In vivo glucose utilization by individual tissues during nonlethal hypermetabolic sepsis

Febrile sepsis was induced in rats by repeated s.c. injections of live Escherichia coli bacteria. Glucose utilization of different tissues was investigated in vivo by using the 2-deoxyglucose tracer technique. In septic rats the rate of glucose utilization was increased in macrophage-rich tissues, including the liver (2.7-fold), spleen (2.4-fold), and ileum (1.6-fold), compared with tissues from time-matched nonseptic animals. A smaller increase in glucose utilization was evident in the abdominal muscle (1.3-fold) and in the white portion of the quadriceps muscle (1.3-fold). Changes were not significant in nine other tissues, including the brain. We postulate that in sepsis the mononuclear phagocyte system may be responsible for most of the increment of glucose utilization, and the latter provides metabolic support for the increased antibacterial activity of these cells.     

Upregulation of glucose metabolism by granulocyte-monocyte colony-stimulating factor

Alterations of glucose metabolism were investigated for 6 hours following an intraarterial injection of murine recombinant granulocyte-monocyte colony-stimulating factor (GM-CSF) (30 micrograms/kg body weight). GM-CSF resulted in a transient elevation of plasma glucose. The rate of whole body glucose appearance, as measured by infusion of [6-3H] glucose, was increased by about 10% between 0.5 and 3 hours following GM-CSF injection. In vivo glucose utilization of individual tissues was investigated by the tracer 2-deoxyglucose technique. At 30 min, GM-CSF increased glucose utilization by 80-90% in liver and lung, and 50-60% in skin and spleen. At 3 and 6 hours, glucose utilization by these tissues returned toward control levels except for lung. There was a 40-50% increase in glucose utilization by skeletal muscle 30 min after GM-CSF which was sustained for 6 hours. Glucose utilization of testis, ileum and kidney did not change significantly. Plasma concentrations of insulin, glucagon and tumor necrosis factor were not altered in response to GM-CSF. These findings indicate that some of the acute metabolic effects of a short-term administration of GM-CSF are observed in macrophage-rich tissues, and suggest that GM-CSF may be involved in the metabolic upregulation of immunologically active tissues.     

Evidence for an inducible glucose transport system in Kluyveromyces lactis.

To find the cause of delayed glucose oxidation in succinate-grown Kluyveromyces lactis, glucose transport was studied in glucose- and in succinate-grown cells. The initial rate of 2-deoxyglucose (2-dGlc) accumulation, as well as the appearance of 2-deoxyglucose 6-phosphate, was higher in the glucose-grown cells. In both cell types, 2-dGlc was apparently transported in the free form to be phosphorylated intracellularly. In glucose-grown cells the level of free 2-dGlc in the pool was always less than the external concentration. Exchange transport in starved, poisoned cells loaded with unlabeled 2-dGlc was 140-fold greater in glucose- than in succinate-grown cells, probably beacuse of the presence of an inducible transport component. The development of the increased rate of transport in a succinate-grown uracil-requiring auxotroph after transfer to glucose depends on the presence of uracil.     

Tissue glucose utilization during epinephrine-induced hyperglycemia

The aim of this study was to investigate glucose utilization by individual tissues during epinephrine infusion. First, the applicability of the 2-deoxyglucose (2-DG) tracer technique during in vivo hyperglycemia was investigated in model systems in vitro. Epitrochlearis muscle and spleen cells were incubated with 1.25-20 mM glucose. The discrimination against 2-[14C]DG in glucose metabolic pathways, expressed by the lumped constant, remained unchanged over this wide range of glucose concentrations. It was concluded that in vivo hyperglycemia does not preclude the application of the 2-DG method. In a series of in vivo experiments, chronically catheterized conscious rats fasted for 24 h and were infused with epinephrine (0.2 microgram.kg-1.min-1), which produced a two-fold increase in plasma glucose concentration. 2-[14C]DG was injected 30 min after starting the epinephrine infusion and glucose utilization rates of individual tissues were calculated based on the concentration of phosphorylated 2-DG in samples excised at 70 min. The epinephrine infusion increased glucose utilization rates by 40-160% in hindlimb muscles, skin, ileum, liver, spleen, lung, epididymal fat, and kidney, although no change was found in the brain. Mass action of the increased plasma glucose is likely to play an important role in the enhanced rate of glucose utilization.     

