Rosa Hendler

Substrate Turnover during Prolonged Exercise in Man: SPLANCHNIC AND LEG METABOLISM OF GLUCOSE, FREE FATTY ACIDS, AND AMINO ACIDS

Arterial concentrations and substrate exchange across the leg and splanchnic vascular beds were determined for glucose, lactate, pyruvate, glycerol, individual acidic and neutral amino acids, and free fatty acids (FFA) in six subjects at rest and during 4 h of exercise at approximately 30% of maximal oxygen uptake. FFA turnover and regional exchange were evaluated using (14)C-labeled oleic acid. The arterial glucose concentration was constant for the first 40 min of exercise, but fell progressively thereafter to levels 30% below basal. The arterial insulin level decreased continuously, while the arterial glucagon concentration had risen fivefold after 4 h of exercise. Uptake of glucose and FFA by the legs was markedly augmented during exercise, the increase in FFA uptake being a consequence of augmented arterial levels rather than increased fractional extraction. As exercise was continued beyond 40 min, the relative contribution of FFA to total oxygen metabolism rose progressively to 62%. In contrast, the contribution from glucose fell from 40% to 30% between 90 and 240 min. Leg output of alanine increased as exercise progressed. Splanchnic glucose production, which rose 100% above basal levels and remained so throughout exercise, exceeded glucose uptake by the legs for the first 40 min but thereafter failed to keep pace with peripheral glucose utilization. Total estimated splanchnic glucose output was 75 g in 4 h, sufficient to deplete approximately 75% of liver glycogen stores. Splanchnic uptake of gluconeogenic precursors (lactate, pyruvate, glycerol, alanine) had increased 2- to 10-fold after 4 h of exercise, and was sufficient to account for 45% of glucose release at 4 h as compared to 20-25% at rest and at 40 min of exercise. In the case of alanine and lactate, the increase in precursor uptake was a consequence of a rise in splanchnic fractional extraction. It is concluded that during prolonged exercise at a low work intensity (a) blood glucose levels fall because hepatic glucose output fails to keep up with augmented glucose utilization by the exercising legs; (b) a large portion of hepatic glycogen stores is mobilized and an increasing fraction of the splanchnic glucose output is derived from gluconeogenesis; (c) blood-borne substrates in the form of glucose and FFA account for a major part of leg muscle metabolism, the relative contribution from FFA increasing progressively; and (d) augmented secretion of glucagon may play an important role in the metabolic adaptation to prolonged exercise by its stimulatory influence on hepatic glycogenolysis and gluconeogenesis.

Insulin resistance in uremia.

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.

Regulation of Splanchnic and Peripheral Glucose Uptake by Insulin and Hyperglycemia in Man

