constant (2.9 mM). The simulated experiment then did show a transient acceleration in uptake comparable in magnitude to that observed experimentally, except that the relaxation time was more than 10-fold longer in the simulations. Measurements of the temperature dependence of the inhibition of cytochalasin B binding by maltose and 3-O-methylglucose indicated that apparent sugar affinity is sensitive to carrier orientation at low temperatures, whereas at more physiologic temperatures the intrinsic dissociation constant predominated.

Effect of insulin and adrenergic agonists on glucose transport of porcine adipocytes

1. Insulin increased basal 2-deoxyglucose uptake in isolated swine adipocytes by 75%. In the absence of insulin, isoproterenol did not inhibit basal 2-deoxyglucose transport. 2. Adenosine deaminase plus isoproterenol or theophylline alone reduced insulin effect by 10 and 40%, respectively. Isoproterenol alone or with 2-chloroadenosine did not inhibit insulin effect on glucose transport activity. 3. Insulin effect was inhibited by isoproterenol in the presence of theophylline but not in the presence of adenosine deaminase. 4. These results suggest that catecholamines do not counter-regulate basal and insulin-stimulated glucose transport in swine adipocytes.

Stimulation of glucose transport by insulin and norepinephrine in isolated rat brown adipocytes

The effects of insulin and norepinephrine on glucose transport, glucose uptake, and cell respiration were investigated in isolated rat brown adipocytes. Glucose transport and uptake were determined using [U-14C]-D-glucose and 2-deoxy-[1,2-3H]-D-glucose, respectively. Brown adipocyte respiration was measured polarographically. Dose-response experiments revealed that insulin stimulated D-glucose transport and 2-deoxyglucose uptake between 10(-11) and 10(-7) M with a maximal four- to sixfold stimulation. In the absence of insulin, norepinephrine concentrations ranging from 10(-7) to 10(-7) M also enhanced glucose transport and uptake with a maximal two- to fourfold stimulation. Experiments with alpha- and beta-adrenergic agonists and antagonists showed that the effect of norepinephrine was predominantly mediated via beta-adrenergic pathways. Dibutyryl cyclic AMP and 3-isobutyl-1-methylxanthine also increased glucose transport, suggesting that the effects of norepinephrine are cyclic AMP dependent. Moreover, norepinephrine (10(-8) M) enhanced insulin sensitivity for glucose transport [half-maximum velocity constant (1/2 V max)] but failed to potentiate insulin responsiveness (Vmax). On the other hand, insulin (10(-9) M) had no effect on basal respiration but rapidly inhibited the calorigenic effect of norepinephrine (10(-7) M) by greater than 50%. These results demonstrate that 1) in the absence of insulin, physiological concentrations of norepinephrine stimulate glucose transport via beta-adrenergic pathways, 2) the neurohormone synergistically potentiates brown adipocyte submaximal insulin responses for glucose transport, and 3) insulin counteracts the effects of norepinephrine on brown adipocyte thermogenesis despite the fact that both hormones enhance glucose uptake.

Network thermodynamic analysis and stimulation of isotonic solute-coupled volume flow in leaky epithelia: an example of the use of network theory to provide the qualitative aspects of a complex system and its verification by stimulation

A network thermodynamic model was developed to provide insights into the nature of isotonic solute-coupled volume flow in "leaky" epithelia, where the transepithelial volume flow is assumed to be primarily through the cellular pathway. The coupled flows of solute and volume at each membrane in this four membrane model are described by the practical phenomenological equations as developed by Kedem & Katchalsky (1958). The model contains one permeable non-electrolyte solute (s) and a fixed amount of an impermeable non-electrolyte (i) inside the cell. The cell is assumed to be capable of volume regulation under the steady-state experimental conditions simulated. A solute-pump, located in the basolateral membrane, uses feedback regulation to adjust Cs in the cell in order to maintain cell volume at or near control levels in all simulations. Model behavior is, in general, very consistent with experimental observations with respect to tonicity and magnitude of volume flow over a wide range of experimental conditions. Examination of the parameter space suggests the following important features when isotonic solute-coupled volume flow moves primarily through the cellular pathway: (1) the apical membrane reflection coefficient must be less than that of the basolateral membrane; (2) the basement membrane reflection coefficient must be small; (3) the apical membrane solute permeability and reflection coefficient are the two most "sensitive" parameters and need to vary in an inverse manner in order to maintain isotonicity when both solute and volume flows increase; and (4) relationships (1) and (3) above imply the need for at least two separate solute pathways in the apical membrane, one that is shared with volume flow and one that is not.

Cold exposure potentiates the effect of insulin on in vivo glucose uptake

The effects of cold exposure (48 h at 4 degrees C) and insulin injection (0.5 U/kg iv) on the rates of net 2-[3H]deoxyglucose uptake (Ki) in peripheral tissues were investigated in warm-acclimated rats (25 degrees C). Cold exposure and insulin treatment independently increased Ki values in skeletal muscles (soleus, extensor digitorum longus, and vastus lateralis), heart, white adipose tissue (subcutaneous, gonadal, and retroperitoneal), and brown adipose tissue (P less than 0.01). The effects of cold exposure were particularly evident in brown adipose tissue where the Ki increased greater than 100 times. When the two treatments were combined (insulin injection in cold-exposed rats), it was found that cold exposure synergistically enhanced the maximal insulin responses for glucose uptake in brown adipose tissue, all white adipose tissue depots, and skeletal muscles investigated. The results indicate that cold exposure induces an "insulin-like" effect on Ki that does not appear to be specifically associated with shivering thermogenesis in skeletal muscles, because that effect was observed in all insulin-sensitive tissues. The data also demonstrate that cold exposure significantly potentiates the maximal insulin responses for glucose uptake in the same tissues. This potentialization may result from an enhanced responsiveness of peripheral tissues to insulin, possibly occurring at metabolic steps lying beyond the insulin receptor and an increased tissue blood flow augmenting glucose and insulin availability and thereby amplifying glucose uptake.

