The effect of metabolic control on leucine metabolism in type 1 (insulin-dependent) diabetic patients

Stepwise Discovery of Insulin Effects on Amino Acid and Protein Metabolism

A clear effect of insulin deficiency and replacement on body/muscle mass was a landmark observation at the start of the insulin age. Since then, an enormous body of investigations has been produced on the pathophysiology of diabetes mellitus from a hormonal/metabolic point of view. Among them, the study of the effects of insulin on body growth and protein accretion occupies a central place and shows a stepwise, continuous, logical, and creative development. Using a metaphor, insulin may be viewed as a director orchestrating the music (i.e., the metabolic effects) played by the amino acids and proteins. As a hormone, insulin obviously does not provide either energy or substrates by itself. Rather, it tells cells how to produce and utilize them. Although the amino acids can be released and taken up by cells independently of insulin, the latter can powerfully modulate these movements. Insulin regulates (inhibits) protein degradation and, in some instances, stimulates protein synthesis. This review aims to provide a synthetic and historical view of the key steps taken from the discovery of insulin as an "anabolic hormone", to the in-depth analysis of its effects on amino acid metabolism and protein accretions, as well as of its interaction with nutrients.

The Effect of Glucagon on Protein Catabolism During Insulin Deficiency: Exchange of Amino Acids Across Skeletal Muscle and the Splanchnic Bed

Transient insulin deprivation with concurrent hyperglucagonemia is a catabolic state that can occur in type 1 diabetes. To evaluate glucagon's catabolic effect in the setting of its glucogenic effect, we measured the regional exchanges of amino acid metabolites (amino-metabolites) across muscle and splanchnic beds in 16 healthy humans during either somatostatin followed by glucagon or saline infusion alone. Despite a twofold or greater increase in the regional exchange of amino-metabolites by glucagon, whole-body kinetics and concentrations of amino acids (AA) remained stable. Glucagon increased the splanchnic uptake of not only gluconeogenic but also essential (EAA) AA while increasing their release from the muscle bed. Regional tracer-based kinetics and 3-methylhistidine release indicate that EAA release from muscle is likely caused by reduced protein synthesis rather than increased protein degradation. Furthermore, many metabolites known to affect insulin action and metabolism were altered by hyperglucagonemia including increase in branched-chain AA and keto acids of leucine and isoleucine in arterial plasma. Further, an increase in arterial concentrations of α-aminoadipic acid arising from increased conversion from lysine in the splanchnic bed was noted. These results demonstrate that hyperglucagonemia during hypoinsulinemia increases net muscle protein catabolism and substantially increases the exchange of amino metabolites across splanchnic and muscle beds.

Insulin Regulation of Proteostasis and Clinical Implications

Maintenance and modification of the cellular proteome are at the core of normal cellular physiology. Although insulin is well known for its control of glucose homeostasis, its critical role in maintaining proteome homeostasis (proteostasis) is less appreciated. Insulin signaling regulates protein synthesis and degradation as well as posttranslational modifications at the tissue level and coordinates proteostasis at the organism level. Here, we review regulation of proteostasis by insulin in postabsorptive, postprandial, and diabetic states. We present the effects of insulin on amino acid flux in skeletal muscle and splanchnic tissues, the regulation of protein quality control, and turnover of mitochondrial protein pools in humans. We also review the current evidence for the mechanistic control of proteostasis by insulin and insulin-like growth factor 1 receptors based on preclinical studies. Finally, we discuss irreversible posttranslational modifications of the proteome in diabetes and how future investigations will provide new insights into mechanisms of diabetic complications.

Exercise and type 1 diabetes (T1DM)

Physical exercise is firmly incorporated in the management of type 1 diabetes (T1DM), due to multiple recognized beneficial health effects (cardiovascular disease prevention being preeminent). When glycemic values are not excessively low or high at the time of exercise, few absolute contraindications exist; practical guidelines regarding amount, type, and duration of age-appropriate exercise are regularly updated by entities such as the American Diabetes Association and the International Society for Pediatric and Adolescent Diabetes. Practical implementation of exercise regimens, however, may at times be problematic. In the poorly controlled patient, specific structural changes may occur within skeletal muscle fiber, which is considered by some to be a disease-specific myopathy. Further, even in well-controlled patients, several homeostatic mechanisms regulating carbohydrate metabolism often become impaired, causing hypo- or hyperglycemia during and/or after exercise. Some altered responses may be related to inappropriate exogenous insulin administration, but are often also partly caused by the "metabolic memory" of prior glycemic events. In this context, prior hyperglycemia correlates with increased inflammatory and oxidative stress responses, possibly modulating key exercise-associated cardio-protective pathways. Similarly, prior hypoglycemia correlates with impaired glucose counterregulation, resulting in greater likelihood of further hypoglycemia to develop. Additional exercise responses that may be altered in T1DM include growth factor release, which may be especially important in children and adolescents. These multiple alterations in the exercise response should not discourage physical activity in patients with T1DM, but rather should stimulate the quest for the identification of the exercise formats that maximize beneficial health effects.

A negative feedback model for a mechanism based description of longitudinal observations. Application for bone turnover biomarkers

In modern medicine the diagnosis and prognosis of an abnormal metabolic condition is based on blood borne measurements involving one or more biomarker.

OBJECTIVE

This paper reports the development of a minimal negative feedback model for the description of longitudinal biomarkers concentrations for treatment of osteoporosis in postmenopausal women.

METHODS

Literature data were obtained from double-blind, placebo-controlled clinical trial over three years. There were four treatment groups: 1) Placebo, 2) Alendronate, 3) Conjugated Estrogen, and/or 4) Combination therapy. The negative feedback model consists of a biomarker and a companion controller. By considering the above basal biomarker values it is shown that the dynamics can be described by a second order differential equation without the involvement of biomarker production rate. The second order differential equation is also analogous to classical negative feedback servomechanism model with two parameters ω(n) and ξ. It was assumed that the rate constants defining the negative feedback model were equal which would set ξ to 0.707 with only ω(n) to be estimated.

RESULTS

ω(n) was estimated for both lumbar spine bone mineral density (BMD) and bone-specific alkaline phosphatase (BAP) in four treatments groups. The t(½) of BMD and BAP were estimated at 26.8 (0.30) and 9.4 (0.30) days respectively.

CONCLUSIONS

The negative feedback model of BMD supports the mechanism whereby Conjugated Estrogen and Alendronate decrease the clearance rate constant of BMD analogous to increased apoptosis of osteoclasts. The linked negative feedback models facilitate a mechanism based prediction of BMD using the concentrations of the bone turnover marker BAP.

Effects of type 2 diabetes and insulin on whole-body, splanchnic, and leg protein metabolism

CONTEXT

Type 2 diabetes (T2D) is characterized by insulin resistance to glucose metabolism. Most studies suggest that protein metabolism is unaffected by T2D, but regional protein metabolism and response to multiple doses of insulin have not been examined.

OBJECTIVE

Our objective was to determine whether insulin regulation of splanchnic and leg protein metabolism are affected by T2D during hyperglycemia and graded insulin levels.

DESIGN AND SETTING

We conducted a cross-sectional study at an academic medical center.

PARTICIPANTS

T2D and non-T2D adults were matched for age (62 yr) and body mass index (30 kg/m(2)).