Evidence for an inducible glucose transport system in Kluyveromyces lactis

To find the cause of delayed glucose oxidation in succinate-grown Kluyveromyces lactis, glucose transport was studied in glucose- and in succinate-grown cells. The initial rate of 2-deoxyglucose (2-dGlc) accumulation, as well as the appearance of 2-deoxyglucose 6-phosphate, was higher in the glucose-grown cells. In both cell types, 2-dGlc was apparently transported in the free form to be phosphorylated intracellularly . In glucose-grown cells the level of free 2-dGlc in the pool was always less than the external concentration. Exchange transport in starved, poisoned cells loaded with unlabeled 2-dGlc was 140-fold greater in glucose- than in succinate-grown cells, probably because of the presence of an inducible transport component. The development of the increased rate of transport in a succinate-grown uracil-requiring auxotroph after transfer to glucose depends on the presence of uracil.     

Interleukin-1 induced increases in glucose utilization are insulin mediated

Interleukin-1 (IL-1) is known to modulate a variety of the acute-phase responses to infection. Since an enhanced rate of whole-body glucose utilization is a consistent feature of the hypermetabolic phase of infection, the purpose of the present study was to determine whether IL-1 could increase glucose uptake and whether that increase was dependent on the concomitant elevation in plasma insulin. Glucose utilization (Rg) of different tissues was investigated in vivo by the 2-deoxyglucose tracer technique. Human purified IL-1 was administered to chronically, catheterized conscious rats and increased the plasma insulin levels and the Rg in macrophage-rich tissues, including the lung, spleen, liver and skin. IL-1 also increased Rg in skeletal muscle and diaphragm. To eliminate the insulin-stimulated increase in Rg, somatostatin (SRIF) was infused 1 h prior to IL-1. SRIF prevented the IL-1 induced increase in insulin and tissue glucose utilization. IL-1 administration to streptozotocin-induced diabetic rats also failed to increase Rg in any tissue examined. These data suggest that the administration of IL-1 increases organ glucose utilization by insulin-dependent mechanisms.     

Kupffer cells play a major role in insulin-mediated hepatic glucose uptake in vivo.

The effect of insulin on the in vivo glucose utilization by different hepatic cells was investigated using the euglycemic, hyperinsulinemic clamp, combined with the 2-deoxyglucose tracer technique. Rats were infused with insulin at a rate of 2.8 or 9.0 mU/min/kg for 220 min, resulting in plasma concentrations of the hormone of about 80 microU/ml and 340 microU/ml, respectively. Glucose use by the whole liver was elevated by more than 200% following insulin. However, glucose uptake by the parenchymal cells was only elevated by 50-60%. By contrast nonparenchymal cells were more responsive to insulin. Glucose uptake by endothelial cells was increased 100% and Kupffer cells displayed the most marked response to insulin showing a 3- to 6-fold increase in glucose uptake. These data indicate that the sinusoidal nonparenchymal cells are the major sites of the insulin-mediated increased glucose utilization by the liver.     

Rates and tissue sites of noninsulin- and insulin-mediated glucose uptake in diabetic rats.

The purpose of the present study was to determine whether streptozotocin-induced diabetes alters the rates and tissue distribution of insulin-mediated glucose uptake (IMGU) and noninsulin-mediated glucose uptake (NIMGU). In vivo glucose disposal was assessed using the tracer [U-14C]-2-deoxyglucose technique in chronically catheterized conscious rats. For nondiabetic animals, rates of NIMGU were determined during severe insulinopenia (less than 5 microU/ml), induced by the infusion of somatostatin, under both euglycemic (6 mM) and hyperglycemic (17 mM) conditions. In diabetic rats, in which a severe insulin deficiency already existed, NIMGU was determined under basal hyperglycemic conditions and during euglycemic conditions produced by inhibiting hepatic glucose output. IMGU was determined in both groups using the euglycemichyperinsulinemic clamp technique. Glucose uptake was consistently higher (50-280%) in all tissues removed from diabetic rats under basal conditions, compared with tissues from control animals in the basal state. When control animals were rendered insulinopenic, glucose uptake by the skeletal muscle, heart, and diaphragm was reduced 30-60%, indicating that the uptake by these tissues occurred by both insulin- and noninsulin-mediated mechanisms. Glucose disposal by the other tissues sampled was entirely due to NIMGU under basal conditions. When blood glucose levels were elevated from 6 to 17 mM in control animals, NIMGU increased in all tissues (60-280%) except the brain. Rates of NIMGU were essentially identical between control and diabetic animals, under either euglycemic or hyperglycemic conditions, when glucose uptake was determined under the same steady-state plasma glucose levels. In contrast to the normal rate of NIMGU by muscle, IMGU by the skeletal muscle and heart from diabetic rats were reduced under mild hyperinsulinemic conditions (100 microU/ml), compared with control animals. Furthermore, in response to a maximal, stimulating dose of insulin (500 microU/ml), IMGU was impaired in the diaphragm, liver, lung, spleen, skin, and kidney removed from diabetic animals. These results indicate that the majority of glucose disposal under basal postabsorptive conditions occurs by NIMGU in both control and diabetic rats. Furthermore, while IMGU was selectively impaired in this model of insulin-dependent diabetes, the rates and tissue distribution of NIMGU were unaltered when glucose uptake was determined under similar plasma glucose levels.     

