In order to quantitate the pathways by which liver glycogen is repleted, we administered [1-13C]glucose by gavage into awake 24-h fasted rats and examined the labeling pattern of 13C in hepatic glycogen. Two doses of [1-13C]glucose, 1 and 6 mg/g body wt, were given to examine whether differences in the plasma glucose concentration altered the metabolic pathways via which liver glycogen was replenished. After 1 and 3 h (high-dose group) and after 1 and 2 h (low-dose group), the animals were anesthetized and the liver was quickly freeze-clamped. Liver glycogen was extracted and the purified glycogen hydrolyzed to glucose with amyloglucosidase. The distribution of the 13C-label was subsequently determined by 13C-nuclear magnetic resonance spectroscopy. The percent 13C enrichment of the glucosyl units in glycogen was: 15.1 +/- 0.8%(C-1), 1.5 +/- 0.1%(C-2), 1.2 +/- 0.1%(C-3), 1.1 +/- 0.1%(C-4), 1.6 +/- 0.1%(C-5), and 2.2 +/- 0.1%(C-6) for the high-dose study (n = 4, at 3 h); 16.5 +/- 0.5%(C-1), 2.0 +/- 0.1%(C-2), 1.3 +/- 0.1%(C-3), 1.1 +/- 0.1%(C-4), 2.2 +/- 0.1%(C-5), and 2.4 +/- 0.1%(C-6) in the low-dose study (n = 4, at 2 h). The average 13C-enrichment of C-1 glucose in the portal vein was found to be 43 +/- 1 and 40 +/- 2% in the high- and low-dose groups, respectively. Therefore, the amount of glycogen that was synthesized from the direct pathway (i.e., glucose----glucose-6-phosphate----glucose-1-phosphate----UDP-glucose---- glycogen) was calculated to be 31 and 36% in the high- and low-dose groups, respectively. The 13C-enrichments of portal vein lactate and alanine were 14 and 14%, respectively, in the high-dose group and 11 and 8%, respectively, in the low-dose group. From these enrichments, the minimum contribution of these gluconeogenic precursors to glycogen repletion can be calculated to be 7 and 20% in the high- and low-dose groups, respectively. The maximum contribution of glucose recycling at the triose isomerase step to glycogen synthesis (i.e., glucose----triose-phosphates----glycogen) was estimated to be 3 and 1% in the high- and low-dose groups, respectively. In conclusion, our results demonstrate that (a) only one-third of liver glycogen repletion occurs via the direct conversion of glucose to glycogen, and that (b) only a very small amount of glycogen synthesis can be accounted for by the conversion of glucose to triose phosphates and back to glycogen; this suggests that futile cycling between fructose-6-phosphate and fructose-1,6-diphosphate under these conditions is minimal. Our results also show that (c) alanine and lactate account for a minimum of between 7 and 20% of the glycogen synthesized, and that (d) the three pathways through which the labeled flux is measured account for a total of only 50% of the total glycogen synthesized. These results suggest that either there is a sizeable amount of glycogen synthesis via pathway(s) that were not examined in the present experiment or that there is a much greater dilution of labeled alanine/lactate in the oxaloacetate pool than previously appreciated, or some combination of these two explanations.

We examined the ability of physiological hyperinsulinemia to enhance potassium and glucose uptake by splanchnic and peripheral tissues in 12 chronically uremic subjects by using the euglycemic insulin clamp technique in combination with hepatic and femoral venous catheterization. In control subjects, the decline in plasma potassium concentration averaged 0.95 +/- 0.05 meq/liter. Splanchnic (67 +/- 10.3 mu eq/min) and leg (22.2 +/- 1.4 mu eq/min) potassium uptake accounted for 43 and 59%, respectively, of the total amount of potassium that was translocated from the extracellular to intracellular fluid compartment. In uremic individuals, the decline in plasma potassium concentration (0.98 +/- 0.10) was similar to controls. Likewise, the mean splanchnic (66.6 +/- 6.1 mu eq/min) and leg (22.4 +/- 1.6 mu eq/min) potassium uptakes were similar to controls. These results indicate that insulin-mediated potassium uptake is not altered by uremia. In contrast, insulin-mediated glucose uptake is markedly impaired. These observations suggest that the various actions of insulin can be differentially impaired by uremia and that steps distal to the insulin receptor must be responsible for the insulin resistance.