INTERVENTIONS

Glucose was maintained at approximately 9 mmol/liter while insulin was infused at three progressively higher rates, achieving circulating concentrations of approximately 150, 350, and 700 pmol/liter, respectively.

MAIN OUTCOME MEASURES

Protein kinetics were measured using labeled phenylalanine (Phe) and tyrosine (Tyr).

RESULTS

Whole-body protein breakdown and synthesis rates were higher in T2D but declined with increasing insulin in both groups. Leg Phe and Tyr appearance and disappearance and estimates of protein breakdown and synthesis, respectively, were higher in T2D but did not decline significantly with insulin, resulting in similar net balance between groups. Splanchnic response to insulin was blunted in T2D, shown by a smaller reduction in rates of disappearance and net balance of Phe and Tyr as insulin increased. Splanchnic conversion of Phe to Tyr was lower in T2D and less sensitive to insulin, whereas nonsplanchnic Phe to Tyr tended to be higher in T2D.

CONCLUSIONS

T2D results in higher whole-body, splanchnic, and leg protein turnover and blunts the insulin-mediated suppression of splanchnic protein anabolism under hyperglycemic, hyperinsulinemic conditions.

Autophagy in the brains of young patients with poorly controlled T1DM and fatal diabetic ketoacidosis

Semi-quantitative neuroradiologic studies, quantitative neuron density studies and immunocytochemistry markers of oxidative stress and neuroinflammation indicate neuronal injury and deficits in young patients with chronic poorly controlled type 1 diabetes mellitus (T1DM). Present data suggest that pathogenesis of the neuronal deficits in young patients, who die as the result of diabetic ketoacidosis (DKA) and brain edema (BE), does not involve apoptosis, a prominent form of regulated cell death in many disease states. To further address this we studied mediators of macroautophagy, endoplasmic reticulum (ER) stress and apoptosis. In all areas studied we demonstrated increased levels of macroautophagy-associated proteins including light chain-3 (LC3) and autophagy related protein-4 (Atg4), as well as increased levels of the ER-associated glucose-regulated protein78/binding immunoglobulin protein (GRP78/BiP) in T1DM. In contrast, cleaved caspase-3 was rarely detected in any T1DM brain regions. These results suggest that chronic metabolic instability and oxidative stress may cause alterations in the autophagy-lysosomal pathway but not apoptosis, and macroautophagy-associated molecules may serve as useful candidates for further study in the pathogenesis of early neuronal deficits in T1DM.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

Effect of insulin on whole body protein metabolism in children with type 1 diabetes

PURPOSE OF REVIEW

Untreated type 1 diabetes (T1D) is associated with abnormalities in protein metabolism, leading to protein loss. These alterations can be particularly detrimental in children, affecting both normal growth and development. A better understanding of the effects of insulin on protein metabolism in children with T1D is essential for optimizing therapy and minimizing consequences of the disease.The aim of the present review is to outline the effects of insulin on whole body protein metabolism in T1D, focusing particularly on studies in children with T1D.

RECENT FINDINGS

Whole body protein degradation and amino acid oxidation are enhanced in children with T1D. Insulin reduces the rates at which body proteins are degraded. Whole body protein synthesis is either unaffected or reduced by insulin, even when insulin is administered together with amino acids to prevent insulin-dependent hypoaminoacidemia. Provision of insulin with oral nutrients improves protein balance by inhibiting whole body protein degradation, but does not affect protein synthesis.

SUMMARY

In children with T1D the anticatabolic effects of insulin on whole body protein metabolism are mainly exerted through a reduction in rates at which body proteins are degraded. Nutritional factors enhancing the anabolic effect of insulin need to be further elucidated.

Elimination of infused branched-chain amino-acids from plasma of patients with non-obese type 2 diabetes mellitus

Increased plasma levels of branched-chain amino-acids (BCAA) have been demonstrated in poorly controlled diabetes mellitus, and related to absolute or relative insulin deficiency. To study the pathogenesis of this alteration, the elimination of BCAA from plasma was measured in 8 patients with non-obese type 2 diabetes mellitus and in 8 age-matched control subjects during steady-state BCAA concentrations induced by a primed-continuous infusion. Fasting BCAA levels were increased by 40-50% in patients with diabetes. The plasma clearances of valine, isoleucine, and leucine, calculated as infusion rate divided by steady-state concentration, were reduced by 20% in diabetics, despite 50% hyperinsulinemia (P < 0.01). Basal BCAA levels and BCAA clearance were negatively correlated (r(2) = 0.46 - 0.56). The endogenous basal appearance rates of BCAA, estimated by the basal concentrations multiplied by the plasma clearances, were normal in diabetics, and there was no difference in the apparent volumes of distribution of BCAA. The increased basal concentration of BCAA in poorly controlled type 2 diabetics (693 [SD 114; n = 8] mumol/l vs 479 [88; n = 8] in controls (P < 0.005) is attributable to changes in plasma clearances, without any change in the efflux of BCAA into plasma. This may be due to insulin resistance.

The effects of growth hormone and/or testosterone on whole body protein kinetics and skeletal muscle gene expression in healthy elderly men: a randomized controlled trial

CONTEXT

Alterations of protein turnover may contribute to the progressive decline of muscle mass with aging.

OBJECTIVE

Our objective was to examine the effects of near-physiological recombinant human GH and/or testosterone (T) administration to older men on whole body protein kinetics and muscle gene expression.

DESIGN, SETTINGS, AND PARTICIPANTS

A 6-month randomized, double-blind, placebo-controlled trial in 21 healthy elderly men aged 65-75 yr, was performed. Participants were randomized to receive placebo GH and placebo T, rhGH and placebo T (GH), T and placebo GH (T), or rhGH and T (GHT).

INTERVENTIONS

The leucine rate of appearance (index of proteolysis), nonoxidative leucine disposal rate (an index of protein synthesis), and leucine oxidation rate were measured with an infusion of l-[1-(13)C] leucine. Muscle biopsies for the measurement of gene expression were performed. Body composition and aerobic capacity (maximal oxygen capacity) were measured.

RESULTS

Serum IGF-I levels increased significantly with GH and GHT (P < 0.001) compared with placebo. T increased significantly only in the T group (P = 0.028). Leucine rate of appearance and nonoxidative leucine disposal rate increased with GH (P = 0.015, P = 0.019) and GHT (P = 0.017, P = 0.02), but leucine oxidation did not change significantly in any treatment group. Midthigh muscle mass and maximal oxygen capacity increased (P < 0.04) with GHT only. Expression of muscle function genes did not change significantly, but the within-group comparisons revealed a significant increase of androgen receptor expression in the GHT group (P = 0.001).

CONCLUSION

This study showed that 6-month treatment with low-dose GH alone or with T in healthy elderly men produces comparable increments in whole body protein turnover and protein synthesis.

Insulin resistance of protein metabolism in type 2 diabetes

OBJECTIVE

We previously demonstrated that 1) obesity impairs and 2) sex influences insulin sensitivity of protein metabolism, while 3) poor glycemic control in type 2 diabetes accelerates protein turnover in daily fed-fasted states. We hypothesized that type 2 diabetes alters the insulin sensitivity of protein metabolism and that sex modulates it.