Glucose transporter content and glucose uptake in skeletal muscle constructs engineered in vitro

Engineered muscle may eventually be used as a treatment option for patients suffering from loss of muscle function. The metabolic and contractile function of engineered muscle has not been well described; therefore, the purpose of this experiment was to study glucose transporter content and glucose uptake in engineered skeletal muscle constructs called myooids. Glucose uptake by way of 2-deoxyglucose and GLUT-1 and GLUT-4 transporter protein content was measured in basal and insulin-stimulated myooids that were engineered from soleus muscles of female Sprague-Dawley rats. There was a significant increase in the basal 2-deoxyglucose uptake of myooids compared with adult control (fivefold), contraction-stimulated (3.4-fold), and insulin-stimulated (threefold) soleus muscles (P = 0.0001, 0.0001, and 0.0001, respectively). In addition, there was a significant increase in the insulin-stimulated 2-deoxyglucose uptake of myooids compared with adult control soleus muscles in basal conditions (6.5-fold) and adult contraction-stimulated (4.5-fold) and insulin- stimulated (3.9-fold) soleus muscles (P = 0.0001, 0.0001, and 0.0001, respectively). There was a significant 30% increase in insulin-stimulated compared with basal 2-deoxyglucose uptake in the myooids. The myooid GLUT-1 protein content was 820% of the adult control soleus muscle, whereas the GLUT-4 protein content was 130% of the control soleus muscle. Myooid GLUT-1 protein content was 6.3-fold greater than GLUT-4 protein content, suggesting that the glucose transport of the engineered myooids is similar in several respects to that observed in both fetal and denervated skeletal muscle tissue.     

Evidence that activation of 2-deoxy-D-glucose transport in rat thymocyte suspensions results from enhanced coupling between transport and hexokinase activity.

1. Suspensions of rat thymocytes accumulate free 2-deoxy-D-glucose (2-dGlc) within the cytosol to a concentration approx. 25-fold above the external concentration. This active accumulation was enhanced by 40 nM-phorbol 12-myristate 13-acetate (phorbol). 2. The Km for zero-trans uptake in control cells was 2.3 +/- 0.14 mM and Vmax. was 0.41 +/- 0.08 mumol/min per 10(10) cells (n = 6). In cells treated with phorbol (40 nM) the Km for zero-trans uptake was 1.2 +/- 0.13 mM and Vmax. 0.46 +/- 0.03 mumol/min per 10(10) cells (n = 6). The Km was decreased significantly by phorbol (P less than 0.01). 3. Phorbol-dependent activation of thymocytes delayed exit of free 2-dGlc into sugar-free solution and prevented exchange exit. Activation had no effect on 3-O-methyl D-glucoside (3-OMG) exit. 4. Coupling of 2-dGlc transport to hexokinase activity was determined by observing the effects of various concentrations of unlabelled cytosolic 2-dGlc on influx of labelled 2-dGlc into the hexose phosphate pool. In control cells this coupling was 0.81 +/- 0.02 and in phorbol-activated cells it was 0.92 +/- 0.01 (P less than 0.01). 5. The high-affinity inhibitor of hexokinase, mannoheptulose, inhibited uptake of 2-dGlc in both control and phorbol-treated cells. These data are consistent with a model for activation of sugar transport in which hexokinase activity is integrated with the sugar transporter at the endofacial surface. The results suggest that phorbol increases the degree of coupling transport with hexokinase activity, thereby leading to an increase in the rate of uptake of 2-dGlc, a decrease in exit of free 2-dGlc from the cytosol and an increase in free 2-dGlc accumulation.     

