Tritiated Glucose Research Articles

Glucose turnover rate and peripheral insulin sensitivity in alcoholic patients without liver damage.

Glucose intolerance is frequently found in alcoholic patients and an impaired insulin response has been documented in them. To look for alternative mechanisms that could explain this intolerance, a glucose turnover using tritiated glucose and an euglycemic glucose clamp were performed to measure the glucose production rate and peripheral insulin sensitivity, respectively. Two groups of recently abstinent chronic male alcoholic patients without evidence of liver damage were studied. The glucose turnover technique showed a higher basal glucose production rate in alcoholics, compared with normal volunteers (2.83 +/- 0.29 vs. 1.84 +/- 0.22 mg/kg/min); an intravenous ethanol load significantly increased this rate. The euglycemic glucose clamp did not show peripheral insulin resistance in alcoholics, compared with controls.

Peripheral and hepatic resistance to insulin and hepatic resistance to glucagon in uraemic subjects. Studies at physiologic and supraphysiologic hormone levels.

To characterize endogenous glucose production in uraemia, nondialyzed uraemic patients and controls were exposed to two major modulating hormones, insulin and glucagon. Nineteen uraemic and 15 healthy subjects underwent either a 2-step (insulin infusion rates: 0.45 and 1.0 mU.kg-1.min-1) or a 3-step (insulin infusion rates: 0.1, 0.2 and 0.3 mU.kg-1.min-1) sequential euglycaemic insulin clamp. Average steady state serum insulin concentrations were almost identical during all five infusion rates in uraemic patients (16, 22, 26, 31 and 66 mU/l) and controls (15, 19, 24, 33 and 68 mU/l). At all steps, insulin infusion was accompanied by significantly lower glucose disposal rates [( 3(-3)H]glucose) in uraemic patients compared with controls (P less than 0.05 or less). Moreover, the restraining potency of insulin on endogenous glucose production was much more prominent in healthy than in uraemic subjects at the lowest three infusion rates (0.6 +/- 1.0 versus 1.4 +/- 0.3 (mean +/- 1 SD), -0.3 +/- 0.7 versus 0.7 +/- 0.3, and -1.1 +/- 0.7 versus 0.2 +/- 0.6 mg.kg-1.min-1; P less than 0.05, P less than 0.01 and P less than 0.01, respectively), implying a shift to the right of the dose-response curve in uraemia. In contrast, basal values were comparable (2.4 +/- 0.3 versus 2.2 +/- 0.6 mg.kg-1.min-1) as the difference vanished at higher infusion rates, i.e. peripheral insulinaemia above approximately equal to 30 mU/l. Another 7 uraemic patients and 7 controls were infused with glucagon at constant rates of 4 or 6 ng.kg-1.min-1, respectively, for 210 min concomitant with somatostatin (125 micrograms/h) and tritiated glucose. The ability of glucagon to elevate plasma glucose was markedly attenuated in uraemic patients compared with controls during the initial 60 min of glucagon exposure. This difference was entirely due to diminished hepatic glucose production (3.5 +/- 0.8 versus 4.8 +/- 1.0 mg.kg-1.min-1; P less than 0.05). In conclusion, in addition to insulin resistance in peripheral tissues, uraemia is also associated with hepatic insulin resistance. Furthermore, glucagon challenge implies impaired early endogenous glucose release in uraemia suggesting a superimposed hepatic resistance to glucagon.