RESEARCH DESIGN AND METHODS

Hyperinsulinemic ( approximately 570 pmol/l), euglycemic (5.5 mmol/l), and isoaminoacidemic (kept at postabsorptive concentrations) clamps were performed in 17 hyperglycemic type 2 diabetic subjects and 23 subjects without diabetes matched for age and body composition, after 7 days on a inpatient, protein-controlled, isoenergetic diet. Glucose and leucine kinetics were determined using tracers.

RESULTS

In type 2 diabetes, postabsorptive (baseline) glycemia was 8-9 mmol/l, glucose production (R(a)) and disposal (R(d)) were elevated, and once clamped, endogenous glucose R(a) remained greater and R(d) was less (P < 0.05) than in control subjects. Baseline leucine kinetics did not differ despite higher insulin levels. The latter was an independent predictor of leucine flux within each sex. With clamp, total flux increased less (P = 0.016) in type 2 diabetic men, although protein breakdown decreased equally ( approximately 20%) in male groups but less in female groups. Whereas protein synthesis increased in male control subjects and in both female groups, it did not in male subjects with type 2 diabetes. In men, homeostasis model assessment of insulin resistance predicted 44%, and, in women, waist-to-hip ratio predicted 40% of the change in synthesis.

CONCLUSIONS

During our clamp, men with type 2 diabetes have greater insulin resistance of protein metabolism than that conferred by excess adiposity itself, whereas women do not. These results may have implications for dietary protein requirements.

Insulin-associated weight gain in diabetes--causes, effects and coping strategies

Insulin therapy or intensification of insulin therapy commonly results in weight gain in both type 1 and type 2 diabetes. This weight gain can be excessive, adversely affecting cardiovascular risk profile. The spectre of weight gain can increase diabetic morbidity and mortality when it acts as a psychological barrier to the initiation or intensification of insulin, or affects adherence with prescribed regimens. Insulin-associated weight gain may result from a reduction of blood glucose to levels below the renal threshold without a compensatory reduction in calorie intake, a defensive or unconscious increase in calorie intake caused by the fear or experience of hypoglycaemia, or the 'unphysiological' pharmacokinetic and metabolic profiles that follow subcutaneous administration. There is, however, scope for limiting insulin-associated weight gain. Strategies include limiting dose by increasing insulin sensitivity through diet and exercise or by using adjunctive anorectic or insulin-sparing pharmacotherapies such as pramlintide or metformin. Insulin replacement regimens that attempt to mimic physiological norms should also enable insulin to be dosed with maximum efficiency. The novel acylated analogue, insulin detemir, appears to lack the usual propensity for causing weight gain. Elucidation of the pharmacological mechanisms underlying this property might help clarify the mechanisms linking insulin with weight regulation.

Combined growth hormone/insulin-like growth factor I in addition to glutamine-supplemented TPN results in net protein anabolism in critical illness

Protein loss leading to reduced lean body mass is recognized to contribute to the high levels of morbidity and mortality seen in critical illness. This prospective, randomized, controlled study compared the effects of conventional parenteral nutrition (TPN), glutamine-supplemented (0.4 g.kg-1.day-1) TPN (TPNGLN), and TPNGLN with combined growth hormone (GH, 0.2 IU.kg-1.day-1) and IGF-I (160 microg.kg-1.day-1) on protein metabolism in critical illness. Nineteen mechanically ventilated subjects [64 +/- 3 yr, body mass index (BMI) 23.8 +/- 1.3, kg/m2] were initially studied in the fasting state (study 1) and subsequently after 3 days of nutritional with/without hormonal support (study 2). All had recently been admitted to the ICU and the majority were postemergency abdominal surgery (APACHE II 17.5 +/- 1.0). Protein metabolism was assessed using a primed constant infusion of [1-13C]leucine. Conventional TPN contained mixed amino acids, Intralipid, and 50% dextrose. TPNGLN, unlike TPN alone, resulted in an increase in plasma glutamine concentration ( approximately 50%, P < 0.05). Both TPN and TPNGLN decreased the rate of protein breakdown (TPN 15%, P < 0.002; TPNGLN 16%, P < 0.05), but during these treatments the patients remained in a net negative protein balance. Combined treatment with TPNGLN + GH/IGF-I increased plasma IGF-I levels (10.3 +/- 0.8 vs. 48.1 +/- 9.1 nmol/l, study 1 vs. study 2, P < 0.05), and in contrast to therapy with nutrition alone, resulted in net protein gain (-0.75 +/- 0.14 vs. 0.33 +/- 0.12 g protein.kg-1.day-1, study 1 vs. study 2, P < 0.05). Therapy with GH/IGF-I + TPNGLN, unlike nutrition alone, resulted in net positive protein balance in a group of critically ill patients.

Effect of protein restriction on sulfur amino acid catabolism in insulin-dependent diabetes mellitus

Persons with conventionally treated insulin-dependent diabetes mellitus (IDDM) appear to be impaired in their ability to reduce fed-state urea production appropriately in response to dietary protein restriction (Hoffer LJ, Taveroff A, and Schiffrin A. Am J Physiol 272: E59-E67, 1997). To determine whether these conclusions apply to whole body sulfur amino acid (SAA) catabolism, we used samples from this protocol to measure daily urinary sulfate excretion and fed-state sulfate production after a high-protein test meal before and after dietary protein restriction. Eight normal subjects and six IDDM subjects treated with twice-daily intermediate- and short-acting insulin consumed a mixed test meal containing 0.50 g protein/kg after adaptation to 4 days of high protein intake (1.28 g protein/kg body wt) and again after 5 days of dietary protein restriction (0.044 g/kg). Adaptation to protein restriction decreased daily urinary sulfate and urea-N excretion by approximately 80%. Over the first 24 h of protein restriction, urinary sulfate excretion decreased more than urea-N excretion for both the normal and IDDM subjects. Under conditions of a high prior protein intake, fed-state sulfate production was normal for the IDDM subjects; protein restriction reduced fed-state sulfate production by 51% (normal subjects) and 59% (IDDM subjects; not significant). We conclude that whole body SAA metabolism is normal in conventionally treated IDDM before and after dietary protein restriction. SAA catabolism, as measured by fed-state sulfate production, may be a convenient and useful method to determine the extent of whole body protein dysregulation in IDDM.

Synthesis rate of muscle proteins, muscle functions, and amino acid kinetics in type 2 diabetes

Improvement of glycemic status by insulin is associated with profound changes in amino acid metabolism in type 1 diabetes. In contrast, a dissociation of insulin effect on glucose and amino acid metabolism has been reported in type 2 diabetes. Type 2 diabetic patients are reported to have reduced muscle oxidative enzymes and VO(2max). We investigated the effect of 11 days of intensive insulin treatment (T(2)D+) on whole-body amino acid kinetics, muscle protein synthesis rates, and muscle functions in eight type 2 diabetic subjects after withdrawing all treatments for 2 weeks (T(2)D-) and compared the results with those of weight-matched lean control subjects using stable isotopes of the amino acids. Whole-body leucine, phenylalanine and tyrosine fluxes, leucine oxidation, and plasma amino acid levels were similar in all groups, although plasma glucose levels were significantly higher in T(2)D-. Insulin treatment reduced leucine nitrogen flux and transamination rates in subjects with type 2 diabetes. Synthesis rates of muscle mitochondrial, sarcoplasmic, and mixed muscle proteins were not affected by glycemic status or insulin treatment in subjects with type 2 diabetes. Muscle strength was also unaffected by diabetes or glycemic status. In contrast, the diabetic patients showed increased tendency for muscle fatigability. Insulin treatment also failed to stimulate muscle cytochrome C oxidase activity in the diabetic patients, although it modestly elevated citrate synthase. In conclusion, improvement of glycemic status by insulin treatment did not alter whole-body amino acid turnover in type 2 diabetic subjects, but leucine nitrogen flux, transamination rates, and plasma ketoisocaproate level were decreased. Insulin treatments in subjects with type 2 diabetes had no effect on muscle mitochondrial protein synthesis and cytochrome C oxidase, a key enzyme for ATP production.

