Philip Felig

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.

Liver and kidney metabolism during prolonged starvation

This study quantifies the concentrations of circulating insulin, growth hormone, glucose, free fatty acids, glycerol, beta-hydroxybutyrate, acetoacetate, and alpha amino nitrogen in 11 obese subjects during prolonged starvation. The sites and estimated rates of gluconeogenesis and ketogenesis after 5-6 wk of fasting were investigated in five of the subjects. Blood glucose and insulin concentrations fell acutely during the 1st 3 days of fasting, and alpha amino nitrogen after 17 days. The concentration of free fatty acids, beta-hydroxybutyrate, and acetoacetate did not reach a plateau until after 17 days. Estimated glucose production at 5-6 wk of starvation is reduced to approximately 86 g/24 hr. Of this amount the liver contributes about one-half and the kidney the remainder. Approximately all of the lactate, pyruvate, glycerol, and amino acid carbons which are removed by liver and kidney are converted into glucose, as evidenced by substrate balances across these organs.

Amino acid metabolism during prolonged starvation

Plasma concentration, splanchnic and renal exchange, and urinary excretion of 20 amino acids were studied in obese subjects during prolonged (5-6 wk) starvation. Splanchnic amino acid uptake was also investigated in postabsorptive and briefly (36-48 hr) fasted subjects.A transient increase in plasma valine, leucine, isoleucine, methionine, and alpha-aminobutyrate was noted during the 1st wk of starvation. A delayed, progressive increase in glycine, threonine, and serine occurred after the 1st 5 days. 13 of the amino acids ultimately decreased in starvation, but the magnitude of this diminution was greatest for alanine which decreased most rapidly during the 1st week of fasting. In all subjects alanine was extracted by the splanchnic circulation to a greater extent than all other amino acids combined. Brief fasting resulted in an increased arterio-hepatic venous difference for alanine due to increased fractional extraction. After 5-6 wk of starvation, a marked falloff in splanchnic alanine uptake was attributable to the decreased arterial concentration. Prolonged fasting resulted in increased glycine utilization by the kidney and in net renal uptake of alanine. It is concluded that the marked decrease in plasma alanine is due to augmented and preferential splanchnic utilization of this amino acid in early starvation resulting in substrate depletion. Maintenance of the hypoalaninemia ultimately serves to diminish splanchnic uptake of this key glycogenic amino acid and is thus an important component of the regulatory mechanism whereby hepatic gluconeogenesis is diminished and protein catabolism is minimized in prolonged fasting. The altered renal extraction of glycine and alanine is not due to increased urinary excretion but may be secondary to the increased rate of renal gluconeogenesis observed in prolonged starvation.

The glucose-alanine cycle

Abstract Alanine is quantitatively the primary amino acid released by muscle and extracted by the splanchnic bed in postabsorptive as well as prolonged fasted man. The hepatic capacity for conversion of alanine to glucose exceeds that of all other amino acids. Insulin inhibits gluconeogenesis by reducing hepatic alanine uptake. In contrast, in diabetes, an increase in hepatic alanine extraction is observed in the face of diminished circulating substrate. In prolonged fasting, diminished alanine release is the mechanism whereby gluconeogenesis is reduced. In circumstances in which alanine is deficient, such as pregnancy and ketotic hypoglycemia of infancy, fasting hypoglycemia is accentuated. Augmented glucose utilization in exercise and hyperpyruvicemia consequent to inborn enzymatic defects are accompanied by increased circulating levels of alanine. These data thus suggest the existence of a glucose-alanine cycle in which alanine is formed peripherally by transamination of glucose-derived pyruvate and transported to the liver where its carbon skeleton is reconverted to glucose. The rate of recycling of glucose carbon skeletons in this pathway appears to occur at approximately 50% of that observed for the Cori (lactate) cycle.