We investigated the effects of hyperinsulinemia and hyperglycemia on peripheral glucose uptake, hepatic glucose production, and splanchnic glucose uptake in man. Euglycemic and hyperglycemic clamp studies were carried out in 37 healthy subjects in combination with hepatic vein catheterization and [3H-3]glgcose infusion. In the basal state, hepatic glucose production ([3H-3]glucose) exceeded net splanchnic glucose output (catheter) in every subject (2.3 ± 0.04 versus 1.7 ± 0.07 mg/min · kg, P < 0.001), indicating uptake of glucose by the splanchnic region at a rate of 0.6 ± 0.05 mg/ min · kg. In agreement with this estimate, [3H-3]glucose concentration was consistently lower in hepatic venous than in arterial blood, by 3.0 ± 0.2% (P < 0.001). When plasma insulin levels were raised to 37 ± 2, 53 ± 2, 101 ± 2, 428 ± 37, and 1189 ± 14 μU/ml, with maintenance of euglycemia, total glucose uptake rose to 2.9 ± 0.4, 3.9 ± 1.0, 5.1 ± 0.4, 9.9 ±1.1, and 11.8 ± 1.3 mg/min · kg, respectively. The whole body glucose clearance rose significantly above baseline at each hyperinsulinemic plateau (P < 0.05 or less). Hepatic glucose production fell by 68% (P < 0.01) at the lowest hyperinsulinemic level, by 87% at insulin levels of 53 ± 2 μU/ml, and by over 95% with each higher insulin dose. Splanchnic glucose uptake was not significantly increased over basal values at any insulin concentration. When plasma glucose levels were raised to 137 ± 3 and 224 ± 2 mg/dl peripheral plasma insulin levels rose to 20 ± 4 and 55 ± 5 μU/ml, respectively. Total glucose uptake was enhanced (2.5 ± 0.4 and 5.3 ± 1.0 mg/min · kg, P < 0.05 and P < 0.01, respectively). Suppression of hepatic glucose production was <90% at the lower hyperglycemic level, and virtually complete at the higher one. Splanchnic glucose uptake was not changed by mild hyperglycemia (0.5 ± 0.05 mg/min · kg), but rose significantly (1.3 ± 0.3 mg/ min · kg, P < 0.01) with further hyperglycemia. The latter effect resulted primarily from increased glucose delivery to the splanchnic area, since the splanchnic glucose extraction ratio (4.0 ± 0.3%) was not different from baseline (3.0 ± 0.3%). When hyperglycemia (224 ± 1 mg/dl) was combined with a somatostatin infusion, thereby reducing plasma insulin from 15 ± 3 to 10 ± 1 μU/ml (P < 0.01), both total glucose uptake (2.8 ± 0.03 mg/min · kg) and clearance (1.3 ± 0.01 mg/min · kg) were significantly (P < 0.01) lower than in the hyperglycemic studies in which insulin secretion was not blocked. Hepatic glucose production, however, was effectively suppressed (by 74%, P < 0.001), whereas splanchnic glucose uptake was only slightly increased above baseline. Replacement of insulin (via an exogenous infusion at a rate of 0.3 mU/min · kg) restored total glucose uptake, splanchnic glucose uptake, and suppression of hepatic glucose production to the levels seen with hyperglycemia without somatostatin. When hyperglycemia (216 ± 2 mg/dl) was combined with somatostatin and glucagon replacement (no insulin), hepatic glucose production was still suppressed by 47 ± 1% to 1.18 ± 0.01 mg/kg · min (P < 0.001 versus hyperglycemia + SRIF without glucagon replacement). The results indicate that both hyperglycemia and hypoglucagonemia contribute to the decline in hepatic glucose production following somatostatin infusion. In conclusion, hyperinsulinemia alone stimulates glucose uptake by peripheral but not splanchnic tissues. The dose-response characteristics of stimulation of peripheral glucose uptake and inhibition of hepatic glucose production by insulin are very different, the half-maxima being ∼120 and ∼50 μU/ml, respectively. Hyperglycemia enhances glucose uptake by both peripheral and splanchnic tissues, but this action requires an intact endogenous insulin response. In contrast, hyperglycemia can suppress endogenous glucose production even in the presence of low insulin levels.

Insulin Resistance is a Prominent Feature of Insulin-dependent Diabetes

Tissue sensitivity to insulin was examined in 36 control subjects and 19 insulin-dependent diabetics with diabetes of long-standing duration (mean = 10 ± 3 yr) employing the insulin clamp technique (Δ plasma insulin concentration ∼100 βU/ml). Eleven of the diabetics (group I) were studied at their fasting hyperglycemic level (173 mg/dl); the remaining 8 diabetics (group II) were studied after lowering their plasma glucose concentration to euglycemic levels (90 mg/dl). Despite plasma glucose levels that were almost twice as great in the diabetics (group 1,173 versus 91 mg/dl, P < 0.001), insulin-mediated glucose metabolism, 4.77 ± 0.18 mg/kg · min, was reduced by 32% versus controls, 7.03 ± 0.22 mg/kg · min (P < 0.01). When the control subjects were restudied at plasma glucose levels (166 ± 2 mg/dl) that were comparable to those of the diabetics, insulin-mediated glucose metabolism was 12.14 ± 0.96 mg/kg · min (P < 0.01). In diabetics studied at euglycemic levels (group II) insulin-mediated glucose metabolism, 3.39 ± 0.30 mg/kg · min, was reduced even further. The metabolic clearance rate in the 19 diabetics, 3.31 ± 0.23 mg/kg · min, was reduced by 58% compared with controls, 7.83 / 0.25 (P < 0.001). These results emphasize the severe degree of insulin resistance that exists in the insulin-dependent diabetics. Basal hepatic glucose production in the diabetic group, 2.96 ± 0.24 mg/kg · min, was 26% greater than in the controls, 2.35 ± 0.04 (P < 0.001). The fasting plasma glucose concentration displayed a strong positive correlation (r = 0.857, P < 0.001) with basal hepatic glucose production and was weakly and inversely correlated (r = −0.413, P = 0.07) with the basal glucose clearance. Following hyperinsulinemia, however, suppression of hepatic glucose production was ∼ 9 5% in both diabetics and controls, suggesting that peripheral tissues are primarily responsible for the observed impairment in insulin-mediated glucose uptake. The present results indicate that impaired insulin action is a common feature of insulin-dependent diabetics, despite daily insulin requirements (35 ± 2 U/day) that would not clinically characterize them as being insulin resistant.