Diabetic ketoacidosis depletes plasma tryptophan

Diabetic ketoacidosis (DKA) is a severe metabolic disturbance of insulin-dependent diabetes mellitus (IDDM) which has a significant effect on amino acid metabolism. Amino acids serve as precursors for various neurotransmitters which are involved in affective disorders, and patients with IDDM are known to have an increased prevalence of affective disorders. We monitored the plasma concentrations of 23 amino acids in six adolescents prior to treatment of DKA and at 6, 24 and 120 hours after initiation of treatment. The well-known increase in the concentrations of the glucogenic amino acids and the decrease in the branched-chain amino acids were observed in response to treatment of DKA. Low levels of tryptophan were found prior to treatment of DKA. Treatment increased the plasma tryptophan levels, but the mean concentration remained low throughout the sampling period. Only the glutamate-derived amino acids (glutamate, proline and glutamine) from the Krebs cycle pool were significantly affected by treatment. Glutamine declined initially, but recovered as the plasma pH normalized. Our results indicate that DKA causes a depletion of plasma tryptophan. This depletion may predispose some patients with IDDM to have affective disorders secondary to a neurotransmitter imbalance.

Tissue-specific regulation of mitochondrial and cytoplasmic protein synthesis rates by insulin

In vivo studies have reported conflicting effects of insulin on mixed tissue protein synthesis rates. To test the hypothesis that insulin has differential effects on synthesis rates of various protein fractions in different organs, we infused miniature swine (n = 8 per group) with saline, insulin alone (at 0.7 mU/kg(-1). min(-1)), or insulin plus an amino acid mixture for 8 h. Fractional synthesis rate (FSR) of mitochondrial and cytoplasmic proteins in liver, heart, and skeletal muscle, as well as myosin heavy chain (MHC) in muscle, were measured using L-[1-(13)C]leucine as a tracer. The FSR of mitochondrial and cytoplasmic proteins were highest in liver, followed by heart and then muscle. Mitochondrial FSR in muscle was higher during insulin and insulin plus amino acid infusions than during saline. Insulin had no significant effect on FSR of MHC in muscle. In contrast, FSR of both mitochondrial and cytoplasmic proteins were not stimulated by insulin in liver. Insulin also did not increase FSR of mitochondrial in heart, whereas insulin and amino acid stimulated FSR of cytoplasmic protein. In conclusion, insulin stimulates the synthesis of muscle mitochondrial proteins, with no significant stimulatory effect on synthesis of sarcoplasmic and MHC. These results demonstrate that insulin has different effects on synthesis rates of specific protein fractions in the liver, heart, and skeletal muscle.

Whole body protein kinetics in women: effect of pregnancy and IDDM during anabolic stimulation

The effects of pregnancy and type 1 diabetes [insulin-dependent diabetes mellitus (IDDM)] on protein metabolism are still uncertain. Therefore, six normal and five IDDM women were studied during and after pregnancy, using [(13)C]leucine and [(2)H(5)]phenylalanine with a hyperinsulinemic-euglycemic clamp and amino acid infusion. Fasting total plasma amino acids were lower in pregnancy in normal but not IDDM women (2,631 +/- 427 vs. 2,057 +/- 471 and 2,523 +/- 430 vs. 2,500 +/- 440 micromol/l, respectively). Whole body protein breakdown (leucine) increased in pregnancy [change in normal (delta N) and IDDM women (delta D) 0.59 +/- 0.40 and 0.48 +/- 0.26 g. kg(-1). day(-1), both P < 0.001], whereas reductions in protein breakdown due to insulin/amino acids (delta N -0.57 +/- 0.19, delta D -0.58 +/- 0.20 g. kg(-1). day(-1), both P < 0.001) were unaffected by pregnancy. Protein breakdown in IDDM women was not higher than normal, and neither pregnancy nor type 1 diabetes altered the insulin sensitivity of amino acid turnover. Nonoxidized leucine disposal (protein synthesis) increased in pregnancy (delta N 0.67 +/- 0.45, delta D 0.64 +/- 0.34 g. kg(-1). day(-1), both P < 0.001). Pregnancy reduced the response of phenylalanine hydroxylation to insulin/amino acids in both groups (delta N -1.14 +/- 0.74, delta D -1. 12 +/- 0.77 g. kg(-1). day(-1), both P < 0.05). These alterations may enable amino acid conservation for protein synthesis and accretion in late pregnancy. Well-controlled type 1 diabetes caused no abnormalities in the regulation of basal or stimulated protein metabolism.

The effect of insulin on human small intestinal mucosal protein synthesis

BACKGROUND & AIMS

Insulin deficiency was recently shown to stimulate splanchnic protein synthesis in vivo, whereas insulin enhances small intestinal mucosal cell proliferation in vitro. Because insulin is a postprandial hormone, it was hypothesized that it has an important role in regulating small intestinal protein synthesis in humans.

METHODS

Small intestinal mucosal protein synthesis was measured in C-peptide-negative patients with type 1 diabetes mellitus during insulin deprivation (n = 6) and during insulin treatment (n = 6) and in nondiabetic control subjects (n = 6). Mucosal protein synthesis was measured from the increment of [(13)C]leucine enrichment in endoscopically obtained duodenal mucosa samples during a primed continuous infusion of L-[1-(13)C]leucine.

RESULTS

During insulin treatment, the rate of mucosal protein synthesis in patients with type 1 diabetes was similar (1.32% +/- 0.05%/h) to that of nondiabetic controls (1.33% +/- 0.06%/h). However, during insulin deprivation, the mucosal protein synthesis rate in patients with type 1 diabetes was significantly lower (1.15% +/- 0.33%/h) than during either insulin treatment (P = 0.01) or in nondiabetic controls (P = 0.04).

CONCLUSIONS

These studies show that insulin is required for the maintenance of normal rates of protein synthesis in small intestinal mucosa. Because protein synthesis is an essential component of the remodeling process of this fast turning over tissue, the decline in the synthesis rate of small intestinal mucosa during insulin deprivation may be a contributing factor in the development of gastrointestinal complications that occur in poorly controlled type 1 diabetic patients.

