Contribution of insulin resistance to catabolic effect of prednisone on leucine metabolism in humans

Stress and Diabetes Mellitus: Pathogenetic Mechanisms and Clinical Outcome

Evidence suggests that psychological and physical stress are relevant triggering factors for the onset of type 1 diabetes (T1D) and type 2 diabetes (T2D). The underlying mechanisms involve a complex neuroendocrine structure, involving the central nervous system and the periphery. Psychological stress leads to an increase of serum glucocorticoid concentrations and catecholamines release increasing the insulin need and the insulin resistance. According to the β-cell stress hypothesis, also causes of increased insulin demand, such as rapid growth, overweight, puberty, low physical activity, trauma, infections, and glucose overload, are potentially relevant factors in development of T1D. It has also been demonstrated that chronic stress and obesity form a vicious circle which leads to a definitive metabolic failure, increasing the risk of developing T2D. In this review, we will provide the most recent data concerning the role of stress in the outcomes of T1D and T2D, with a focus on the role of physical and psychological stress on the onset of T1D.

The effects of glucagon and the target of rapamycin (TOR) on skeletal muscle protein synthesis and age-dependent sarcopenia in humans

BACKGROUND AND AIMS

Human target of rapamycin (TOR) is a kinase that stimulates protein synthesis in the skeletal muscle in response to amino acids and physical activity.

METHODS

A comprehensive literature search was conducted on the PubMed database from its inception up to May 2021 to retrieve information on the effects of TOR and glucagon on muscle function. Articles written in English regarding human subjects were included.

RESULTS

l-leucine activates TOR to initiate protein synthesis in the skeletal muscle. Glucagon has a crucial role suppressing skeletal muscle protein synthesis by increasing l-leucine oxidation and the irreversible loss of this amino acid. Glucagon-induced l-leucine oxidation suppresses TOR and attenuates the ability of skeletal muscle to synthesize proteins. Conditions associated with increased glucagon secretion typically feature reduced ability to synthesize proteins in the skeletal muscle that may evolve into sarcopenia. Animal protein ingestion, unlike vegetable protein, stimulates glucagon secretion. High intake of animal protein increases l-leucine oxidation and promotes the use of amino acids as fuel. Sarcopenia and arterial stiffness characteristically occur together in conditions featuring insulin resistance, such as aging. Insulin resistance mediates the relationship between aging and sarcopenia and arterial stiffness. The loss of skeletal muscle fibers that characterizes sarcopenia is followed by collagen and lipid accumulation. Likewise, insulin resistance is associated with arterial stiffness and intima-media thickening due to adaptive accretion of collagen and lipids in the arterial wall.

CONCLUSIONS

Human TOR participates in the pathogenesis of sarcopenia and arterial stiffness, although its effects remain to be fully elucidated.

Hypercortisolism and altered glucose homeostasis in obese patients in the pre-bariatric surgery assessment

AIMS

Hypothalamus-pituitary-adrenal (HPA) axis hyperactivity was suggested to be associated with the metabolic syndrome (MS), obesity and diabetes. The aim of this study was to test whether hypercortisolism was associated with altered glucose homeostasis and insulin resistance, hypertension and dyslipidemia in a homogeneous population of obese patients.

MATERIALS/METHODS

In retrospective analysis of a set of data about obese patients attending the outpatient service of a single obesity centre between January 2013 and January 2020, 884 patients with BMI >30 kg/m² were segregated in two subgroups: patients with urinary free cortisol (UFC) higher than normal (UFC+; n = 129) or within the normal range (UFC-; n = 755).

RESULTS

The overall prevalence of UFC+ was 14.6% and double test positivity (morning cortisol >1.8 mcg/dL following overnight dexamethasone suppression test, ODST) was detected in 1.0% of patients. Prediabetes (OR 1.74; 95%CI 1.13-2.69; p = 0.012) and diabetes (OR 2.03; 95%CI 1.21-3.42; p = 0.008) were associated with higher risk of UFC+ when analysis was adjusted for confounding variables. Conversely, hypertension and dyslipidemia were not related to UFC+. Within the individuals with normal FPG and HbA1c, those with higher estimated insulin resistance (HOMA2-IR) maintained a higher risk of UFC+ (OR 2.84, 95%CI 1.06-7.63; p = 0.039) and this relationship was weakened only when the body fat percentage was included into the model.