Exploration of apical sodium transport mechanisms in an epithelial model by network thermodynamic simulation of the effect of mucosal sodium depletion: I. Comparison of three different apical sodium permeability expressions

A model based on that of Koefoed-Johnsen & Ussing (1958) and elaborated by Hviid Larsen (1978) and Lew et al. (1979), is designed using network thermodynamic theory and used to simulate experiments performed on epithelia. Three different expressions for the apical sodium permeability are tested for their ability to reproduce the saturation of the short-circuit current with increasing mucosal sodium concentration. Using the parameters from the previous models, the sodium entry step is shown to be the rate limiting step. If the apical sodium permeability is constant, there is no saturation of the short-circuit current with increasing mucosal sodium. The saturation of the short-circuit current is simulated with versions of the model which include a variable apical sodium permeability. The phenomenological expressions used for the variable permeabilities are those proposed by Fuchs et al. (1977) and Civan & Bookman (1982). They describe the so-called feedback effect of the mucosal and intracellular sodium concentrations.

Testing models of insulin binding in rat adipocytes using network thermodynamic computer simulations

Many different models have been proposed to explain the complex binding behavior of insulin to its receptor, but a systematic comparison of models with experimental results is lacking. We have used network thermodynamic computer simulations to compare models of insulin binding against the results of several experimental tests designed to differentiate between the models. Six models of insulin binding were tested (simple, diffusion-reaction, conversion, dissociation, heterogeneous site, and two-step intramembrane) against results reported in the literature for isolated rat adipocytes. Although still a matter of experimental controversy, the criteria selected for modeling were curvilinear Scatchard plots, bi-or multi-exponential dissociation, insulin-accelerated dissociation, lack of dependence of the overall dissociation constant on receptor number, and receptor reserve. Using a given set of parameter values most appropriate for each model, none was able to account for all of the observed experimental results. This indicates both the complexity of the binding reaction and the need for further model development. The approach of using computer simulations to systematically test models against experimental results affords not only insight into the critical features of a model enabling it to pass a test, but also indicates potential experiments which might differentiate between models.

The regulation of mononuclear phagocyte entry into S phase by the colony stimulating factor CSF-1

CSF-1 is a hemopoietic growth factor that specifically regulates the survival, proliferation, and differentiation of mononuclear phagocytic cells. Populations of adherent bone marrow-derived macrophages (BMM) devoid of CSF-1 producing cells were used to study regulation by CSF-1 of macrophage entry into S phase. More than 95% of BMM possess the CSF-1 receptor. It was shown that 93-98% of BMM are cycling (S phase 8-9 hr, doubling time 24-28 hr) when cultured in the presence of CSF-1. BMM incubated with 15% FCS in the absence of CSF-1 or in the presence of CSF-1 concentrations inducing survival without proliferation enter a quiescent state. This state is characterized by a reduction in the synthesis of DNA (98%), total protein (35%), ribosomal protein (76%), and histone (96%) compared with the synthetic rate of these components in exponentially growing cells. Addition of CSF-1 to BMM rendered quiescent by removal of CSF-1 stimulated entry into S phase with a lag period of approximately 12 h. This lag period is reduced to 8 hr in BMM made quiescent at concentrations of CSF-1 inducing survival without proliferation, an effect which may be related to the expected higher protein content of these cells (Tushinski and Stanley, J. Cell. Physiol., 116:67-75). Neutralization of CSF-1 by antibody at different times during the lag period indicates that CSF-1 is required for almost the entire lag period for the entry of any cells into S phase. In BMM rendered quiescent by removal of both serum and CSF-1, purified CSF-1 without serum stimulated entry of cells into S phase, whereas serum alone was ineffective. The results are consistent with a primary regulatory role of CSF-1 in mononuclear phagocyte proliferation, survival, and function.

Fluid-phase endocytosis by isolated rat adipocytes

We have developed an assay, which uses radiolabeled sucrose as the marker, to measure the rate of fluid-phase endocytosis in isolated rat adipocytes. In addition, the assay was adapted to allow measurement of the release of sucrose from previously loaded cells (fluid-phase exocytosis). Adipocytes take up sucrose at an approximately linear rate for at least 1.5 hours. A portion of the pinocytosed sucrose is rapidly (half-time about 20 minutes) returned to the medium. The minimal value for fluid uptake by endocytosis is 57 nl/10(6) cells-h at 37 degrees C; this value corresponds to the formation of 110,000 endocytic vesicles of 100-nm diameter per cell per hour and the internalization of about 20% of the plasma membrane per hour. Insulin caused a small and variable increase in the rate of sucrose uptake. The average increase of 31% from 11 experiments is statistically significant at the level of P less than 0.01. A small insulin effect upon the uptake of the calcium complex of [14C]EDTA was also observed. Since this complex was taken up at 2.5 times the rate of sucrose, it probably entered by a combination of fluid-phase and adsorptive pinocytosis. Insulin did not elicit a significant change in the rate of sucrose release from preloaded cells.

Glucose 6-phosphate effects on deoxyglucose, glucose and methylglucose transport in rat adipocytes. Evidence for intracellular regulation of sugar transport by glucose metabolites

This study was undertaken to determine whether glucose 6-phosphate can modulate hexose transport in rat adipocytes. The data demonstrated that glucose 6-phosphate decreased hexose transport rates in adipocytes and in adipose plasma membrane vesicles. This effect was not via competitive inhibition. The data provide direct evidence that hexose transport can be inhibited noncompetitively by glucose 6-phosphate.