In vivo glucose metabolism in the awake rat: Tracer and insulin clamp studies

The goals of this study were twofold: (1) to determine the in vivo dose-response relationship in the conscious, unstressed rat between the plasma insulin concentration and total body glucose uptake, and between plasma insulin and suppression of endogenous glucose production; and (2) to develop a physiologic compartmental model to describe the kinetics of plasma glucose in the rat in the basal state. In order to perform repeat insulin clamp studies in the same rat, chronic catheters were implanted in the aortic arch (via the carotid artery) and in the cardiac atrium (via the jugular vein), exteriorized, and fixed to the back of the skull with a dental cement cap. Insulin was infused at rates of 1.2, 2.4, 4.8, 12, and 24 mU/min · kg, and the plasma glucose was held constant at the basal level by a variable glucose infusion (euglycemic insulin clamp). The resulting steady-state plasma insulin concentrations ranged from 40 to 1,300 μU/mL. The dose-response curve for glucose uptake was sigmoidal in shape: in the basal state, total glucose utilization averaged 6.8 mg/min · kg at an insulin concentration of 9 μU/mL, half-maximal glucose uptake (18.3 mg/kg · min) occurred at a plasma insulin concentration between 70 and 80 μU/mL, and maximal uptake (36.6 mg/kg · min) was seen at an insulin level in excess of 100 μU/mL. Residual endogenous glucose production was evaluated by a prime-continuous infusion of (3- 3H)-glucose. The dose-response curve for suppression of endogenous glucose output also was sigmoidal: basal glucose production in the fasting state (6.8 mg/kg · min) suppressed by 71% at the lowest (1.2 mU/kg · min) insulin infusion, and by greater than 90% in animals receiving 2.4, 4.8, or 12 mU/min · kg infusions. Half-maximal inhibition of glucose production occurred at an insulin concentration between 30 and 40 μU/mL. In additional studies bolus injections of tritiated glucose were rapidly administered through the venous catheter, and the time-course of plasma radioactivity was monitored closely for 120 minutes. The plasma clearance rate of glucose determined by this method was 5.2 ± 0.7 mL/min · kg. The initial distribution volume was 159 ± 12 mL/kg. Two exponential components were identified in the tracer disappearance curve of each animal. Based on stringent statistical criteria, the data were fitted to a two-compartmental model. The total volume of the glucose space in this model was calculated to be 273 ± 16 mL/kg and contained a total amount of glucose equal to 307 ± 16 mg/kg. Of this, 179 ± 8 mg/kg resided in the rapidly exchanging compartment (assumed to be the insulin-independent pool), v only 128 ± 8 mg/kg in the insulin-dependent pool. In conclusion, in the rat as in man the curve relating glucose uptake to plasma insulin concentration is sigmoidal in shape, linear and steep at insulin concentrations between 40 and 100 μU/mL; maximal insulin effect resulted in a sixfold stimulation of glucose metabolism above baseline. Hepatic glucose production is completely suppressed at insulin concentrations greater than 70 μU/mL, with half-maximal suppression occurring at insulin concentrations of 30 to 40 μU/mL.

A small subset of cortical astrocytes in culture accumulates glycogen

We are interested in identifying the target cells for norepinephrine in cerebral cortex and in characterizing the effects of norepinephrine on these target cells. Norepinephrine inhibits the incorporation of tritiated glucose into glycogen in rat cerebral cortex in dissociated cell culture. To identify which cells store glycogen in these cultures we combined glycogen cytochemistry with glial flbrillary acidic protein immunocytochemistry. Using this technique we show that cytochemically detectable glycogen is restricted to a small subset of astrocytes as well as an unidentified cell type which does not contain glial fibrillary acidic protein.These results demonstrate that only a minority of astrocytes in cortical cultures accumulate glycogen. Therefore cortical astrocytes are differentiated with respect to glycogen accumulation, an important metabolic function. We do not know if glycogen accumulation in astrocytes is a constitutive or facultative property. In either case the subset of astrocytes which accumulates glycogen might be one of the major cellular targets for norepinephrine in cerebral cortex.

Insulin action in insulin-dependent diabetics after short-term thiazide therapy.

