Abnormal islet and adipocyte function in young B-cell-deficient rats with near-normoglycemia

Sprague-Dawley male rats were injected at 2 days of age with streptozotocin (SZ). At 4 wk of age the fed plasma glucose concentration of the SZ group was 151 +/- 6 mg/dl as compared with 133 +/- 4 for the control group. The fed plasma insulin values were indistinguishable, however. In response to an intraperitoneal glucose challenge the SZ group had marked glucose intolerance and virtually no rise in plasma insulin. After a meal challenge the SZ group also had glucose intolerance, but plasma insulin responses were similar to those of the control. The pancreata of the 4-wk-old rats were perfused in vitro and the SZ group had essentially no response to glucose, but did respond to arginine. Adipocytes of the 4-wk-old SZ rats had impaired glucose conversion to CO2 similar to that seen in the more hyperglycemic 6-wk-old SZ rats. Castration carried out at about 3 wk of age did not influence the hyperglycemia seen at 6 wk of age and later. These data indicate that 4-wk-old SZ rats, while having near-normal plasma glucose levels and normal plasma insulin values, have clearly abnormal B-cell and adipocyte function. With increasing age and weight gain these SZ rats develop frank hyperglycemia.

In vitro phosphorylation of a synthetic collagen peptide by cyclic AMP-dependent protein kinase

A synthetic tridecapeptide that corresponds closely to amino acid residues 98 to 110 in chick collagen alpha 1(I) contains several determinants of specificity required for recognition and phosphorylation by the cyclic AMP-dependent protein kinase. The peptide Gly-Leu-Hyp-Gly-Nle-Lys-Gly-His-Arg-Gly-Phe-Ser-Gly was predicted to be a substrate for the cyclic AMP-dependent protein kinase because it contained multiple basic amino acids NH2-terminal to a potentially phosphorylatable seryl residue. When tested as a substrate for the enzyme, the peptide was stoichiometrically phosphorylated. Phosphoserine was identified as the only phosphoamino acid in a partial hydrolysate of the phosphorylated peptide. The peptide and several of its analogs were phosphorylated by the cyclic AMP-dependent protein kinase with Km values from 1 to 6 mM and Vmax values from 1 to 3 mumol of phosphate/min/mg of enzyme. Although the Km of the kinase for the collagen peptide was high, these results confirmed the prediction made from knowledge of the substrate specificity of cyclic AMP-dependent protein kinase. The potential for such a phosphorylation reaction to occur in vivo is discussed.

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

The relative contributions of transport and intracellular metabolism of glucose to the control of overall glucose utilization were evaluated in rat adipocytes. Transport of 3-O-methylglucose and hexokinase activity in crude homogenates were measured and the derived kinetic parameters incorporated into network thermodynamic computer simulations. Hexokinase was found to be inhibited in a fully noncompetitive pattern by glucose 6-phosphate (Ki = 0.46 mM). When this feature was incorporated into the computer simulations, they reflected measured rates of overall glucose utilization as well as intracellular glucose 6-phosphate concentrations, both in the presence and absence of insulin. The effect of the hormone was represented in the simulations solely by an increase in the number of hexose carriers. A predominant stimulation of transport rather than metabolism was also suggested by the finding that intracellular glucose concentrations assessed by glucose-induced 3-O-methylglucose counter-transport were higher in the presence than in the absence of insulin over a wide range of extracellular glucose concentrations. Nevertheless, it was also found that insulin induced a significant countertransport gradient while the oxidant H2O2 did not, which suggests that insulin-stimulated metabolism does increase overall glucose utilization independently of effects on transport. These studies show that the kinetic patterns of basal and insulin-stimulated glucose utilization in adipocytes may be generated simply by coupling transport and phosphorylation steps and providing for inhibition of the latter by accumulated glucose 6-phosphate.