Glucose metabolism during leg exercise in man

Arterial concentrations and net substrate exchange across the leg and splanchnic vascular bed were determined for glucose, lactate, pyruvate, and glycerol in healthy postabsorptive subjects at rest and during 40 min of exercise on a bicycle ergometer at work intensities of 400, 800, and 1200 kg-m/min. Rising arterial glucose levels and small decreases in plasma insulin concentrations were found during heavy exercise. Significant arterial-femoral venous differences for glucose were demonstrated both at rest and during exercise, their magnitude increasing with work intensity as well as duration of the exercise performed. Estimated glucose uptake by the leg increased 7-fold after 40 min of light exercise and 10- to 20-fold at moderate to heavy exercise. Blood glucose uptake could at this time account for 28-37% of total substrate oxidation by leg muscle and 75-89% of the estimated carbohydrate oxidation. Splanchnic glucose production increased progressively during exercise reaching levels 3 to 5-fold above resting values at the heavy work loads. Close agreement was observed between estimates of total glucose turnover during exercise based on leg glucose uptake and splanchnic glucose production. Hepatic gluconeogenesis-estimated from splanchnic removal of lactate, pyruvate, glycerol, and glycogenic amino acids-could supply a maximum of 25% of the resting hepatic glucose production but could account for only 6-11% of splanchnic glucose production after 40 min of moderate to heavy exercise. IT IS CONCLUDED THAT: (a) blood glucose becomes an increasingly important substrate for muscle oxidation during prolonged exercise of this type: (b) peripheral glucose utilization increases in exercise despite a reduction in circulating insulin levels: (c) increased hepatic output of glucose, primarily by means of augmented glycogenolysis, contributes to blood glucose homeostasis in exercise and provides an important source of substrate for exercising muscle.

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.

Amino acid balance across tissues of the forearm in postabsorptive man. Effects of insulin at two dose levels

Amino acid balance across skeletal muscle and across subcutaneous adipose tissue plus skin of the forearm has been quantified in postabsorptive man before and after insulin infusion into the brachial artery. Skeletal muscle released significant amounts of alpha amino nitrogen after an overnight fast. Most individual amino acids were released. Alanine output was by far the greatest. The pattern of release probably reflects both the composition of muscle protein undergoing degradation and de novo synthesis of alanine by transamination. A significant correlation was observed between the extent of release of each amino acid and its ambient arterial concentration. Elevation of forearm insulin in eight subjects from postabsorptive (12 muU/ml) to high physiologic levels (157 muU/ml) in addition to stimulating muscle glucose uptake blocked muscle alpha amino nitrogen release by 74%. Consistent declines in output were seen for leucine, isoleucine, tyrosine, phenylalanine, threonine, glycine, and alpha-aminobutyric acid. Alanine output was insignificantly affected. Doubling forearm insulin levels (from 10 to 20 muU/ml) in eight subjects increased muscle glucose uptake in three and blocked alpha amino nitrogen output in two although both effects were seen concurrently in only one subject. Changes in net amino acid balance after insulin could be accounted for by increased transport of amino acids into muscle cells or retardation of their exit. It is likely that ambient arterial amino acid concentrations are established and maintained primarily by the extent of muscle amino acid release. The individual amino acids whose outputs from forearm muscle decline after forearm insulinization correspond well with those whose levels fall systematically after systemic insulinization. This suggests that declines in amino acid levels after systemic insulinization are due to inhibition of muscle release. Doubling basal insulin approaches the threshold both for blockade of muscle amino acid output and stimulation of glucose uptake, effects which appear to occur independently.

Amino acid metabolism in exercising man.