The influence of short-term thiazide treatment on peripheral tissue and liver sensitivity to insulin in insulin-dependent diabetes mellitus was determined by the euglycemic insulin clamp technique. A sequential three-step hyperinsulinemic clamp was performed in six insulin-dependent diabetics before and after 2 wk of hydroflumethiazide (HFT) administration in a daily dose of 75 mg. Insulin was infused at rates of 0.5, 2.0, and 4.0 mU X kg-1 X min-1, and each dose was given for at least 120 min. Glucose uptake during the last 30 min of each step was almost identical in the two situations (2.7 +/- 0.6 vs. 2.4 +/- 0.5 mg X kg-1 X min-1, 9.6 +/- 0.9 vs. 9.7 +/- 1.2 mg X kg-1 X min-1, and 12.0 +/- 1.3 vs. 12.6 +/- 1.5 mg X kg-1 X min-1). Serum insulin levels were also similar, and blood glucose was kept at 100 +/- 3, 99 +/- 4, and 97 +/- 3 mg/dl before thiazides and at 93 +/- 6, 93 +/- 6, and 94 +/- 6 mg/dl after thiazides. Another five insulin-dependent diabetics were infused with tritiated glucose followed by insulin infusion at two rates: 0.45 and 1.0 mU X kg-1 X min-1. Basal glucose output was comparable before and after thiazides (3.63 +/- 0.24 vs. 2.97 +/- 0.26 mg X kg-1 X min-1), as was the liver response to increasing insulin concentrations. The metabolic state as assessed by HbA1c and fasting blood glucose did not differ in the two experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hyperglycemia on glucose production and utilization in humans. Measurement with [23H]-, [33H]-, and [614C]glucose.

Studies with tritiated isotopes of glucose have demonstrated that hyperglycemia per se stimulates glucose utilization and suppresses glucose production in humans. These conclusions rely on the assumption that tritiated glucose provides an accurate measure of glucose turnover. However, if in the presence of hyperglycemia the isotope either loses its label during "futile" cycling or retains its label during cycling through glycogen, then this assumption is not valid. To examine this question, glucose utilization and glucose production rates were measured in nine normal subjects with a simultaneous infusion of [23H]glucose, an isotope that may undergo futile cycling but does not cycle through glycogen; [614C]glucose, an isotope that may cycle through glycogen but does not futile cycle; and [33H]glucose, an isotope that can both undergo futile cycling and cycle through glycogen. In the postabsorptive state at plasma glucose concentration of 95 mg X dl-1, glucose turnover determined with [614C]glucose (2.3 +/- 0.1 mg X kg-1 X min-1) was greater than that determined with [33H]glucose (2.1 +/- 0.1 mg X kg-1 X min-1, P = 0.002) and slightly less than that determined with [23H]glucose (2.7 +/- 0.2 mg X kg-1 X min-1, P = 0.08). Plasma glucose was then raised from 95 to 135 to 175 mg X dl-1 while insulin secretion was inhibited, and circulating insulin, glucagon, and growth hormone concentrations were maintained constant by infusion of these hormones and somatostatin. Glucose production and utilization rates determined with [614C]glucose continued to be less than those determined with [23H]glucose and greater than those seen with [33H]glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of glucose analogues into cultured cerebral microvessel endothelium.

Tritiated glucose analogues 3-O-methylglucose (3-OMG) and 2-deoxyglucose (2-DG) were used to study glucose uptake properties in established lines of cultured mouse cerebral microvessel endothelium. Uptake of both analogues was similar in terms of rate and absolute amount for the first two minutes. Thereafter, intracellular accumulation of 2-DG continued at a more rapid rate because of intracellular phosphorylation of this substrate. The uptake of 3-OMG uptake was temperature-dependent, independent of Na+, and not inhibited by ouabain or 2,4-dinitrophenol. Phloretin and cytochalasin B both significantly inhibited 3-OMG uptake. Other hexoses in high concentration acted as competitive inhibitors at the endothelial cell membrane. Pre-incubation of cells with 50 mM D-glucose resulted in higher levels of 3-OMG accumulation than in control cells (counter-transport phenomenon). In contrast to findings at the blood-brain barrier in vivo, insulin was found to stimulate 3-OMG uptake. Maximal stimulation of approximately 3-fold was found at ambient insulin concentrations of 1,000 ng/ml or higher. The findings provide support at the cellular level for some components of the model of carrier-mediated glucose transport across the blood-brain barrier which has been postulated to exist in vivo. The effect of insulin is discussed in the light of new data that show stimulation of glucose analogue transport into isolated cerebral capillaries.