The effect of insulin-stimulated pentose phosphate cycle activity on cellular glutathione content in rat adipocytes

Insulin is known to stimulate intracellular H2O2 production in rat adipocytes. This H2O2 could in turn stimulate the pentose phosphate cycle by oxidizing GSH and shifting the redox state of the cells. However, insulin had no effects on cell GSH content or GSSG in buffer other than as related to changes in medium glucose. On the contrary, in the presence of an active pentose phosphate cycle, insulin tended to reverse the fall in glutathione content induced with the catalase inhibitor 3-amino-1,2,4-triazole, the oxidant t-butyl hydroperoxide and the sulfhydryl blocker N-ethylmaleimide. It was also found that insulin-stimulated H2O2 production could be blocked under conditions in which the effect of the hormone on the pentose phosphate cycle persisted. These results suggest that stimulation of the pentose phosphate cycle by insulin is not related to increased H2O2 generation, rather that activation of the pentose phosphate cycle by the hormone may provide NADPH for regeneration of depleted GSH.

The simple model of adipocyte hexose transport. Kinetic features, effect of insulin, and network thermodynamic computer simulations

Kinetic studies of the rat adipocyte hexose transport system were performed using the integrated rate approach and these compared to the simple carrier model of transport. Equilibrium exchange 3-O-methylglucose entry and exit studies showed directional symmetry with Km = overall dissociation constants = 8-10 mM. Comparison of zero-trans and equilibrium exchange entry also revealed similar Km and Vmax values. Insulin pretreatment increased the maximal rate of transport at 20 mM 3-O-methylglucose about 5- to 6-fold with each procedure. Studies of glucose-induced steady state 3-O-methylglucose countertransport provided evidence that carrier permeability and not carrier-substrate dissociation was rate limiting for overall transport. These data, therefore, indicate equal mobility of the loaded and unloaded carriers. Network thermodynamic computer simulations of the simple carrier model using kinetic parameters derived from zero-trans experiments provided good fits of actual data. The effect of insulin was best represented by an increase in total number of carrier units. It is concluded that the adipocyte hexose carrier displays bidirectional symmetry, limitation of transport by carrier movement rather than substrate-carrier interaction, equal rates of movement of loaded and unloaded carriers, and adherence to a simple carrier model in which insulin increases the total number of carrier units.

Rat adipocyte utilization of different substrates: effects of cell size and the control of lipogenesis

The metabolism of labeled glucose, pyruvate and acetate was compared in adipocytes isolated from old, obese rats (greater than 500 g) and young, lean rats (130-150 g). The larger cells from old, obese rats had markedly reduced rates of glucose, pyruvate and acetate conversion to glyceride-fatty acids, indicating that large cell fatty acid formation is reduced at some point beyond the entry of pyruvate and acetate into glucose metabolism. No evidence of a primary block in the pentose phosphate cycle of cells from old, obese rats was found. In spite of diminished glucose metabolism to several products in the large cells, both basal and insulin-stimulated rates of glyceride-glycerol synthesis from glucose and pyruvate were similar in each cell type. This indicates a relative diversion of carbon flow to alpha-glycerophosphate and reesterification in the large cells. Addition of low concentrations of glucose increase glyceride-fatty acid synthesis from acetate (both cell types) or pyruvate carbon (small cells), but decreased glyceride-glycerol synthesis from pyruvate carbon (both cell types). The acceleration of small cell fatty acid synthesis from pyruvate carbon by glucose and insulin was shown to be related to provision of NADPH from glucose metabolism in the pentose cycle. These studies indicate that, although the block in lipogenesis in adipocytes from old, obese rats appears to reside in the pathway of fatty acid synthesis itself, provision of additional alpha-glycerophosphate or NADPH from glucose metabolism may, under certain conditions, increase lipogenesis in cells from old, obese and young, lean rats.

The mechanism of the insulin-like effects of ionic zinc

The insulin-like effects of ionic zinc (Zn2+) were studied in isolated rat adipocytes. Concentrations of Zn2+ between 250 and 1000 microM stimulated 3-O-methylglucose transport and glucose metabolism to CO2, glyceride-fatty acid, and glyceride-glycerol. Selective stimulation of the pentose phosphate cycle was observed since a Zn2+-induced increase in glucose carbon 1 oxidation persisted even when glucose transport was blocked with 50 microM cytochalasin B or when transport was no longer rate-limiting for metabolism at high concentrations of glucose. Enhanced pentose phosphate cycle activity may have been due to a selective inhibition of glutathione reductase by the ion, which was also accompanied by a fall in cellular glutathione content. Zn2+ also inhibited lipolysis stimulated by the beta-adrenergic agent ritodrine in the absence of glucose. The effects of Zn2+ on glucose oxidation and stimulated rates of lipolysis were inhibited by extracellular catalase, indicating that they were largely a result of H2O2 generation. The H2O2 production appeared for the most part to be caused by zinc-catalyzed autoxidation of sulfhydryl groups present on external cell membranes, although involvement of sulfhydryl groups on bovine serum albumin in the buffer could also have contributed. The insulin-like effects of Zn2+ in adipocytes are therefore caused not only by direct effects of the ion on intracellular metabolism but also by indirect effects related to H2O2 generation.