Effect of growth hormone treatment on postprandial protein metabolism in growth hormone-deficient adults

Growth hormone (GH) treatment of GH-deficient adults increases lean body mass. To investigate this anabolic effect of GH, body composition and postabsorptive and postprandial protein metabolism were measured in 12 GH-deficient adults randomized to placebo or GH treatment. Protein metabolism was measured after an infusion of [1-13C]leucine before and after a standard meal at 0 and 2 mo. After 2 mo, there was an increase in lean body mass in the GH group (P < 0. 05) but no change in the placebo group. In the postabsorptive state, there was increased nonoxidative leucine disappearance (NOLD; a measure of protein synthesis) and leucine metabolic clearance rate and decreased leucine oxidation in the GH group (P < 0.05) but no change in the placebo group. After the meal, there was an increase in NOLD and oxidation in all studies (P < 0.05), but the increase in NOLD, measured as area under the curve, was greater in the GH group (P < 0.05). This study clearly demonstrates for the first time that the increase in protein synthesis in the postabsorptive state after GH treatment of GH-deficient adults is maintained in the postprandial state.

Nutritional effects of dietary protein restriction in insulin-dependent diabetes mellitus

The effects of dietary protein deprivation in insulin-dependent diabetes mellitus (IDDM) have been investigated in a merely rudimentary fashion in human subjects. Moderate dietary protein restriction of 0.6 g/(kg ideal body weight.d) over 3 mo in free-living IDDM patients produces increased adiposity during weight maintenance and decreased muscle strength. These effects might have been predicted from studies of protein deprivation in diabetic subjects, indicating impairment of nitrogen retention. The clinical consequences of dietary protein restriction in IDDM may be more complex than described to date. This is suggested by the overriding paradox that the actions of insulin on protein synthesis are inconsistent among in vitro, animal and human in vivo models. The inconsistency and the observation that insulin deficiency in humans accelerates both proteolysis and protein synthesis imply that knowledge about insulin, diabetes and protein metabolism in humans is inadequate and should be studied in increasing detail. Better understanding of the clinical consequences of dietary protein restriction in diabetes, both beneficial and adverse, is likely to come from future studies incorporating clinically relevant levels of insulin deficiency and protein deprivation into studies of bodily function, clinical outcomes and specific examination of the metabolism of individual proteins.

Protein metabolism in insulin-dependent diabetes mellitus

Patients with insulin-dependent diabetes are in a catabolic state without insulin replacement. The mechanism of insulin's anticatabolic effect has been investigated in whole-body and regional tracer kinetic studies. Whole-body studies have demonstrated that there are increases in both protein breakdown and protein synthesis during insulin deprivation. Because the magnitude of the increase in protein breakdown is greater than the magnitude of the increase in protein synthesis, there is a net protein loss during insulin deprivation. Regional studies have shown that insulin replacement inhibits protein breakdown and synthesis in splanchnic tissue but only inhibits protein breakdown in skeletal muscle. Because the increase in protein synthesis in splanchnic tissues is greater than the increase in protein breakdown, insulin deprivation results in a net accretion of protein in the splanchnic bed. In contrast, in skeletal muscle, there is a net increase in protein breakdown during insulin deprivation, resulting in a net release of amino acids. There are no human data concerning the site of protein accretion in the splanchnic bed or the specific protein whose synthesis is increased during insulin deprivation. It appears that insulin exerts its overall anticatabolic effect in insulin-dependent diabetes mainly through the inhibition of muscle protein breakdown.

Hormonal regulation of human muscle protein metabolism

A continuous turnover of protein (synthesis and breakdown) maintains the functional integrity and quality of skeletal muscle. Hormones are important regulators of this remodeling process. Anabolic hormones stimulate human muscle growth mainly by increasing protein synthesis (growth hormone, insulin-like growth factors, and testosterone) or by decreasing protein breakdown (insulin). Unlike in growing animals, insulin's main anabolic effect on muscle protein in adult humans is an inhibition of protein breakdown. Protein synthesis is stimulated only in the presence of a high amino acid supply. A combination of the stress hormones (glucagon, glucocorticoids, and catecholamines) cause muscle catabolism, but the effects of the individual hormones on human muscle and their mechanisms of action remain to be clearly defined. Although thyroid hormone is essential during growth, both an excess and a deficiency cause muscle wasting by yet unknown mechanisms. A greater understanding of the regulation of human muscle protein metabolism is essential to elucidate mechanisms of muscle wasting.

Metabolic adaptation to protein restriction in insulin-dependent diabetes mellitus

Eight normal subjects, four subjects with intensively treated insulin-dependent diabetes mellitus (IDDM), and six subjects with conventionally treated IDDM consumed a test meal of 0.5 g protein and 10 kcal per kg body weight, first while adapted to a conventional diet high in protein, and then again after 5 days of dietary protein restriction. Metabolic N balance (N consumed minus urea production) and net protein utilization were measured over the 9 h after consumption of the test meal, as was recovery in urea of 15N from a tracer dose of [15N]alanine included in each test meal. After the first test meal, N balance and net protein utilization were similar and close to zero for all groups. After the second test meal, N balance and net protein utilization became positive for all groups (P < 0.05) but significantly less so (P < 0.05) for the conventionally treated than for the normal and intensively treated diabetic subjects. 15N recovery in urea was reduced for all groups after the second test meal (P < 0.05) but probably less effectively (P < 0.09) for the conventionally treated diabetic subjects. Metabolic adaptation to protein restriction may be less effective than normal in conventionally treated IDDM.

Protein metabolism in diabetes mellitus

Insulin deficiency is a protein catabolic state. In vivo studies have shown that insulin enhances short-side-chain amino acid intracellular uptake, stimulates transcription and translation of RNA, increases the gene expression of albumin and other proteins and inhibits liver protein breakdown enzymes. In IDDM patients most of the whole-body protein turnover studies have shown that insulin deficiency increases protein breakdown and increases amino acid oxidation and that these effects are reversed by insulin treatment. Recent studies have demonstrated that a substantial increase in leucine transamination during insulin deprivation contributes to leucine catabolism in IDDM patients. Protein synthesis in the insulin-deprived state is also increased although to a lesser extent than protein breakdown, and this increased whole-body protein synthesis is reduced with an insulin infusion; thus the effects of insulin are largely mediated through its effects on protein breakdown. The metabolic derangements in diabetes frequently involve disturbances in substrates and hormones other than insulin. The observed effects of insulin deficiency in diabetic patients vary in different body compartments; most of the effects of insulin on protein synthesis appear to occur in non-muscular tissues especially in the splanchnic area. In addition, insulin has a differential effect on hepatic protein synthesis, i.e. inhibits fibrinogen synthesis and promotes albumin synthesis. Insulin's anticatabolic effect in IDDM patients is largely due to its inhibition of protein breakdown. The net protein anabolism due to insulin occurs largely in skeletal muscle. In patients with NIDDM these effects are not noted, presumably because of residual endogenous insulin secretion. In fact, treatments that result in improvement of glucose metabolism in obese NIDDM patients do not affect protein metabolism.

Effects of dietary protein restriction on regional amino acid metabolism in insulin-dependent diabetes mellitus

We studied subjects with insulin-dependent diabetes mellitus (IDDM) and controls by administering primed continuous infusions of L-[1-13C,15N)]leucine and L-[2,3-13C2]alanine to measure whole body and forearm metabolism of these amino acids during ample protein intake and again after 4 wk of moderately restricted protein intake. Decreased rates of whole body protein degradation, leucine transamination, leucine oxidation, and increased forearm alanine release produced by dietary protein restriction occurred equivalently in IDDM subjects under short-term tightly managed glycemia and in controls. Dietary protein restriction did not affect whole body alanine appearance or forearm leucine appearance, disposal, or balance in IDDM subjects or controls. IDDM subjects differed from controls only in that normal forearm leucine balance was maintained at higher rates of leucine appearance and disposal. We conclude that IDDM subjects adapt normally to dietary protein restriction. Undernutrition during moderate protein deprivation in these patients likely occurs during episodes of poor glycemic control.