Effects of insulin on peripheral and splanchnic glucose metabolism in noninsulin-dependent (type II) diabetes mellitus.

The mechanism(s) and site(s) of the insulin resistance were examined in nine normal-weight noninsulin-dependent diabetic (NIDD) subjects. The euglycemic insulin clamp technique (insulin concentration approximately 100 microU/ml) was employed in combination with hepatic and femoral venous catheterization and measurement of endogenous glucose production using infusion of tritiated glucose. Total body glucose metabolism in the NIDD subjects (4.37 +/- 0.45 mg/kg per min) was 38% (P less than 0.01) lower than in controls (7.04 +/- 0.63 mg/kg per min). Quantitatively, the most important site of the insulin resistance was found to be in peripheral tissues. Leg glucose uptake in the diabetic group was reduced by 45% as compared with that in controls (6.0 +/- 0.2 vs. 11.0 +/- 0.1 mg/kg leg wt per min; P less than 0.01). A strong positive correlation was observed between leg and total body glucose uptake (r = 0.70, P less than 0.001). Assuming that muscle is the primary leg tissue responsible for glucose uptake, it could be estimated that 90 and 87% of the infused glucose was disposed of by peripheral tissues in the control and NIDD subjects, respectively. Net splanchnic glucose balance during insulin stimulation was slightly more positive in the control than in the diabetic subjects (0.31 +/- 0.10 vs. 0.05 +/- 0.19 mg/kg per min; P less than 0.07). The difference (0.26 mg/kg per min) in net splanchnic glucose balance in NIDD represented only 10% of the reduction (2.67 mg/kg per min) in total body glucose uptake in the NIDD group and thus contributed very little to the insulin resistance. The results emphasize the importance of the peripheral tissues in the disposal of infused glucose and indicate that muscle is the most important site of the insulin resistance in NIDD.

Hyperinsulinaemia and insulin insensitivity: studies in subjects with insulinoma.

Hepatic glucose turnover, peripheral insulin sensitivity and insulin receptor binding were measured in four subjects with insulinoma before and 3 months after surgical resection of the insulinoma. Basal hepatic glucose production, quantitated employing a primed constant infusion of tritiated glucose, was low pre-operatively (5.2 +/- 1.7 mumol X kg-1 X min-1) but returned to normal post-operatively (14.9 +/- 2.8; normal subjects 13.9 +/- 0.8 mumol X kg-1 X min-1). Paired euglycaemic dose-response curves were developed for each subject. Insulin sensitivity, expressed as a right shift of the dose-response curve (ED50), was low pre- and post-operatively. However, insulin responsiveness (Vmax) remained normal (pre-operatively 13.9 +/- 2.2, post-operatively 13.8 +/- 0.8, normal subjects 16.7 +/- 0.8 ml X kg-1 X min-1). There was no consistent pattern in monocyte or erythrocyte receptor binding before or after surgery. These data suggest that the chronic hyperinsulinaemia causes suppression of hepatic glucose production, and a state of insulin insensitivity which appears to be due to a post-receptor defect.

Effects of a High-Starch Diet with Low or High Fiber Content on Postabsorptive Glucose Utilization and Glucose Production in Normal Subjects

The effects of fiber, added to a high-starch diet, on glucose and insulin metabolism were studied in seven healthy men in a controlled environment. Diets rich in starch (carbohydrate provided 62% energy) contained either 16 g or 100 g of fiber from several sources of food and were given for 10-day periods. Two parameters of glucose control were measured: glucose metabolism during insulin-glucose infusions (seven subjects) and glucose production measured by infusing tritiated glucose tracer (five subjects). Three sets of studies were carried out in the sequence: low-fiber, high-fiber, low-fiber. The respective mean values (+/- SE) for glucose utilization were 6.70 +/- 1.4, 7.01 +/- 1.02, and 6.77 +/- 1.34 mg/kg X min. Analysis of variance failed to show a significant effect of dietary change. Values for basal glucose production were 2.0 +/- 0.1, 1.9 +/- 0.2, and 2.4 +/- 0.3 mg/kg X min with the low-fiber, high-fiber, and low-fiber diets, respectively, which were not significantly different. The one significant response to the high-fiber diet was a lowering in plasma cholesterol, the mean values for the seven subjects during the three periods being 154 +/- 12, 138 +/- 10, and 156 +/- 13 mg/dl (P less than 0.05).