Triacylglycerol turnover in large and small rat adipocytes: effects of lipolytic stimulation, glucose, and insulin

Rates of lipolysis and reesterification were determined under various conditions in adipocytes from epididymal fat pads of old, spontaneously obese rats and compared to cells from younger, leaner animals. No differences were observed in lipolytic responsiveness to several concentrations (2--50 microM) of the beta-adrenergic agent ritodrine compared to cells from younger, leaner rats. The large cells showed diminished rates of lipolysis, reesterification, and glyceride-glycerol synthesis from glucose of a submaximal but not maximal insulin concentrations, probably reflecting decreased large cell receptor numbers. In both cell types reesterification measured in the presence of ritodrine progressively rose with increasing concentrations of glucose in the medium. At each glucose concentration (0.5--25 mM) rates of reesterification were similar in each cell type. When maximal concentrations of insulin (5 nM) were also added, at low glucose concentrations (less than 5 mM) there was a similar increase in reesterification in large and small cells. No insulin effect in either cell type was observed at high concentrations of glucose. Although fatty acid synthesis from glucose in the large cells was markedly diminished, glyceride-glycerol synthesis was well maintained, correlating well with calculated reesterification rates. In fact, reesterification was found to be quantitatively very important in determining total triacylglycerol turnover in both cell types. High rates of reesterification might not only allow maintenance of triacylglycerol stores, but could also increase metabolic sensitivity to changes in hormonal or substrate concentrations.

Insulin stimulation of Na+ transport and glucose metabolism in cultured kidney cells

A line of toad kidney cells (A6) in continuous culture was evaluated for ion transport and metabolic responses to insulin. The cells were grown on permeable supports to allow access of the medium to both basolateral and apical sides of the epithelium. Insulin, on the basolateral side only, produced an increase in short-circuit current (Isc) that was maximal at 40-60 min. A concentration-dependent increase in Isc and potential difference (PD) was found in the range of 10-3.2 X 10(3) microunits/ml insulin. The maximal stimulation of Isc and PD was approximately six- and twofold, respectively. After insulin exposure Isc was equivalent to net Na+ transport, indicating active Na+ transport stimulation. Insulin was also found to increase the incorporation of radiolabeled glucose into glycogen. Thus A6 cells exhibit both transepithelial transport and metabolic responses to insulin.

Determinants of gastric inhibitory polypeptide and insulin secretion

The plasma concentrations of gastric inhibitory polypeptide (IR-GIP), insulin, and glucose were measured in 5 dogs after administration of various constituents of food. IR-GIP responses produced by galactose and fructose were minimal compared to those of glucose. Corn starch caused a significant early increase in insulin and glucose concentrations, although the rise in IR-GIP was delayed. The responses after starch were delayed, possibly due to the necessity for hydrolytic cleavage. Neither individual fatty acids (palmitic, oleic, and linoleic), monoolein, nor glycerol produced increases in IR-GIP comparable to those observed with lesser amounts (on a molar basis) of corn oil alone. None of four amino acids tested orally (arginine, leucine, alanine, and phenylalanine) caused increases in IR-GIP concentrations, but they increased insulin concentrations. Although glucose-induced IR-GIP responses were not inhibited by the amino acids (1 g/kg) tested, increasing concentrations (up to 1 g/kg) of either alanine or arginine caused progressive inhibition of corn-oil-stimulated IR-GIP responses, in spite of the expected amino-acid-induced rises in insulin concentrations. These results confirm the specificity of the IR-GIP secretory response for glucose and triglyceride, and suggest that certain food constituents (such as amino acids) may modify the GIP secretory response to a meal.