Transport and accumulation of 2-deoxy-D-glucose in wild-type and hexokinase-deficient cultured Chinese-hamster ovary (CHO) cells.

Hexokinase-deficient mutants and wild-type Chinese-hamster ovary cells have been used to investigate the role of hexokinase in uptake and accumulation of 2-D-deoxyglucose (2-dGlc). The evidence for a specific sugar transport system in both types of cells is that there is similar saturable phloretin-sensitive uptake of 2-dGlc and 3-O-methyl-D-glucose (3-OMG) in both types of cell. In wild-type cells, 2-dGlc is accumulated to a tissue:medium ratio of 10- and in the mutant only 3-fold; 3-OMG is not accumulated by either mutant or wild-type cells. The evidence that hexokinase affects the membrane transport process is that the rate of exit of free 2-dGlc from wild-type cells is 5-fold less than from mutant cells, whereas there is no difference in the rate of loss of 3-OMG between mutant and wild-type cells.     

The influence of ATP on sugar uptake mediated by the constitutive glucose carrier of Saccharomyces cerevisiae

The glucose carrier of Saccharomyces cerevisiae transports the phosphorylatable sugars glucose, mannose, fructose and 2-deoxy-D-glucose (2-dGlc) and the non-phosphorylatable sugar 6-deoxy-D-glucose (6-dGlc). Reduction of the ATP concentration by, for example, incubating cells with antimycin A, results in a decrease in uptake of 2-dGlc and fructose. These uptake velocities can be increased again by raising the ATP level. These results establish a role of ATP in sugar transport. Transport of glucose and mannose is less affected by changes in the ATP concentration than 2-dGlc and fructose uptake, while the 6-dGlc transport is independent of the amount of ATP in the cells. Also, reduction of the kinase activity by incubation with xylose diminished transport of 2-dGlc and fructose, while the uptake of glucose and mannose remained unchanged. It is discussed that these results are due to transport-associated phosphorylation with ATP as substrate and the hexokinases and the glucokinase as phosphorylating enzymes.     

High glucose concentrations inhibit glucose phosphorylation, but not glucose transport, in human endothelial cells.

Glucose uptake is autoregulated in a variety of cell types and it is thought that glucose transport is the major step that is subjected to control by sugar availability. Here, we examined the effect of high glucose concentrations on the rate of glucose uptake by human ECV-304 umbilical vein-derived endothelial cells. A rise in the glucose concentration in the medium led a dose-dependent decrease in the rate of 2-deoxyglucose uptake. The effect of high glucose was independent of protein synthesis and the time-course analysis indicated that it was relatively slow. The effect was not due to inhibition of glucose transport since neither the expression nor the subcellular distribution of the major glucose transporter GLUT1, nor the rate of 3-O-methylglucose uptake was affected. The total in vitro assayed hexokinase activity and the expression of hexokinase-I were similar in cells treated or not with high concentrations of glucose. In contrast, exposure of cells to a high glucose concentration caused a marked decrease in phosphorylated 2-deoxyglucose/free 2-deoxyglucose ratio. This suggests the existence of alterations in the rate of in vivo glucose phosphorylation in response to high glucose. In summary, we conclude that ECV304 human endothelial cells reduce glucose utilization in response to enhanced levels of glucose in the medium by inhibiting the rate of glucose phosphorylation, rather than by blocking glucose transport. This suggests a novel metabolic effect of high glucose on cellular glucose utilization.     

Acute inhibition of insulin-stimulated glucose transport by the phosphatase inhibitor, okadaic acid.