Bacterial uptake of algal extracellular products: an experimental approach

Methods for partitioning planktonic algal and bacterial populations were examined. If samples were filtered under gravity through a 1 μm pore size Nuclepore polycarbonate membrane the algae were retained on the filter and most (usually > 80%) of the bacteria were found in the filtrate. The application of any vacuum or pressure during this process resulted in the appearance of chlorophyll in the filtrate. In radiotracer experiments this would lead to an overestimate of 14C incorporation into the bacteria. There was excellent agreement between measurements of partition efficiency by epifluorescence direct counts of bacteria and by uptake of trace quantities of tritiated glucose. The methods were applied to the measurement of algal excretion of dissolved organic matter and its uptake by bacteria over light‐dark cycles. An illuminated water bath was adapted to provide shortened (3–24 h) light‐dark cycles which could be used to examine interactions between individual algal and bacterial species. Estimates of bacterial heterotrophic production based on the assimilation of labelled algal extracellular products were lower than those calculated from the incorporation of SO2‐4.

Secondary tritium and carbon-14 isotope-effects on ion-exchange chromatography of the d-glucose-borate complex

Secondary, hydrogen isotope-effects are observed when d-[1- 3H]glucose, d-[2- 3H]glucose, d-[3- 3H]glucose, d-[5- 3H]glucose, and d-[6- 3H]glucose are each co-chromatographed with nonlabeled d-glucose. A secondary, carbon isotope-effect is also observed when nonlabelled d-glucose is co-chromatographed with d-[U- 14C]glucose; no isotope effect is seen with d-[6- 14C]glucose. These isotope effects have been quantitatively analyzed. The chromatographic separation of radiolabeled glucose from glucose is the result of a differential effect of isotopic substitution on complexation of the hydroxyl oxygen atoms with a proton and boron. Tritium in positions 2, 3, 5, and 6, but not in position 1, apparently increases complexation with a proton (a pK increase), but does not similarly increase complexation with boron. As a result, the tritiated glucose is eluted more rapidly on the ion-exchange resin. Based upon this effect, it is possible to obtain isotopically pure, tritiated glucose by ion-exchange fractionation.

Preliminary ultrastructural and autoradiographic evidence that the trophonema of the sea anemoneActinia fragaceahas a nutritive function

The developing oocyte of the sea anemone Actinia fragacea is associated with a distinct group of gonad epithelial cells which constitute the trophonema. Electron microscopy has shown that the cells of the trophonema extend through a pore in the mesoglea which surrounds the oocyte, and make intimate contact with the oocyte surface. The ooplasm beneath this region of contact differs from the rest of the oocyte in containing numerous small vesicles, but few yolk granules or other organelles. Light microscope autoradiography has shown that oocytes within gonads can take up and incorporate tritiated glucose and leucine from solution. The cells of the trophonema appear more active in precursor incorporation than other gonad epithelial cells. The evidence therefore suggests that these cells have a nutritive function during oogenesis.

Uptake of Organic Matter by Meiofauna Organisms and Interrelationships with Bacteria

Investigations of bacteria and meiofauna in sandy beach sediments of the brackish-water Kiel Fjord and Kiel Bight (Baltic Sea, FRG) indicate inverse interrelationships: high numbers and biornass (carbon) of meiofauna are correlated with low numbers and biornass of bacteria, and vice versa. Bacterial carbon is 4 times as high as meiofauna carbon; this reflects the extreme ecological character of the wave-washed beach sediments studied. Tracer experiments using tritiated glucose reveal different participation of individual groups of meiofauna in the incorporation of labelled material. Oligochaetes (exclusively detritovores) show the highest incorporation rate of organic material, followed by turbellarians and nematodes. There is some indication that particulate organic matter originally derived from bacteria and that their extracellular products represented the major part of the organic material taken up. Laboratory experiments stress the necessity of studying melofauna activity under close-to-natural conditions.