The insulin-like effects of low molecular weight thiols: role of trace metal contamination of commercial thiols

During the investigation of insulin-like effects of low molecular weight thiols in rat adipocytes it was found that thiol autoxidation to H2O2 occurred even in the absence of albumin and cells. Since chromatography of the thiols on a chelating ion exchange resin prevented H2O2 formation, it is likely that trace metal contaminations of the thiol preparations catalyzed thiol autoxidation in oxygenated solutions. Incubation of rat adipocytes with 1 mM dithioerythritol and low concentrations of dialyzed albumin stimulated 2-deoxyglucose transport, glucose metabolism to CO2 and lipids, and formate resin or in most instances incubation with EDTA or catalase prevented these effects. The antilipolytic effect of dithioerythritol was also abolished by catalase. The insulin-like effects of thiols therefore appear to be solely mediated by H2O2 generated during thiol autoxidation catalyzed by trace metal contaminants either of albumin or of the commercial thiol preparation itself.

S aureus endocarditis: a review and plea for early surgery

In a study of 31 cases and a review of the literature, Staphylococcus aureus endocarditis was distinguished from that due to other organisms by the absence of prior valvular disease, by the presence of debilitating illness or acute onset, and by a toxic fulminant course. Availability of semi-synthetic penicillins decreased mortality from 90% to about 50%, with death due to heart failure rather than sepsis. Valvular replacement may improve survival if employed at the first signs of cardiac decompensation, rather than after medical therapy has failed to stabilize a downhill course.

The insulin-like effect of hydrogen peroxide on pathways of lipid synthesis in rat adipocytes

In addition to the well known insulin-like effects of certain concentrations of H2O2 on glucose transport and oxidation in isolated rat adipocytes, the present work demonstrates that lipid synthesis from glucose is also enhanced over a narrow range of H2O2 concentrations (0.15 to 0.5 mM) added to the incubation medium. As in the case of insulin, H2O2 was found to stimulate greater glucose incorporation into glyceride-fatty acid than incorporation into glyceride-glycerol. As part of a multifaceted regulation of lipogenesis, H2O2, like insulin, increased the amount of pyruvate dehydrogenase in the active form without increasing the total amount of pyruvate dehydrogenase. Pyruvate dehydrogenase activity increased within 5 min of H2O2 incubation, reached a maximum at 15 min and declined thereafter as the H2O2 disappeared from the incubation medium. While medium glucose per se was found to activate the enzyme, it is unlikely that the effect of H2O2 was mediated by the known enhancement of glucose transport since the effects on the enzyme were maximal in the absence of glucose in the incubation medium. These findings add to the growing list of insulin effects that are reproduced by H2O2, and strengthen the hypothesis that assigns H2O2 the role of "second messenger" of insulin.

Insulin-stimulated intracellular hydrogen peroxide production in rat epididymal fat cells

Insulin stimulation of hydrogen peroxide production by rat epididymal fat cells was investigated by studying the oxidation of formate to CO2 by endogenous catalase. Under optimal concentrations of formate (0.1 to 1 mM) and glucose (0.275 mM), insulin stimulated formate oxidation 1.5- to 2.0-fold. Inhibitors of catalase activity, including nitrite and azide, inhibited both basal and insulin-stimulated formate oxidation at concentrations that did not interfere with insulin effects on glucose C-1 oxidation or glucose H-3 incorporation into lipids. The addition of exogenous catalase increased formate oxidation only slightly, while exogenous H2O2 (0.5 mM) stimulated formate oxidation by endogenous catalase strongly. These data indicate that the insulin-stimulated H2O2 production was intracellular. Insulin dose-response curves for formate oxidation were identical with those for glucose H-3 incorporation into lipids. The dependence of relative insulin effects on the logarithm of the glucose concentration was bell-shaped for formate oxidation and correlated highly with the coresponding dependences of glucose C-1 oxidation and glucose H-3 incorporation into lipids. This suggests that insulin stimulation of intracellular H2O2 production is linked to glucose metabolism. Since it is known that extracellular H2O2 can mimic insulin in several respects, these observations suggest that H2O2 may act as a "second messenger" for the observed effects of insulin.