CONCLUSIONS

In obese patients, hypercortisolism was more frequent across the entire spectrum of altered glucose homeostasis including the very early stages; this relation could not be detected for the other criteria of the MS, as waist, hypertension and atherogenic dyslipidemia.

Steroid-induced hyperglycemia: An underdiagnosed problem or clinical inertia? A narrative review

Corticosteroids are widely diffused drugs. An important side effect is the impairment of glycemic control both in patients with known diabetes and in normoglycemic ones potentially leading to steroid-induced diabetes mellitus (SIDM). In this review based on papers released on PubMed, MEDLINE, and EMBASE from January 2015 to October 2017, we summarized and discussed main updates about the definition, the diagnosis, and the pathophysiology of steroid-induced hyperglycemia (SIH), with a look to new therapies. Main alterations responsible for the diabetogenic effect of corticosteroids are a negative impact on insulin sensitivity along with a derangement on insulin secretion, explaining the typical post-prandial hyperglycemia linked to the promotion of gluconeogenesis. An early and precise diagnosis of SIH and/or SIDM is necessary, but current criteria do not seem sensible enough. As an afterthought, the treatment should be reasoned and tailored according to proposed glycemic thresholds and patient comorbidities, choosing between antidiabetic oral drugs and insulin, the latter being preferable among hospitalized patients. SIDM and SIH are frequent problems, but often underdiagnosed due to old diagnostic criteria. Dedicated guidelines universally shared are mandatory in order to harmonize the treatment of these conditions, thus overtaking single therapeutic strategies mostly arising from literature.

Glucocorticoid-induced hyperglycemia

Corticosteroid-induced hyperglycemia is a common medical problem that can lead to frequent emergency room visits, hospital admissions and prolonged hospital stay, in addition to the well known morbidity associated with hyperglycemia. However, the diagnosis and treatment of corticosteroid-induced hyperglycemia is surprisingly undervalued by most professionals, probably because of the lack of quality studies to determine specific strategies of action. In the present review, we discuss the pathophysiology of corticosteroid-induced hyperglycemia, focusing on diverse patterns of hyperglycemia induced by the different formulations, and provide clues for diagnosis based on the duration of treatment and the administration schedule of corticosteroids. We propose a treatment strategy based on both the pathophysiology of the process and the mechanism of action of different corticosteroids, and take into account dosing and administration timing to predict the duration of therapy. Finally, we propose treatment goals that differ slightly between the transient and continuous use of corticosteroids based on evidence from clinical practice guidelines of diabetes care both in ambulatory and hospital settings.

Severe hypernatremia from a urea-induced diuresis due to body protein wasting in an insulin-resistant type 2 diabetic patient

CONTEXT

Hypernatremia is encountered after pituitary or hypothalamic surgery and typically is secondary to vasopressin deficiency resulting in increased free water clearance with inadequate water replacement.

OBJECTIVE

We report a type 2 diabetic patient with severe hypernatremia (Na⁺ = 161 mEq/L) after hypothalamic surgery. Unexpectedly, this was accompanied by persistent urinary hypertonicity and negative total but positive electrolyte free water clearance.

MAIN OUTCOME MEASURE

Measurement of urinary electrolytes and urea revealed that an osmotic diuresis induced by urea derived principally by breakdown of endogenous protein was causative. Body protein losses over 48 hours were estimated to exceed 2 kg of lean mass. High-dose glucocorticoid, insulin resistance, and a postsurgical catabolic stress likely contributed.

CONCLUSION

In surgically severely stressed individuals, proteolysis of endogenous protein can strongly impact body water metabolism and contribute to severe hypernatremia.