The effect of starvation on leucine, alanine and glucose metabolism in obese subjects

The relationship between changes in ketone concentrations and leucine metabolism (seven obese subjects), glucose and alanine metabolism (seven obese subjects) was investigated using radioisotopic techniques after 12 h, 60 h and 2 weeks starvation. Leucine metabolism was also measured in five lean subjects after 12 h and 60 h starvation. In the obese subjects leucine concentration increased after 60 h starvation and leucine metabolic clearance rate, glucose and alanine concentration decreased (P < 0.05). Glucose and alanine production rate (Ra) decreased after 2 weeks (P < 0.05) but there was no change in leucine Ra after 60 h or 2 weeks. In the lean subjects leucine concentration, production rate and oxidation rate were increased after 60 h (P < 0.005, P < 0.05, P < 0.05). Ketone concentration was inversely related to alanine Ra (r = -0.51, P < 0.02) but was not related to measurements of protein metabolism in the obese subjects. This study demonstrates that the effect of short-term starvation on protein metabolism differs in lean and obese subjects. The decrease in glucose Ra during long-term starvation may be in part due to a decreased supply of alanine for gluconeogenesis.

A computer program for estimating the influence of the body bicarbonate pool during CO2 breath tests

Mathematical representation of physiological systems is a useful means of monitoring various parameters and factors of such systems. The use of compartmental models for the study of physiological systems has led to the introduction of several new techniques for solving problems, concerning the kinetics between the individual sites of these systems. The computer program, presented here, allows the calculation of the influence of a mammillary model, representing the bicarbonate pool, on the expiration rate of the label, administered during a breath test. It consists of two modules, one for performing non-linear least squares regression fitting of discrete measurements from a breath test (model output) and a second one for estimating the corresponding label released into the bicarbonate pool (model input), based on the measurements of expired air. Data, derived from metabolic studies using oral administration of naturally labelled 13C-glucose in man, under various metabolic conditions, was used as an input to this program.

Insulin increases the daily food intake of diabetic rats on high and low fat diets

The effects of insulin dose and diet composition on daily food intake were investigated by IV infusion of insulin in doses of 2 to 5 U/day into diabetic rats consuming either a high CHO or high fat diet. The daily food intake of the diabetic rats on both diets increased significantly over baseline levels (p < .01) at the low insulin doses and was maintained at these elevated levels through the 5 U/day dose. Insulin increased the rate of weight gain from Ig/day during baseline to 2 and 2.5 g/day in high CHO and high fat fed diabetics (p < .01). These results show that treatment of diabetic rats with continuous low doses of IV insulin results in a 40% increase in daily food intake regardless of the diet consumed and this increase is accompanied by an increase in rate of body weight gain. While the high fat fed diabetics were relatively hypoglycemic, these increases in intake are not the result of insulin-induced hypoglycemia, since blood glucose concentrations are significantly elevated when the increases occur at the lower insulin doses (p < .01). Thus, peripheralinsulin infused at physiological levels stimulates rather than inhibits daily food intake.

Glucose and fat metabolism in adults with growth hormone deficiency

OBJECTIVE

Adults with long-standing GH deficiency have a decreased lean body mass and an increased fat mass. We investigated the effects of the abnormal body composition on glucose turnover and fuel metabolism.

DESIGN

Cross-sectional analysis.

PATIENTS

Twenty-four adults with acquired GH deficiency and a wide range of adiposity (body mass index from 18.8 to 42.3 kg/m2).

MEASUREMENTS

In the post-absorptive state glucose turnover was measured following intravenous injection of 3-3H-glucose and leucine oxidation was assessed following intravenous injection of 1-14C-leucine. Glucose and fat oxidation were calculated from indirect calorimetry using protein oxidation derived from leucine oxidation.

RESULTS

Total glucose turnover was 692 +/- 146 mumol/min (mean +/- SD) and increased with height (r = 0.70, P = 0.0003) and with lean body mass (LBM) (r = 0.80, P < 0.0001). Glucose turnover expressed per kg LBM was in the published normal range (14.2 +/- 2.1 mumol/kg LBM min). Glucose oxidation was 47 +/- 27% of glucose turnover and increased with LBM (r = 0.59, P = 0.008) but not with height (r = 0.32, NS). Glucose turnover increased with increasing fat oxidation (r = 0.61, P = 0.006). The non-oxidative part of glucose turnover was positively correlated with fat oxidation (r = 0.82, P = 0.0001) and inversely with the respiratory quotient (r = -0.81, P < 0.0001). Ketone body concentration (r = 0.55, P = 0.03), but not free fatty acid levels (r = 0.21, NS), correlated with fat oxidation. Fasting plasma glucagon levels were elevated (35 +/- 13 vs 9 +/- 19 pmol/l (published controls)) and inversely related to lean body mass (r = -0.44, P = 0.04).

CONCLUSIONS

Adults with GH deficiency studied after an overnight fast have changes in glucose and fuel metabolism seen in normal subjects after more prolonged fasting suggesting that adults with hormone deficiency have reduced carbohydrate stores.

Effects of insulin-like growth factor-I (IGF-I), insulin and combined IGF-I-insulin infusions on protein metabolism in dogs

The effect of infusions of recombinant insulin-like growth factor-I (IGF-I) (34, 103 or 688 pmol min-1 kg-1), insulin (3.4, 10.3 or 68.8 pmol min-1 kg-1) or combined infusions (34 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin or 103 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin) on protein metabolism, using an infusion of [1-14C]leucine was investigated in anaesthetized fasted dogs. Leucine concentration, production rate (measure of protein degradation), oxidation rate and non-oxidative disappearance rate (measure of protein synthesis) were decreased in a similar dose dependent manner by the IGF-I and insulin infusions (P < 0.01). The decrease in these measurements of leucine metabolism were greater following 34 pmol IGF-I + 3.4 pmol insulin than with either component infused alone (P < 0.05). Free fatty acid concentrations were decreased by all insulin doses (P < 0.01) but only by 103 and 688 pmol min-1 kg-1 insulin-like growth factor (P < 0.05, P < 0.01). These data demonstrate that IGF-I, like insulin, has a dose dependent effect on protein metabolism and that combined insulin and IGF-I infusions have additive effects on protein metabolism.