Characterisation of bound residues of [3H]triforine in barley grain grown in the field

AbstractBarley was grown in an experimental field and at growth stage J (during the stem extension stage when the second node of the stem was formed and the next‐to‐last leaf was just visible), the aerial part of the plants was sprayed with an aqueous emulsion of a mixture of triforine and [3H]triforine (uniformly labelled in the piperazine ring) at the normal rate of 250 g triforine ha−1. The barley was harvested when ripe, and the grain was analysed separately. Extraction of the grain with methanol left methanol‐insoluble solids containing an amount of radioactivity (the bound residue) which represented 75% of the total radioactivity incorporated into the grain. Methanol acidified with hydrochloric acid extracted a further 7% of the triforine‐derived bound residues in the form of radioactive iminodiacetic acid (1.1%), glycine (3.3 %), serine (0.9%), ethanolamine (0.2%) and unidentified compounds (1.5 %); in the grain, these compounds or their precursors had thus been complexed to grain constituents. Aqueous 0.03M sodium hydroxide extracted a further 27% of the total tritium which had been incorporated by means of α chemical bonds into the protein fraction; acid hydrolysis of the proteins yielded radioactive glycine (9.2%), serine (3.9%) and unidentified compounds (13.9%) which could have been a mixture of a large number of other amino‐acids. The plant solids (which contained 41% of the total tritium) left after the alkaline aqueous extractions, were processed and separated into tritiated cellulose (4%) and starch (37%) fractions. The starch was hydrolysed and the resulting glucose was converted into the osazone (34%). After being recrystallised several times, the osazone contained a constant specific radioactivity, indicating that [3H]glucose was present. This glucose may be considered as having a carbon skeleton mainly originating biochemically from some metabolites of [3H]triforine; this tritiated glucose may also be considered as indirect evidence of the biochemical oxidation, by transamination, of the metabolites of [3H]triforine; in barley grain, the tritiated glucose (or at least a part of it) was anabolised into proteins. No piperazine was observed in the bound residues in the grain.

Glucose intolerance and aging: evidence for tissue insensitivity to insulin.

The relative contributions of impaired insulin secretion and of impaired tissue sensitivity to insulin to the glucose intolerance of aging were examined in 84 healthy volunteers, ranging in age from 21 to 84 yr, employing the hyperglycemie and euglycemic insulin clamp techniques, respectively. HYPERGLYCEMIC CLAMP. The blood glucose concentration was acutely raised and was maintained at 125 mg/dl above basal levels for 2 h. Since the glucose concentration was held constant, the glucose infusion rate was an index of glucose metabolism (M). In young subjects, M averaged 9.48 ± 0.40 mg/kg · min compared with 6.48 ± 0.28 in old subjects (P < 0.001). When all subjects were considered together, a progressive age-related decline in M was observed (r = –0.665, P < 0.001). The plasma insulin response (I) was biphasic, with an early burst within the first 6 min, followed by a phase of gradually increasing insulin concentration. No difference in either the early or late phases of insulin secretion was observed between young and old subjects. Consequently, the M/l (×100) ratio, an index of tissue sensitivity to endogenous insulin, decreased from 14.90 ± 1.01 to 10.98 ± 0.81 mg/kg min per μU/ml (P < 0.005). EUGLYCEMIC INSULIN CLAMP. The plasma insulin concentration was acutely raised and was maintained at about 100 μU/ml above basal levels by a primed continuous infusion of insulin. The blood glucose concentration was held constant at the basal level by a variable glucose infusion. M/I (×100), again, was a measure of tissue sensitivity to insulin (exogenous) and was decreased in old (4.95 ± 0.31 mg/kg · min per μU/ml) versus young (6.95 ± 0.45) subjects (P < 0.001). Hepatic glucose production was measured with tritiated glucose during the euglycemic clamp study; it declined similarly in young (to 0.13 ± 0.05 mg/kg · min) and old (to 0.09 ± 0.03 mg · min) subjects. In conclusion, under the present experimental conditions, employing intravenous glucose and/or insulin, impaired tissue sensitivity to insulin is the primary factor responsible for the decrease in glucose tolerance observed with advancing age. Since hepatic glucose production is normally suppressed by insulin in old subjects, the site of insulin resistance must reside in peripheral tissues. Beta cell response to glucose, as determined by the hyperglycemie clamp technique, cannot account for the age-related decline in M.