Characterization of proinsulin-insulin intermediates in human plasma

This work addressed the problem of heterogeneity of immunoreactive insulin (IRI) in human plasma. Subjects with normal glucose tolerance were given 75g of an oral glucose solution, followed in 30 min by an intravenous infusion of 30g of arginine over 30 min. At the end of the infusion blood was withdrawn for analysis. IRI was extracted from plasma of individual subject by immunosorbent columns and was fractionated by gel filtration, disc gel electrophoresis and isoelectric focusing. Human IRI components were identified by molecular size, immunoreactivity with a human proinsulin antibody, sensitivity to trypsin, and by comparison of electrophoretic mobility and isoelectric point with porcine pancreatic products, after suitable correction for electric charge and molecular weight differences. The pattern of IRI heterogeneity was the same among six healthy subjects. Heterogeneity of proinsulin-size IRI in circulation was more marked than that of insulin-size material. Proinsulin and desdipeptide proinsulin were present in approximately equal amounts accompanied by minor amounts of split proinsulin and monodesamido-desdipeptide proinsulin. Insulin-size IRI contained over 80% insulin. Minor amounts of monodesamidoinsulin and diarginylinsulin were observed in some cases. The types of IRI components observed in plasma are evidence in support of a physiologic role of trypsin-and carboxypeptidase B-like enzymes in the conversion of proinsulin to insulin. Moreover, this study provides a base line for investigation of abnormalities in proinsulin-to-insulin conversion that may be associated with certain pathologic states.

The effect of endogenous gastric inhibitory polypeptide on glucose-induced insulin secretion in mild diabetes

Diabetic subjects have previously been found to have increased secretion of immunoreactive gastric inhibitory polypeptide (IR-GIP) in response to oral glucose or triglyceride compared with normal subjects. In the present study, a group of 19 diabetics was selected with milder glucose intolerance (fasting plasma glucose = 109 mg. per deciliter) than those reported previously. In these diabetics no enhancement of IRGIP responses was found to either 75 gm. oral glucose or 50 ml. oral corn oil when compared with 13 subjects with normal carbohydrate tolerance. Some of the subjects were obese and, when assessed independently from glucose tolerance, obesity was associated with higher integrated IRGIP responses to oral glucose but not to oral corn oil. The degree of obesity (92 to 154 per cent ideal body weight) did not correlate with fasting or stimulated IRGIP responses of the 32 subjects. One hour after ingestion of 50 ml. corn oil, when IRGIP concentrations were maximal but glucose and insulin remained unaffected, the diabetic subjects showed increased relative acute-phase insulin secretion to a 20 gm. pulse of intravenous glucose compared with that from a glucose pulse alone (p < 0.01). Subjects with normal carbohydrate tolerance had no increase in relative acute insulin response to glucose after triglyceride priming. The relative second-phase insulin response (10 to 30 min.) increased significantly with triglyceride priming in both diabetics (p < 0.01) and nondiabetics (p < 0.05). Although glucose disappearance rates (Kg) correlated with relative insulin responses, the increase in insulin secretion with triglyceride priming was not associated with an increase in Kg. Although obese subjects show increased stimulated GIP responses to oral glucose, patients with mild diabetes have normal GIP secretion to oral glucose and to triglyceride, but show a GIP-associated increase in both phases of glucose-induced insulin secretion.

N epsilonB29-(+)-biotinylinsulin and its complexes with avidin. Synthesis and biological activity

Insulin was modified with d-biotin-N-hydroxysuccinimide ester in dimethylformamide. Mono-, di-, and triacylated insulins were separated by preparative isoelectric focusing. Monoacylated derivatives (isoelectric point 5.1) were fractionated twice on DEAE-cellulose to yield pure N epsilonB29-biotinylinsulin. The structure of the product was established by amino acid analysis before and after deamination. N epsilonB29-biotinylinsulin had biological activity indistinguishable from insulin on glucose oxidation and lipid synthesis assays using isolated rat epididymal fat cells. Complexes of N epsilonB29-biotinylinsulin with avidin, having essentially all but one binding site filled with biotin, were prepared in order to obtain a 1:1 insulin:avidin ration. The elicited identical maximal biological responses, but showed a potency decreased to 5% of that of insulin. Such complexes conjugated with ferritin will provide a useful tool in the development of electron microscopic stains of insulin receptors.