Short-term prednisone use antagonizes insulin's anabolic effect on muscle protein and glucose metabolism in young healthy people

Glucocorticoids cause muscle atrophy and weakness, but the mechanisms for these effects are unclear. The purpose of this study was to test a hypothesis that prednisone (Pred) counteracts insulin's anabolic effects on muscle. A randomized, double-blind cross-over design was used to test the effects of 6 days either Pred (0.8 mg x kg(-1) x day(-1)) or placebo use in seven healthy young volunteers. Protein dynamics were measured across the leg using stable isotope tracers of leucine (Leu) and phenylalanine (Phe) after overnight fast and during a hyperinsulinemic (1.5 microU x min(-1) x kg FFM(-1)) euglycemic clamp with amino acid replacement. Fasting glucose, amino acids, insulin, and glucagon were higher (P < 0.01) on Pred vs. placebo, whereas leg blood flow was 18% lower. However, basal whole body and leg kinetics of Leu and Phe were unaltered by Pred. Insulin infusion increased leg glucose uptake in both trials but was 65% lower with Pred than with placebo. Insulin in both trials similarly suppressed whole body flux of Leu and Phe. Importantly, insulin increased net Leu and Phe balance across the leg and the balance between muscle protein synthesis and breakdown, but these changes were 45-140% lower (P < 0.03) in Pred than in placebo. The present study demonstrates that short-term Pred use in healthy people does not alter whole body or leg muscle protein metabolism during the postaborptive state but causes muscle insulin resistance for both glucose and amino acid metabolism, with a blunted protein anabolism. This interactive effect may lead to muscle atrophy with continued use of glucocorticoids.

Novel insights into glucocorticoid-mediated diabetogenic effects: towards expansion of therapeutic options?

At pharmacological concentrations, glucocorticoids (GCs) display potent anti-inflammatory effects, and are therefore frequently prescribed by physicians to treat a wide variety of diseases. Despite excellent efficacy, GC therapy is hampered by their notorious metabolic side effect profile. Chronic exposure to increased levels of circulating GCs is associated with central adiposity, dyslipidaemia, skeletal muscle wasting, insulin resistance, glucose intolerance and overt diabetes. Remarkably, many of these side-effects of GC treatment resemble the various components of the metabolic syndrome (MetS), in which indeed subtle disturbances in the hypothalamic-pituitary-adrenal (HPA) axis and/or increased tissue sensitivity to GCs have been reported. Recent developments have led to renewed interest in the mechanisms of GC's diabetogenic effects. First, 'selective dissociating glucocorticoid receptor (GR) ligands', which aim to segregate GC's anti-inflammatory and metabolic actions, are currently being developed. Second, at present, selective 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inhibitors, which may reduce local GC concentrations by inhibiting cortisone to cortisol conversion, are evaluated in clinical trials as a novel treatment modality for the MetS. In this review, we provide an update of the current knowledge on the mechanisms that underlie GC-induced dysmetabolic effects. In particular, recent progress in research into the role of GCs in the pathogenesis of insulin resistance and beta-cell dysfunction will be discussed.

Prevention and management of hyperglycemia after pancreas transplantation

PURPOSE OF REVIEW

Pancreas transplantation is considered the optimal therapy for patients with insulin-dependent diabetes. Successful pancreas transplantation achieves euglycemia and allows freedom from insulin therapy. Long-term allograft success may be limited by the development of impaired glucose metabolism. The objectives of the present review are to summarize the possible reasons for endocrine pancreatic dysfunction and to focus on its prevention and management and emphasize the role of immunosuppression.

RECENT FINDINGS

The diabetogenic effects of current immunosuppressive agents have been well established. Regimens without corticosteroids and calcineurin-inhibitor minimization or avoidance have been promoted. Recent studies have revisited the pathogenesis of type I and type II diabetes and demonstrated common pathways, including apoptosis induction, for the exhaustion and destruction of the pancreatic islets.

SUMMARY

The immunosuppressive regimens in pancreatic transplantation should be designed and appropriately modified according to the graft immunological and metabolic conditions. New molecules that are able to preserve islet function and maintain optimal insulin secretion should be considered for pancreas transplant recipients.