Insulin Sensitivity and Insulin Binding to Monocytes in Maturity-Onset Diabetes

Tissue sensitive to insulin and insulin binding to monocytes were evaluated in 15 nonobese maturity-onset diabetics and in 16 healthy controls. Insulin sensitivity was determined by the insulin clamp technique in which the plasma insulin is acutely raised and maintained 100 muU/ml above the fasting level and plasma glucose is held constant at fasting levels by a variable glucose infusion. The amount of glucose infused is a measure of overall tissue sensitivity to insulin. In the diabetic group, the fasting plasma glucose concentration (168+/-4 mg/dl) was 85% greater than controls (P < 0.01) whereas the plasma insulin level (15+/-1 muU/ml) was similar to controls. During the insulin clamp study, comparable plasma insulin levels were achieved in the diabetics (118+/-5) and the controls (114+/-5 muU/ml). However, the glucose infusion rate in the diabetics (4.7+/-0.4 mg/kg.min) was 30% below controls (P < 0.01). Among the diabetics, the glucose infusion rate correlated directly with the fasting plasma glucose level (r = 0.57, P < 0.05). In five diabetic subjects, glucose metabolism was similar to controls, and these diabetics had the highest fasting glucose levels. When they were restudied after prior normalization (with insulin) of the fasting plasma glucose (100+/-1 mg/dl), the glucose infusion rate during the insulin clamp was 30% lower than observed in association with hyperglycemia (P < 0.01). Studies that employed tritiated glucose to measure endogenous glucose production indicated comparable 90-95% inhibition of hepatic glucose production during hyperinsulinemia in the diabetic and control subjects.(125)I-insulin binding to monocytes in the diabetics (5.5+/-0.6%) was 30% below that in controls (P < 0.01). Insulin binding to monocytes and insulin action as determined with the insulin clamp were highly correlated in both control (r = 0.67, P < 0.01), and diabetic subjects (r = 0.88, P < 0.001). We conclude that (a) tissue sensitivity to physiologic hyperinsulinemia is reduced in most maturity-onset diabetics; (b) this decrease in sensitivity is located, at least in part, in extrahepatic tissues; (c) the resistance to insulin may be mediated by a reduction in insulin binding; and (d) in maturity-onset diabetics with normal tissue sensitivity to insulin, hyperglycemia may be a contributing factor to the normal rates of insulin-mediated glucose uptake.

The role of microfauna in detrital utilization by the polychaete, Nereis succinea (Frey and Leuckart)

Net incorporation rates of detritus derived from carbon-14 labelled seaweed, Gracilaria foliifera (Harvey) Taylor, by the polychaete, Nereis succinea (Frey and Leuckart), were measured in the presence and absence of the ciliate, Aspidisca sp. Net incorporation rates averaged 79 in the absence and 129 μg dry wt detritus·mg −1 dry wt worm·day −1 in the presence of ciliates. Ciliates did not significantly affect oxidation rates. In the presence of Nereis succinea, however, oxidation rates approximately doubled due to respiration and feeding activity. The increase in net incorporation probably resulted from a combination of stimulation of bacteria by grazing, fragmentation of detritus by ciliates, and ingestion of ciliates. The rate of heterotrophic uptake of tritiated glucose was used as a relative measure of microbial activity in the sediment. Glucose flux was greatest when N. succinea and Aspidisca were present together. Our data clarify the regulatory rôle of the microfaunal community in the transfer of energy from detritus to the macrodetritivore and the rôle of macrofauna in enhancing microbial activity in the sediment through bioturbation.