The evidence appears overwhelming that insulin resistance is the major cause of the carbohydrate intolerance observed in chronically uremic subjects and that the primary site of this insulin resistance resides in peripheral tissues, muscle. Diminished lipoprotein lipase activity, perhaps related to decreased insulin sensitivity, is in large part responsible for the hypertriglyceridemia. However, there is some evidence that increased VLDL synthesis also contributes to the disturbance in lipid metabolism. This latter abnormality may be related to the high circulating insulin levels that result from the insulin resistance. In contrast to glucose and lipid metabolism, the plasma amino acid and potassium lowering effects of insulin are normal in uremic individuals. Whether there is an impaired ability of insulin to stimulate protein synthesis and inhibit protein degradation remains to be delineated.

Insulin resistance in uremia mediated by postbinding defects. Uremic subjects exhibit reduced glucose uptake in response to physiological increments in plasma insulin concentration. The relative contributions of insulin binding versus postbinding defects to this insulin resistance are not known. Because insulin normally requires only a small portion of its receptors to exert maximal biological effects on glucose uptake, it should be possible to overcome the insulin resistance of uremia, if it is due to a binding defect, by utilizing sufficiently high insulin levels to draw on the large reserve of spare receptors. If postbinding defects are present, however, then a decreased insulin response would be expected at high insulin concentrations. To determine whether or not the insulin resistance of uremia could be overcome by very high insulin levels, the dose-response relationship between plasma insulin concentration and insulin-mediated glucose uptake in seven chronically uremic and nine control subjects was evaluated. Insulin was infused at rates of 20, 40, 200, and 400 mU/min·m 2 , and the plasma glucose concentration was held constant at the basal level by a variable glucose infusion (euglycemic insulin clamp technique). Under these steady state conditions of euglycemia, the glucose infusion rate plus endogenous glucose production (measured with 3 H-3-glucose) provides a measure of the amount of glucose taken up by the entire body. The steady state plasma insulin concentrations during the four insulin infusion protocols in the uremic subjects were consistently higher than in the controls. Despite the higher insulin levels in the uremics, the total amount of glucose taken up by the body was reduced by nearly 50% at each insulin plateau compared to controls. These results indicate that the in vivo dose-response curve relating plasma insulin concentration to glucose metabolism was shifted to the right in uremics and failed to normalize at high insulin levels. Basal hepatic glucose production was similar in uremic and control subjects. During the 20 and 40 mU/min·m 2 insulin clamp studies, suppression of hepatic glucose production in uremics (83 and 94%, respectively) was similar to controls (88 and 98%, respectively). Specific 125 I-insulin binding to circulating monocytes was measured in nine uremic and ten control subjects. No difference in insulin binding between the uremics and controls was observed. In normals, insulin binding correlated directly with glucose uptake and inversely with the fasting plasma insulin concentration. In uremics, no correlation was observed between insulin binding and glucose metabolism or between insulin binding and fasting plasma insulin concentration. It is concluded that in chronic renal failure insulin resistance is caused primarily by a postbinding defect in insulin action. La resistance a l'insuline dans l'uremie mediee par des anomalies survenant apres la liaison. Les uremiques ont une diminution de la captation du glucose en reponse a des augmentations physiologiques de la concentration plasmatique d'insuline. On ne connait pas le role relatif des anomalies de la liaison de l'insuline et de celles survenant apres la liaison dans cette resistance a l'insuline. Puisque, normalement, l'insuline n'a besoin que d'un petit nombre de recepteurs pour avoir ses effets biologiques maximum sur la captation du glucose, il devrait etre possible de depasser cette resistance a l'insuline d l'uremie, si elle etait due a un defaut de liaison, en utilisant des concentrations d'insuline suffisamment elevees pour faire appel aux grandes quantites