Management of hyperglycaemia after pancreas transplantation: are new immunosuppressants the answer?

Pancreas transplantation is considered the optimal therapy for patients with diabetes mellitus who reach end-stage renal disease. Despite achievement of euglycaemia after this procedure, the progression to impaired pancreatic function and metabolic exhaustion still represents one of the major concerns that increase the risk of graft loss. This paper reviews the possible mechanisms that can induce post-transplant hyperglycaemia, including those related to immunosuppression and those non-related, and the new strategies available for minimising or preventing this complication. Different aetiologies can induce pancreatic dysfunction. Technical complications, acute pancreatitis and delayed graft function, mostly related to impaired insulin secretion, are considered the early causes for abnormal glucose control. In general, acute rejection does not affect the endocrine portion of the pancreas graft because islet destruction occurs later than the inflammation of the exocrine components. Hyperinsulinaemia and insulin resistance represent the main concern for the progression of blood glucose intolerance. The anastomotic techniques of the exocrine portion of the pancreas and the immunosuppressive regimens are of critical importance for the development of impaired glucose metabolism. Hyperinsulinaemia, as a result of the fact that systemic-enteric or systemic-bladder drainages reducing the hepatic clearance of insulin, has led to the introduction of more physiological techniques using portal drainage of the endocrine secretions. Experimental and clinical data have shown that many of the current immunosuppressants account, to a large degree, for the increased risk of the development of post-transplant hyperglycaemia. The most common maintenance regimen in pancreatic transplantation still consists of triple therapy with a combination of corticosteroids, calcineurin inhibitors (either ciclosporin [cyclosporine] or tacrolimus), and mycophenolate mofetil (MMF).The diabetogenic effects of corticosteroids and calcineurin inhibitors have resulted in the need for protocols able to minimise their use. Recent studies have shown the safety and efficacy of steroid-sparing or -free regimens. Sirolimus has shown powerful immunosuppressive potency in absence of nephrotoxicity and diabetogenicity. Multicentre and single-centre reports have demonstrated that both calcineurin inhibitor withdrawal and avoidance were possible when sirolimus was used in a concentration-controlled fashion, with low-dose corticosteroids and MMF. Although the experience with sirolimus in pancreatic transplantation is still limited, the results are promising. Patients affected by diabetic gastroparesis seem to better tolerate a regimen with sirolimus and low-dose tacrolimus than one with tacrolimus in combination with MMF.For successful, long-term results of pancreatic transplantation, it is crucial to combine donor selection, technical aspects, modified anastomotic techniques and new therapeutic approaches designed to minimise the metabolic and non-metabolic adverse effects of the immunosuppressive regimens.

Effect of short-term prednisone use on blood flow, muscle protein metabolism, and function

Glucocorticoids can cause muscle atrophy, but the effect on muscle protein metabolism in humans has not been adequately studied to know whether protein synthesis, breakdown, or both are altered. We tested the effect of 6 d of oral prednisone (Pred, 0.5 mg/kg.d) on muscle protein metabolism and function. Six healthy subjects (three men/three women, 22-41 yr) completed two trials (randomized, double-blind, cross-over) with Pred and placebo. Fasting glucose, insulin, IGF-I, and glucagon were higher on Pred vs. placebo, whereas IGF-II and IGF binding protein-1 and -2 were lower. Whole-body amino acid fluxes, blood urea nitrogen, and urinary nitrogen loss were not statistically different between trials. Leg blood flow was 25% lower on Pred leading to 15-30% lower amino acid flux among the artery, vein, and muscle. However, amino acid net balance and rates of protein synthesis and breakdown were unchanged, as were synthesis rates of total mixed, mitochondrial, sarcoplasmic, and myosin heavy chain muscle proteins. Muscle mitochondrial function, muscle strength, and resting energy expenditure were also unchanged. These results demonstrate that a short-term moderate dose of prednisone affects glucose metabolism but has no effect on whole-body or leg muscle protein metabolism or muscle function.