Glucose Intolerance in Uremia

The relative contributions of impaired insulin secretion and of tissue insensitivity to insulin to the carbohydrate intolerance of uremia were investigated in 10 chronically uremic subjects. Two types of glucose-clamp experiments were performed in each patient before and after 10 wk of thrice weekly hemodialysis. In both types the blood glucose concentration was maintained at a constant level by the periodic adjustment of a variable glucose infusion with a negative feedback formula.Hyperglycemic clamp. The blood glucose concentration was acutely raised and maintained 125 mg/dl above basal levels for 2 h. Since the glucose concentration was held constant, the glucose infusion rate is an index of glucose metabolism (M). After dialysis M increased in all patients from an average of 4.23 to 6.30 mg/kg body wt per min (P < 0.001). The plasma insulin responses (I) both pre- and postdialysis were biphasic with an early burst within the first 2-5 min, followed by a phase of gradually increasing insulin concentration. After dialysis the plasma insulin response diminished slightly. Consequently, the M/I ratio, an index of tissue sensitivity to endogenous insulin, increased postdialysis in all subjects by an average of 92% (P < 0.01). Euglycemic clamp. The plasma insulin concentration was acutely raised and maintained by a primecontinuous insulin infusion. The blood glucose concentration was held constant at the basal level by a variable glucose infusion as above. M/I again is a measure of tissue sensitivity to insulin (exogenous) and increased in all patients postdialysis by an average of 57% (P < 0.01). In two patients hepatic glucose production was measured with tritiated glucose during the euglycemic clamp and declined by 84% predialysis. A similar decrease (82%) was observed postdialysis. Thus, both the hyperglycemic and euglycemic clamp techniques demonstrated tissue insensitivity to insulin to be the dominant carbohydrate defect in uremia. The surprising apparent lack of consistency in the change in beta cell response postdialysis is explained by the strong inverse correlation between beta cell sensitivity to glucose and tissue sensitivity to insulin (r = -0.920; P < 0.001). Those individuals who showed the most striking improvement in tissue sensitivity to insulin actually decreased their serum insulin response to hyperglycemia; those whose improvement in tissue sensitivity was more modest showed increases in beta cell responses.

Insulin Binding to Monocytes and Insulin Action in Human Obesity, Starvation, and Refeeding

Insulin binding to monocytes and insulin action in vivo was examined in 14 obese subjects during the postabsorptive state and after starvation and refeeding. Tissue sensitivity to insulin was evaluated with the euglycemic insulin clamp technique. The plasma insulin concentration is acutely raised and maintained 100 muU/ml above the fasting level, and plasma glucose is held constant by a variable glucose infusion. The amount of glucose infused is a measure of tissue sensitivity to insulin and averaged 285+/-15 mg/m(2) per min in controls compared to 136+/-13 mg/m(2) per min in obese subjects (P <0.001). (125)I-Insulin binding to monocytes averaged 8.3+/-0.4% in controls vs. 4.6+/-0.5% in obese subjects (P < 0.001). Insulin binding and insulin action were highly correlated in both control (r = 0.86, P < 0.001) and obese (r = 0.94, P < 0.001) groups. Studies employing tritiated glucose to measure glucose production indicated hepatic as well as extrahepatic resistance to insulin in obesity. After 3 and 14 days of starvation, insulin sensitivity in obese subjects decreased to 69+/-4 and 71+/-7 mg/m(2) per min, respectively, whereas (125)I-insulin binding increased to 8.8+/-0.7 and 9.0+/-0.4%. In contrast to the basal state, there was no correlation between insulin binding and insulin action. After refeeding, tissue sensitivity increased to 168+/-14 mg/m(2) per min (P < 0.001) whereas insulin binding fell to 5.0+/-0.3%. We conclude that (a) in the postabsorptive state insulin binding to monocytes provides an index of in vivo insulin action in nonobese and obese subjects and, (b) during starvation and refeeding, insulin binding and insulin action changes in opposite directions suggesting that postreceptor events determine in vivo insulin sensitivity.