de recepteurs de reserve. Au contraire, si des anomalies apres la liaison existent, on pourrait s'attendre a une diminution de la reponse insulinique pour de fortes concentrations d'insuline. Afin de determiner si la resistance insulinique de l'uremie peut etre depassee a de tres fortes concentrations d'insuline, la relation effet-dose entre la concentration plasmatique d'insuline et la captation du glucose dependant de l'insuline chez sept uremiques chroniques et neuf sujets controles etait etudie. L'insuline etait perfusee a des debits de 20, 40, 200 et 400 mU/min·m 2 , et la glycemie etait maintenue constante a sa valeur de base par une perfusion de glucose a debit variable (technique du clamp insulinique euglycemique). Dans ces conditions a l'equilibre euglycemique, le debit de perfusion de glucose plus la production de glucose endogene (mesuree avec du 3 H-3-glucose) fournit une mesure de la quantite de glucose utilisee par l'organisme entier. Les concentrations plasmatiques d'insuline a l'etat d'equilibre pendant les quatre protocoles de perfusion insulinique chez les uremiques etaient significativement plus hautes que chez les controles. Malgre les plus fortes concentrations d'insuline chez les uremiques, la quantite totale de glucose retenue par l'organisme etait diminuee d'environ 50% a chaque plateau insulinique, par rapport aux controles. Ces resultats indiquent que la courbe dose-reponse in vivo entre la concentration plasmatique d'insuline et le metabolisme du glucose etait decalee vers la droite chez les uremiques, et ne s'est pas normalisee a de fortes concentrations d'insuline. La production hepatique de base de glucose etait identique chez les uremiques et les controles. Au cours des etudes avec des debits d'insuline de 20 et 40 mU/min·m 2 , la suppression de la production hepatique de glucose chez les uremiques etait identique a celle des controles. La liaison specifique de 125 I insuline a des monocytes circulants a ete mesuree chez neuf uremiques et dix controles. Il n'a pas ete observe de difference entre la liaison de l'insuline chez les uremiques et chez les controles. Chez les normaux, la liaison insulinique etait directement correlee avec l'entree du glucose, et inversement correlee avec la concentration plasmatique d'insuline a jeun. Chez les uremiques, nous n'avons observe aucune correlation entre la liaison de l'insuline et le metabolisme du glucose ni entre la liaison de l'insuline et la concentration plasmatique d'insuline a jeun. Il est conclu qu'au cours de l'insuffisance renale chronique, la resistance a l'insuline est principalement due a un defaut de l'action de l'insuline en aval de sa liaison.

Tissue sensitivity to insulin was examined with the euglycemic insulin clamp technique in 17 chronically uremic and 36 control subjects. The plasma insulin concentration was raised by approximately 100 microU/ml and the plasma glucose concentration was maintained at the basal level with a variable glucose infusion. Under these steady-state conditions of euglycemia, the glucose infusion rate is a measure of the amount of glucose taken up by the entire body. In uremic subjects insulin-mediated glucose metabolism was reduced by 47% compared with controls (3.71 +/- 0.20 vs. 7.38 +/- 0.26 mg/kg . min; P less than 0.001). Basal hepatic glucose production (measured with [3H]-3-glucose) was normal in uremic subjects (2.17 +/- 0.04 mg/kg . min) and suppressed normally by 94 +/- 2% following insulin administration. In six uremic and six control subjects, net splanchnic glucose balance was also measured directly by the hepatic venous catheterization technique. In the postabsorptive state splanchnic glucose production was similar in uremics (1.57 +/- 0.03 mg/kg . min) and controls (1.79 +/- 0.20 mg/kg . min). After 90 min of sustained hyperinsulinemia, splanchnic glucose balance reverted to a net uptake which was similar in uremics (0.42 +/- 0.11 mg/kg . min) and controls (0.53 +/- 0.12 mg/kg . min). In contrast, glucose uptake by the leg was reduced by 60% in the uremic group (21 +/- 1 vs. 52 +/- 8 mumol/min . kg of leg wt; P less than 0.005) and this decrease closely paralleled the decrease in total glucose metabolism by the entire body. These results indicate that: (a) suppression of hepatic glucose production by physiologic hyperinsulinemia is not impaired by uremia, (b) insulin-mediated glucose uptake by the liver is normal in uremic subjects, and (c) tissue insensitivity to insulin is the primary cause of insulin resistance in uremia.