Insulin is thought to exert its effects on cellular function through the phosphorylation or dephosphorylation of specific regulatory substrates. We have analyzed the effects of okadaic acid, a potent inhibitor of type 1 and 2A protein phosphatases, on the ability of insulin to stimulate glucose transport in rat adipocytes. Insulin and okadaic acid caused a 20-25- and a 3-6-fold increase, respectively, in the rate of 2-deoxyglucose accumulation by adipose cells. When added to cells previously treated with okadaic acid, insulin failed to stimulate 2-deoxyglucose accumulation beyond the levels observed with okadaic acid alone. Treatment of cells with okadaic acid did not inhibit the effect of insulin to stimulate tyrosine autophosphorylation of its receptor. These results indicate that okadaic acid potently inhibits the effects of insulin to stimulate glucose uptake and/or utilization at a step after receptor activation. To clarify the mechanism of inhibition by okadaic acid, the intrinsic activity of the plasma membrane glucose transporters was analyzed by measuring the rate of uptake of 3-O-methylglucose by adipose cells, and the concentration of adipocyte/skeletal muscle isoform of the glucose transporter (GLUT-4) in plasma membranes isolated from these cells. Insulin caused a 15-20-fold stimulation of 3-O-methylglucose uptake and a 2-3-fold increase in the levels of GLUT-4 detected by immunoblotting of isolated plasma membranes; okadaic acid caused a 2-fold increase in 3-O-methylglucose uptake, and a 1.5-fold increase in plasma membrane GLUT-4. Pretreatment of cells with okadaic acid blocked the effect of insulin to stimulate 3-O-methylglucose uptake and to increase the plasma membrane concentration of GLUT-4 beyond the levels observed with okadaic acid alone. These results indicate that the effect of okadaic acid to inhibit the effect of insulin on glucose uptake is exerted at a step prior to the recruitment of glucose transporters to the cell surface, and suggest that a phosphatase activity may be critical for this process.     

Early cellular responses to diverse growth stimuli independent of protein and RNA synthesis.

Serum, elevated pH, excess Zn++, 9,10 dimethyl-1,2 dibenzanthracene (DMBA) and insulin accelerate the progress of growth-inhibited chick embryo cells into the S-period of DNA synthesis. A comparative study was made of their capacity to elicit other cellular responses within two hours after their application. All the agents studied stimulated the uptake of the glucose analogue 2-deoxy-D-glucose (2-dGlc). Elevated pH elicited a more striking increase than the other agents in the uptake of the amino acid analogue alpha-amino isobutyric acid (AIB). The application of subtoxic concentrations of Zn++ or DMBA did not stimulate the uptake of uridine by cells nor its incorporation into RNA when tested at 2 hours. However, it was found that the stimulation of uridine utilization did occur but was delayed several hours. Similarly, the accelerated onset of DNA synthesis was also delayed for several hours by these agents. Insulin acted like serum in stimulating the utilization of 2-dGlc, AIB and uridine. Serum and DMBA were particularly effective in stimulating the utilization of choline. It was concluded that the utilization of 2-dGlc, uridine and thymidine are affected similarly by all the agents, but that there may be differential effects in the utilization of AIB and choline. The inhibition of RNA synthesis by actinomycin D did not prevent the relative stimulation of 2-dGlc, AIB and choline utilization by serum and pH treatment. The inhibition of protein synthesis by cycloheximide did not prevent the relative stimulation of 2-dGlc and choline utilization by serum and pH treatment. It partially blocked the increased uptake of AIB and had erratic effects on the utilization of uridine. It was concluded that neither RNA nor protein synthesis is required for some, if not all, the early responses to growth stimuli measured here. The inhibited cell appears to be a poised system which carries out a programmed array of reactions characteristic of the cell type following perturbation by a variety of unrelated agents. In vivo specificity is provided by the physiological reagents available (i.e., hormones) and their capacity to interact with different cell types.     

1,2-Diacylglycerol and ceramide levels in rat skeletal muscle and liver in vivo. Studies with insulin, exercise, muscle denervation, and vasopressin