Effect of ketone infusions on amino acid and nitrogen metabolism in man.

To evaluate the role of hyperketonemia in the hypoalaninemia and decreased protein catabolism of prolonged starvation, Na dl-beta-hydroxybutyrate was administered as a primed continuous 3-6-h infusion in nonobese subjects and in obese subjects in the postabsorptive state and after 3 days and 3-5 1/2 wk of starvation. An additional obese group received 12-h ketone infusions on 2 consecutive days after 5-10 wk of fasting. The ketone infusion in nonobese and obese subjects studied in the postabsorptive state resulted in total blood ketone acid levels of 1.1-1.2 mM, a 5-15 mg/100 ml decrease in plasma glucose, and unchanged levels of insulin, glucagon, lactate, and pyruvate. Plasma alanine fell by 21% (P smaller than 0.001) in 3 h. In contrast, other amino acids were stable or varied by less than 10%. Infusions lasting 6 h reduced plasma alanine by 37%, reaching levels comparable to those observed in prolonged starvation. Equimolar infusions of NaC1 and/or administration of NaHCO3 failed to alter plasma alanine levels. During prolonged fasting, plasma alanine, which had fallen by 40% below prefast levels, fell an additional 30% in response to the ketone infusion. In association with repeated prolonged (12 h) infusions in subjects fasted 5-10 wk, urinary nitrogen excretion fell by 30%, returning to base line after cessation of theinfusions and paralleling the changes in plasma alanine. Ketone infusins resulted in two- to fourfold greater increments in blood ketone acids in fasted as compared to postabsorptive subjects. It is concluded that increased blood ketone acid levels induced by infusions of Na DL-beta-hydroxybutyrate result in hypoalaninemia and in nitrogen conservation in starvation. These data suggest that hyperketonemia may be a contributory factor in the decreased availability or circulating alanine and reduction in protein catabolism characteristic of prolonged fastings9

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.

Hyperglucagonemia and blood glucose regulation in normal, obese and diabetic subjects.

Glucagon was infused to maintain plasma concentrations three to six times the basal level (300 to 600 pg per milliliter) into 16 normal and seven non-diabetic obese subjects. Hyperglucagonemia caused only a transient rise of 5 to 10 mg per 100 ml in basal glucose levels and had no effect on oral glucose tolerance or plasma insulin. In three patients with adult and two with juvenile-onset diabetes on maintenance insulin, hyperglucagonemia maintained for two to four days caused no change in plasma glucose of ketone concentration. In contrast, in nine insulin-withdrawn patients the glycemic response to hyperglucagonemia was five to 15 times greater (P less then 0.05) than in normal controls. Hyperglucagonemia does not cause glucose intolerance in normal subjects or bring about deterioration of diabetic control when insulin is available. Glucagon in the insulin-deprived patient can worsen the diabetic state. These findings suggest the primary role of insulin deficiency in the diabetogenic action of glucagon.

Hyperglucagonemia in Laennec's cirrhosis. The role of portal-systemic shunting.