Leucine kinetics during a brief fast in diabetes in pregnancy

The effect of diabetes in pregnancy on leucine turnover and oxidation was examined in 12 insulin-dependent diabetic (IDDM) subjects and 12 gestationally diabetic (GDM) subjects during the third trimester of pregnancy. The data were compared with those in normal pregnant women studied during the same time period and reported previously. Eight of the IDDM subjects were on continuous subcutaneous insulin infusion (insulin pump), and four were on conventional twice-daily insulin treatment. Of the GDM group, seven were on insulin therapy and five were on dietary management. Leucine kinetics were quantified using [1-13C]leucine tracer in combination with respiratory calorimetry and measurement of lean body mass using the H2[18O] dilution method. In addition, glucose kinetics were measured in insulin-treated subjects using [6,6(2)H2]glucose tracer. Despite rigorous metabolic control, fasting plasma glucose (IDDM 5.5 +/- 1.9 mmol/L [P < .05], GDM 4.7 +/- 1.3 [P < .01], controls 3.6 +/- .6, mean +/- SD) and hemoglobin A1 ([HbA1] IDDM 7.9 +/- 1.9%, GDM 7.5% +/- 2.1%) levels were higher in diabetic subjects. Although total insulin levels were higher in insulin-treated diabetic subjects, free-insulin concentrations were similar in all groups. Rates of excretion of urinary urea nitrogen and respiratory quotients were also similar. The rate of glucose turnover was lower in insulin-treated subjects compared with normals. Leucine flux, a measure of the rate of protein breakdown, and leucine oxidation were higher in IDDM and insulin-treated GDM subjects. The rate of leucine oxidation was increased in conventionally managed IDDM and insulin-treated GDM subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

The opposing effects of insulin and hyperglycemia in modulating amino acid metabolism during a glucose tolerance test in lean and obese subjects

An intravenous glucose tolerance test (IVGTT) was administered in both lean (n = 9) and obese (n = 14) volunteers to ascertain the importance of the dynamic interactions between insulin and glucose on plasma concentrations of two amino acids known to be primarily used by skeletal muscle, namely leucine and isoleucine, and one amino acid, phenylalanine, which is primarily metabolized by the liver. After a 30-minute basal period, each subject received a bolus injection of glucose (0.3 g/kg IV) followed 20 minutes later by a bolus injection of tolbutamide (300 mg). Blood samples were drawn frequently for 180 minutes after the glucose infusion to determine plasma concentrations of glucose, insulin, leucine, isoleucine, and phenylalanine. Plasma glucose and insulin concentrations during the basal period were increased by 10% and 100%, respectively, in obese compared with lean individuals (P < .05), whereas phenylalanine, isoleucine, and leucine levels were similar between groups. During the IVGTT, plasma glucose level initially increased by twofold and slowly returned to basal level thereafter, whereas insulin level responded to glucose and tolbutamide infusions in a typical biphasic manner. Plasma leucine and isoleucine levels did not change from basal levels during the first 60 minutes of the IVGTT as hyperglycemic hyperinsulinemia prevailed in both groups. However, when plasma glucose had returned to near-basal levels, plasma leucine and isoleucine levels began to decrease, reaching a plateau of approximately 20% and 35% below basal, and plasma insulin level remained elevated in the lean and obese individuals, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Creatinine height index and lean body mass in adult patients with insulin-dependent diabetes mellitus followed for 7 years from onset

The 24-hour urinary creatinine excretion value can be used as an index of protein nutrition; the creatinine height index and lean body mass can be estimated from this value. On the basis of longitudinally measured 24-hour urinary creatinine excretions during the initial 7 years of type 1 diabetes in an incidence cohort of 147 adult patients, we studied creatinine height index and lean body mass and possible relationships to sequential measurements of glycated hemoglobin (HbA1c). The patients were divided into four groups according to their glycemic control during these 7 years: I, HbA1c < 7.4% (n = 37); II, HbA1c 7.4% to 8.2% (n = 37); III, HbA1c 8.3% to 8.9% (n = 38); IV, HbA1c > 8.9% (n = 35). One year after the onset of diabetes, height indices were as follows (% of normal values, median and quartiles): I, 104% (90 to 116); II, 101% (78 to 105); III, 121% (92 to 128); IV, 87% (78 to 109) ([IV] < [I to III]; p < .05). During the following 6 years no significant differences in height index were observed among the four groups of patients at any point in time. Slightly higher calculated lean body mass values were found in the most well-controlled patients, but otherwise no differences were found in lean body mass. It is concluded that, apart from the first year, indices of protein nutrition remain normal during the initial 7 years of type 1 diabetes, even in patients with poor glycemic control.

Leucine metabolism in patients with Cushing's syndrome before and after successful treatment

OBJECTIVE

Results from studies on the effect of glucocorticosteroids on protein turnover in both rat and man have been conflicting. The aim of this study was to investigate the primary cause of muscle wasting in patients with Cushing's syndrome.

DESIGN

Studies of whole body 1(-14)C-leucine turnover in patients with Cushing's syndrome before and after successful treatment, and in control subjects.

PATIENTS

Eleven patients with Cushing's syndrome before and after (n = 5) treatment and 11 control subjects.

MEASUREMENTS

Whole body 1(-14)C-leucine turnover to determine leucine metabolic clearance rate, leucine production rate, leucine oxidation rate and leucine incorporation into protein.

RESULTS

Plasma leucine concentration (mean +/- SEM 100 +/- 6 mumol/l), leucine metabolic clearance rate (9.97 +/- 0.11 mumol/min/kg), leucine turnover (0.98 +/- 0.11 mumol/min/kg) and leucine incorporation into protein (0.71 +/- 0.09 mumol/min/kg) were all significantly reduced in patients with Cushing's syndrome compared with control subjects (122 +/- 6 mumol/l, P < 0.05; 13.61 +/- 1.27 mumol/min/kg, P < 0.05; 1.65 +/- 0.12 mumol/min/kg, P < 0.05; 1.46 +/- 0.10 mumol/min/kg, P < 0.001, respectively). Leucine oxidation rate was similar in the patients with Cushing's syndrome and control subjects. When leucine metabolism was expressed in terms of lean body mass (LBM) in five patients with Cushing's syndrome and 11 control subjects, leucine MCR, leucine turnover and leucine oxidation were not significantly different in the two groups. However, leucine incorporation into protein was significantly reduced (P < 0.001) in the patients with Cushing's syndrome (1.07 +/- 0.20 mumol/min/kg LBM) compared with control subjects (1.95 +/- 0.11 mumol/min/kg LBM).

CONCLUSION

We conclude from these studies that the muscle wasting associated with Cushing's syndrome is primarily due to a reduction in protein synthesis.

Insulin action on protein metabolism

On the basis of the preceding observations, the following sequence of events can be postulated during insulin deficiency or excess. The main feature of insulin deficiency is the disruption of protein balance in muscle that rapidly leads to emaciation and wasting. Muscle protein degradation is greatly enhanced while increased amino acid availability maintains protein synthesis. In splanchnic tissues, both degradation and synthesis are increased but with an altered pattern, so that the levels of some proteins are increased (e.g. proteins of the acute-phase response), while those of others are decreased (e.g. albumin). As a result, intracellular protein content in liver is maintained but secretion of plasma proteins is abnormal. In healthy subjects, an acute increase in insulin concentration, as occurs after a meal, leads to a rapid suppression of protein breakdown in the splanchnic area. If hyperinsulinaemia is not supported by an exogenous amino acid supply, as might occur during a protein-free meal or experimentally during euglycaemic hyperinsulinaemic clamping, the plasma as well as muscle free amino acid concentration drops, owing to reduced splanchnic release. With reduced amino acid availability, insulin is not anabolic in muscle. If amino acid concentrations are maintained at normal or high levels, e.g. following a mixed meal, a net protein deposition in muscle may occur, primarily because of a stimulation of synthesis and possibly owing to inhibition of breakdown.