Studies on BC3H-1 myocytes suggest that the insulin-induced increase in cellular diacylglycerol level mediates the insulin-stimulated glucose transport in these cells (Standaert, M. L., Farese, R. V., Cooper, D. R., and Pollet, R. J. (1988) J. Biol. Chem. 263, 8696-8705). The present study tested whether diacylglycerol could mediate the insulin-induced and exercise-induced increases in glucose uptake by rat skeletal muscle in vivo. Glucose uptake by calf muscles of the rat was assessed by measuring cellular 2-deoxyglucose uptake in vivo. Diacylglycerol and ceramides in muscles frozen in situ were assayed with diacylglycerol kinase. Intravenous injection of 0.1 unit of insulin/rat resulted in a 6-fold increase in muscle 2-deoxyglucose uptake during the subsequent 25-min period. In contrast, no statistically significant changes in muscle diacylglycerol or ceramide levels were observed at 2, 5, 10, and 25 min after insulin injection. When calf muscles of the hindlimb were exercised in vivo for 25 min by electrical stimulation inducing one contraction/s, 2-deoxyglucose uptake by muscles was increased 15-fold. However, no statistically significant changes in muscle diacylglycerol or ceramide content were observed at 5, 10, 15, and 25 min of exercise. Although the findings do not exclude the possibility of a compartmentalized increase in diacylglycerol level, the present data suggest that diacylglycerol is not a mediator of the insulin-induced or exercise-induced augmentation of glucose uptake by skeletal muscle in vivo. Since interruption of nerve supply to the muscles makes the muscles insulin resistant (Turinsky, J., (1987) Am. J. Physiol. 252, R531-R537), the effect of denervation on diacylglycerol and ceramide levels in calf muscles of the rat was also examined. The denervation resulted in 21, 51, and 117% increases in muscle diacylglycerol levels at 3, 16, and 32 days after denervation, respectively. No statistically significant changes in muscle ceramide levels were observed at any postdenervation interval. Finally, the measured lipids were studied in muscles and livers of rats infused with supraphysiological doses of vasopressin (86 pmol/min). In controls, diacylglycerol concentrations of the muscles and liver did not significantly differ, but the liver exhibited a 5-fold higher level of ceramides than the muscles. Infusion of vasopressin for 5 min did not have a statistically significant effect on diacylglycerol concentration of the liver but continuation of the same infusion for 10 min resulted in a 63% increase in liver diacylglycerol. The 10-min infusion had no effect on muscle diacylglycerol concentration or ceramide levels in any of the tissues studied.(ABSTRACT TRUNCATED AT 400 WORDS)     

Uptake and incorporation of glucose and mannose by whole cells of Bacteroides thetaiotaomicron.

Glucose uptake by whole-cell suspensions of the obligate anaerobe Bacteroides thetaiotaomicron was two- to fourfold higher under aerobic conditions than during incubation under atmospheres of N(2) or H(2) gas. The O(2)-stimulated uptake activity was lost rapidly (>70% in 5 h) when cell suspensions were incubated aerobically, but this loss was prevented by the addition of crude catalase. Catalase had no apparent effect on cell viability during these incubations. Glucose uptake activity was strongly inhibited by a 10-fold excess of mannose or galactose but not by methyl-alpha-d-glucoside, fructose, or lactose. Both glucose and mannose were rapidly incorporated into polyglucose after uptake. The O(2)-stimulated glucose uptake was not inhibited by cyanide, azide, 2,4-dinitrophenol, or 2-N-heptyl-4-hydroxyquinoline-N-oxide. However, p-chloromercuribenzoate, menadione, and sodium fluoride inhibited uptake by 88, 67, and 55%, respectively. All attempts to detect phosphoenolpyruvate-phosphotransferase activity for glucose, methyl-alpha-d-glucoside, and 2-deoxyglucose were negative. The bacteria contained hexokinase activity and a complete glycolytic Embden-Meyerhof pathway.     

Glucose 6-phosphate stimulation of MgATP-dependent Ca2+ uptake by rat kidney microsomes

(1) The features of MgATP-dependent Ca2+ accumulation under stimulation with glucose 6-phosphate were studied in rat kidney microsomes. (2) Ca2+ accumulated in the presence of MgATP alone does not exceed approx. 2 nmol/mg protein. (3) Glucose 6-phosphate markedly stimulates Ca2+ accumulation, up to steady-state levels approx. 15-fold higher than in its absence. (4) The hydrolysis of glucose 6-phosphate by glucose-6-phosphatase is essential for the stimulation, as shown by inhibiting the glucose 6-phosphate hydrolysis with adequate concentrations of vanadate. Inorganic phosphate is accumulated in microsomal vesicles during glucose 6-phosphate-stimulated Ca2+ uptake in equimolar amounts with respects to Ca2+. (5) Increasing concentrations of glucose 6-phosphate result in increasing stimulations of Ca2+ uptake, until a maximal Ca2(+)-loading capacity of approx. 27 nmol/mg microsomal protein is reached. It is suggested that the enlargement of the kidney microsomal Ca2+ pool induced by glucose 6-phosphate (an important metabolite in kidney) might play a role in the regulation of Ca2+ homeostasis in kidney tubular cells.