Abstract Plasma pancreatic glucagon concentrations were determined in 18 cirrhotic patients in the basal state and after the infusion of alanine, 0.15 g per kilogram of body weight in two to four minutes. In the cirrhotic patients, basal concentrations of glucagon were two to five times normal, and the glucagon response to alanine administration was three to 13 times greater than in 11 control subjects. Glucagon levels were highest in four patients with portacaval anastomosis, were less elevated in 10 cirrhotic patients with spontaneous portal-systemic shunting and were normal in four cirrhotic patients without portal-systemic shunting. Glucagon concentrations correlated closely in all three groups with the degree of ammonia intolerance. Elevations of plasma tyrosine, methionine and phenylalanine were observed in the cirrhotic group, whereas the levels of branched chain amino acids (valine, leucine and isoleucine) were reduced. Hyperglucagonemia may contribute to the glucose intolerance observed in cirrho...

Influence of Glucocorticoids on Glucagon Secretion and Plasma Amino Acid Concentrations in Man

Plasma concentrations of glucagon, insulin, glucose, and individual plasma amino acids were measured in normal nonobese and obese subjects before and after 3 days of dexamethasone treatment (2 mg/day) and in patients with Cushings syndrome. The subjects were studied in the basal postabsorptive state and following the infusion of alanine (0.15 g/kg) or ingestion of a protein meal. In nonobese subjects dexamethasone treatment resulted in a 55% increment in basal glucagon levels and in a 60-100% increase in the maximal glucagon response to alanine infusion or protein ingestion. In obese subjects, basal glucagon rose by 110% following dexamethasone, while the response to alanine increased fourfold. In patients with Cushings syndrome basal glucagon levels were 100% higher and the glucagon response to alanine infusion was 170% greater than in normal controls.Dexamethasone treatment in normal subjects resulted in a 40% rise in plasma alanine concentration which was directly proportional to the rise in basal glucagon. The remaining 14 amino acids were unchanged. In the patients with Cushings syndrome alanine levels were 40% higher than in normal controls and were directly proportional to basal glucagon concentrations. No other plasma amino acids were significantly altered in the group with Cushings syndrome. It is concluded that (a) glucocorticoids increase plasma glucagon concentration in the basal state and in response to protein ingestion or aminogenic stimulation; (b) this effect of glucocorticoids occurs in the face of obesity and persists in chronic hypercorticism; (c) hyperalaninemia is a characteristic of acute and chronic glucocorticoid excess, and may in turn contribute to steroid-induced hyperglucagonemia; and (d) increased alpha cell secretion may be a contributory factor in the gluconeogenic and diabetogenic effects of glucocorticoids.

Influence of Oral Glucose Ingestion on Splanchnic Glucose and Gluconeogenic Substrate Metabolism in Man

To evaluate the role of splanchnic and peripheral tissues in the disposal of an oral glucose load, splanchnic exchange of glucose, lactate, pyruvate, glycerol and amino acids was determined in ten healthy subjects in the basal state and for three hours following the oral ingestion of 100 gm. of glucose. Following glucose ingestion, splanchnic glucose output rose rapidly, reaching values two to three times the basal rate at fifteen minutes and returning to baseline by ninety minutes. A secondary rise in splanchnic glucose output occurred at 150 minutes and coincided with a secondary increment in arterial glucose. Total splanchnic glucose output over three hours was 40 ± 3 gm., representing a total increase of only 15 ± 3 gm. above basal splanchnic glucose output. The peak rise in blood glucose was directly proportional to the increase in splanchnic glucose output. Arterial concentrations of alanine, lactate and pyruvate rose by 15, 65 and 80 per cent, respectively, following oral glucose. These arterial elevations were preceded by a 75–100 per cent inhibition of splanchnic uptake of alanine and lactate; in the case of pyruvate there was a reversal from a net uptake in the basal state to a significant net splanchnic output after glucose ingestion. Arterial glycerol fell by 50 per cent and was accompanied by a comparable fall in splanchnic uptake. It is concluded that in normal, postabsorptive man, (a) the major portion of a 100 gm. oral glucose load is retained within the splanchnic bed; (b) only 15 per cent of the ingested glucose is available for disposal by peripheral tissues as increased (abovebasal) glucose utilization; (c) the height and shape of the oral glucose tolerance curve are largely determined by the rate and pattern of splanchnic glucose escape; (d) glucose-induced hyperlactatemia, hyperpyruvicemia and hyperalaninemia are due at least in part, to altered splanchnic exchange of these substrates.