Arterial concentration and net exchange across the leg and splanchnic bed of 19 amino acids were determined in healthy, postabsorptive subjects in the resting state and after 10 and 40 min of exercise on a bicycle ergometer at work intensities of 400, 800, and 1200 kg-m/min. Arterio-portal venous differences were measured in five subjects undergoing elective cholecystectomy. In the resting state significant net release from the leg was noted for 13 amino acids, and significant splanchnic uptake was observed for 10 amino acids. Peripheral release and splanchnic uptake of alanine exceeded that of all other amino acids, accounting for 35-40% of total net amino acid exchange. Alanine and other amino acids were released in small amounts (relative to net splanchnic uptake) by the extrahepatic splanchnic tissues drained by the portal vein. During exercise arterial ananine rose 20-25% with mild exertion and 60-96% at the heavier work loads. Both at rest and during exercise a direct correlation was observed between arterial alanine and arterial pyruvate levels. Net amino acid release across the exercising leg was consistently observed at all levels of work intensity only for alanine. Estimated leg alanine output increased above resting levels in proportion to the work load. Splanchnic alanine uptake during exercise exceeded that of all other amino acids and increased by 15-20% during mild and moderate exercise, primarily as a consequence of augmented fractional extraction of alanine. For all other amino acids, there was no change in arterial concentration during mild exercise. At heavier work loads, increases of 8-35% were noted for isoleucine, leucine, methionine, tyrosine, and phenylalanine, which were attributable to altered splanchnic exchange rather than augmented peripheral release. The data suggest that (a) synthesis of alanine in muscle, presumably by transamination of glucose-derived pyruvate, is increased in exercise probably as a consequence of increased availability of pyruvate and amino groups; (b) circulating alanine serves an important carrier function in the transport of amino groups from peripheral muscle to the liver, particularly during exercise; (c) a glucose-alanine cycle exists whereby alanine, synthesized in muscle, is taken up by the liver and its glucose-derived carbon skeleton is reconverted to glucose.

Synergistic interaction between exercise and insulin on peripheral glucose uptake.

The interaction of exercise and insulin on glucose metabolism was examined in 10 healthy volunteers. Four study protocols were used: study 1: plasma insulin was raised by approximately 100 microunits/ml while plasma glucose was maintained at basal levels for 2 h (insulin clamp). Study 2: subjects performed 30 min of bicycle exercise at 40% of VO2 max. Study 3: an insulin clamp was performed as per study 1. Following 60 min of sustained hyperinsulinemia, however, subjects exercised for 30 min as per study 2. Study 4: subjects were studied as per study 3 except that catheters were inserted into the femoral artery and vein to quantitate leg glucose uptake. During the 60-90 min period of hyperinsulinemia (study 1), glucose uptake averaged 8.73 +/- 0.10 mg/kg per min. With exercise alone (study 2), the increment in peripheral glucose uptake was 1.43 +/- 0.30 mg/kg per min. When hyperinsulinemia and exercise were combined (study 3), glucose uptake averaged 15.06 +/- 0.98 mg/kg per min (P less than 0.01) and this was significantly (P less than 0.001) greater than the sum of glucose uptake when exercise and the insulin clamp were performed separately. The magnitude of rise in glucose uptake correlated closely with the increase in leg blood flow (r = 0.935, P less than 0.001), suggesting that the synergism is the result of increased blood flow and increased capillary surface area to exercising muscle. More than 85% of total body glucose metabolism during studies 1 and 3 was accounted for by skeletal muscle uptake. These results demonstrate that (a) insulin and exercise act synergistically to enhance glucose disposal in man, and (b) muscle is the primary tissue responsible for the increase in glucose metabolism following hyperinsulinemia and exercise.

Effect of protein ingestion on splanchnic and leg metabolism in normal man and in patients with diabetes mellitus.

The inter-organ flux of substrates after a protein-rich meal was studied in seven healthy subjects and in eight patients, with diabetes mellitus. Arterial concentrations as well as leg and splanchnic exchange of amino acids, carbohydrate substrates, free fatty acids (FFA), and ketone bodies were examined in the basal state and for 3 h after the ingestion of lean beef (3 g/kg body wt). Insulin was withheld for 24 h before the study in the diabetic patients. In the normal subjects, after protein ingestion, there was a large amino acid release from the splanchnic bed predominantly involving the branched chain amino acids. Valine, isoleucine, and leucine accounted together for more than half of total splanchnic amino acid output. Large increments were seen in the arterial concentrations of the branched chain amino acids (100-200%) and to a smaller extent for other amino acids. Leg exchange of most amino acids reverted from a basal net outut to a net uptake after protein feeding which was most marked for the branched chain amino acids. The latter accounted for more than half of total peripheral amino acid uptake...