Isoforms of rat apolipoproteins and changes induced by insulin deficiency and fasting

Reasons for the disordered lipoprotein metabolism in insulin deficiency are not completely understood. In this study the apolipoproteins from plasma of fed and fasted streptozotocin-induced insulin-deficient rats were compared with normal control rats. Analysis of the apolipoprotein isoforms by two-dimensional electrophoresis revealed increased proportions of sialylated apo E and of sialylated apo C-III in diabetic rats compared with control rats. Fasting increased the proportion of sialylated apo E but not the proportion of sialylated apo C-III. 3H-labeled leucine was injected into normal and insulin-deficient rats, followed by a chase of unlabeled leucine after 30 min. Blood samples were collected at intervals over 24 h and the apolipoprotein components were separated by two-dimensional electrophoresis. The relative specific activities of sialylated isoforms of apo E were less than the relative specific activities of non-sialylated apo E isoforms. In contrast, sialylated isoforms of apo C-III had higher relative specific activities than non-sialylated apo C-III. No interconversions of apo E or apo C-III isoforms were found within the lipoprotein fractions. In insulin-deficient diabetic rats the relative specific activities of sialylated apo E and apo C-III isoforms were both increased relative to non-sialylated isoforms when compared with control rats. The results of this study suggest that the isoforms of apo E and apo C-III associated with the plasma lipoproteins of diabetic rats are changed in parallel with changes in synthesis of the isoforms. The changes in association with the isoforms of the apolipoproteins possibly contribute to abnormal metabolism of plasma lipoproteins in insulin deficiency.

Effect of insulin on system A amino acid transport in human skeletal muscle

Transmembrane transport of neutral amino acids in skeletal muscle is mediated by at least four different systems (system A, ASC, L, and Nm), and may be an important target for insulin's effects on amino acid and protein metabolism. We have measured net amino acid exchanges and fractional rates of inward (k(in), min-1) and outward (kout, min-1) transmembrane transport of 2-methylaminoisobutyric acid (MeAIB, a nonmetabolizable amino acid analogue, specific for system A amino acid transport) in forearm deep tissues (skeletal muscle), by combining the forearm perfusion technique and a novel dual tracer ([1-H3]-D-mannitol and 2-[1-14C]-methylaminoisobutyric acid) approach for measuring in vivo the activity of system A amino acid transport. Seven healthy lean subjects were studied. After a baseline period, insulin was infused into the brachial artery to achieve local physiologic hyperinsulinemia (76 +/- 8 microU/ml vs 6.4 +/- 1.6 microU/ml in the basal period, P < 0.01) without affecting systemic hormone and substrate concentrations. Insulin switched forearm amino acid exchange from a net output (-2,630 +/- 1,100 nmol/min per kig of forearm tissue) to a net uptake (1,610 +/- 600 nmol/min per kg, P < 0.01 vs baseline). Phenylalanine and tyrosine balances simultaneously shifted from a net output (-146 +/- 47 and -173 +/- 34 nmol/min per kg, respectively) to a zero balance (16.3 +/- 51 for phenylalanine and 15.5 +/- 14.3 nmol/min per kg for tyrosine, P < 0.01 vs baseline for both), showing that protein synthesis and breakdown were in equilibrium during hyperinsulinemia. Net negative balances of alanine, methionine, glycine, threonine and asparagine (typical substrates for system A amino acid transport) also were decreased by insulin, whereas serine (another substrate for system A transport) shifted from a zero balance to net uptake. Insulin increased k(in) of MeAIB from a basal value of 11.8.10(-2) +/- 1.7.10(-2).min-1 to 13.7.10(-2) +/- 2.2.10(-2).min-1 (P < 0.02 vs the postabsorptive value), whereas kout was unchanged. We conclude that physiologic hyperinsulinemia stimulates the activity of system A amino acid transport in human skeletal muscle, and that this effect may play a role in determining the overall concomitant response of muscle amino acid/protein metabolism to insulin.

Insulin sensitivity of protein and glucose metabolism in human forearm skeletal muscle

Physiologic increases of insulin promote net amino acid uptake and protein anabolism in forearm skeletal muscle by restraining protein degradation. The sensitivity of this process to insulin is not known. Using the forearm perfusion method, we infused insulin locally in the brachial artery at rates of 0.00 (saline control), 0.01, 0.02, 0.035, or 0.05 mU/min per kg for 150 min to increase local forearm plasma insulin concentration by 0, approximately 20, approximately 35, approximately 60, and approximately 120 microU/ml (n = 35). L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine were infused systemically, and the net forearm balance, rate of appearance (Ra) and rate of disposal (R(d)) of phenylalanine and leucine, and forearm glucose balance were measured basally and in response to insulin infusion. Compared to saline, increasing rates of insulin infusion progressively increased net forearm glucose uptake from 0.9 mumol/min per 100 ml (saline) to 1.0, 1.8, 2.4, and 4.7 mumol/min per 100 ml forearm, respectively. Net forearm balance for phenylalanine and leucine was significantly less negative than basal (P < 0.01 for each) in response to the lowest dose insulin infusion, 0.01 mU/min per kg, and all higher rates of insulin infusion. Phenylalanine and leucine R(a) declined by approximately 38 and 40% with the lowest dose insulin infusion. Higher doses of insulin produced no greater effect (decline in R(a) varied between 26 and 42% for phenylalanine and 30-50% for leucine). In contrast, R(d) for phenylalanine and leucine did not change with insulin. We conclude that even modest increases of plasma insulin can markedly suppress proteolysis, measured by phenylalanine R(a), in human forearm skeletal muscle. Further increments of insulin within the physiologic range augment glucose uptake but have little additional effect on phenylalanine R(a) or balance. These results suggest that proteolysis in human skeletal muscle is more sensitive than glucose uptake to physiologic increments in insulin.

Effects of insulin and amino acid infusion on leucine and phenylalanine kinetics in type 1 diabetes

To evaluate the anabolic effects of hyperinsulinemia and hyperaminoacidemia on amino acid (and protein) metabolism in type 1 (insulin-dependent) diabetes mellitus (IDDM), we studied leucine and phenylalanine kinetics in nine IDDM and seven control subjects, both at basal euglycemic conditions and during a euglycemic hyperinsulinemic clamp (approximately 60-80 microU/ml of plasma free insulin), combined with an intravenous infusion of amino acids (AA), which doubled plasma concentrations of most AA. In the basal state, euglycemia was maintained in IDDM subjects at the expense of a peripheral free insulin level (16 +/- 2 microU/ml) greater (P less than 0.05) than controls (9 +/- 1 microU/ml). Despite that, leucine rate of appearance (Ra), alpha-ketoisocaproate oxidation (approximating leucine-carbon oxidation), and nonoxidative leucine disposal, were greater (P less than 0.05) in IDDM than in control subjects. Phenylalanine Ra was slightly but not significantly greater in IDDM vs. control subjects. During the clamp, at comparable plasma free insulin and amino acid concentrations, oxidation was similar in the two groups, endogenous leucine and phenylalanine Ra remained significantly greater (P less than 0.05) in IDDM than in normal subjects, and leucine disposal tended also to be greater in IDDM subjects. Thus, in IDDM subjects maintained at euglycemia, endogenous Ra of essential amino acid(s) (index of endogenous proteolysis) is increased, both in the postabsorptive state and after hyperinsulinemia combined with hyperaminoacidemia, while leucine utilization for protein synthesis is not impaired.