Effects of high doses of glucocorticoids on free amino acids, ribosomes and protein turnover in human muscle

BACKGROUND

Treatment with glucocorticosteroids causes a negative nitrogen balance, but the kinetic mechanisms responsible for this catabolic effect are controversial. We investigated the effects of 60 mg day(-1) prednisolone on protein synthesis and degradation in human skeletal muscle.

MATERIALS AND METHODS

Healthy adults (n = 9) were studied in the postabsorptive state, before and after 3 days of prednisolone treatment. The L-[ring 2,6(-3)H(5)]-phenylalanine tracer technique, concentration and size distribution of the ribosomes, mRNA content of the ubiquitin-proteasome pathway components in muscle, phenylalanine flux across the leg, and the free amino acid concentrations in skeletal muscle were used to study muscle protein metabolism.

RESULTS

The concentrations of most amino acids in arterial blood increased after prednisolone. There were also increased effluxes of phenylalanine, asparagine, arginine, alanine, methionine and isoleucine from the leg. The rate of protein degradation, as measured by the appearance rate (Ra) of phenylalanine, increased by 67% (P = 0.023) which, together with a doubling of the net release of phenylalanine from the leg (P = 0.007), indicated accelerated protein degradation. The pathway was not identified but there was no significant increase in mRNAs' encoding components of the ubiquitin-proteasome pathway. There was a 6% reduction in polyribosomes (P = 0.007), suggesting a decrease in the capacity for protein synthesis, although there was no measured decrease in the rate of protein synthesis.

CONCLUSIONS

These findings indicate that high doses of prednisolone lead to a sharp increase in net protein catabolism, which depends more on enhanced protein breakdown, and an uncertain effect on protein synthesis. The mechanisms stimulating proteolysis and the pathway stimulated to increase muscle protein degradation should be explored.

Growth hormone and insulin-like growth factor-I counteracts established steroid catabolism in rats by effects on hepatic amino-N degradation

BACKGROUND/AIMS

Long-term steroid treatment causes protein wasting. Liver contributes towards this by upregulating ureagenesis. Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are anabolic agents with specific hepatic effects. It is unknown whether IGF-I alone and/or in combination with GH have any effect on established hepatic amino-N catabolism during long-term glucocorticoid treatment.

METHODS

We measured the spontaneous (UNSR) and the substrate standardized rate of urea nitrogen synthesis (STUNSR), N-balance and mRNA levels of urea cycle enzymes in controls (placebo) and four longterm steroid treated groups given (1) prednisolone 4 mg/kg/day during 28 days (St) (2) +GH 1 mg/kg/day from day 21-28 (StGH) (3) +IGF-I 1.5 mg/kg/day 21-28 (StIGF) (4) GH +IGF-I (StGHIGF).

RESULTS

Steroid induced weight loss was stepwisely reversed by IGF-I, GH and both. UNSR, STUNSR and mRNA levels of urea cycle enzymes in the liver increased markedly after steroid treatment, and was normalized after co-administration of GH and IGF-I. N-balance improved after GH and IGF-I administration.

CONCLUSIONS

Our results expands the knowledge of beneficial effects of GH on short-term steroid catabolism to include effects of IGF-I and IGF-I combined with GH on long-term steroid catabolism. Both peptides prevent steroid induced hepatic protein wasting and thereby contribute towards whole body anabolism. The effect in vivo is probably due to an effect of the peptides on urea cycle enzyme mRNA.

Leucine kinetics during simultaneously administered insulin and dexamethasone in preterm infants with severe lung disease

The objective of this study was to determine whether insulin administration would prevent the well-documented catabolic effect of dexamethasone given to preterm infants with chronic lung disease. We studied leucine metabolism in 11 very-low-birth-weight infants before dexamethasone treatment and on d 2, 4, and 7 thereafter. During the first 4 d of dexamethasone, insulin was administered i.v. at a dose of 0.5 (n = 7) or 1.0 (n = 5) IU/kg/d. Leucine turnover was not significantly different between d 0 (337 +/- 41.3 micromol leucine/kg/h), d 2 (288 +/- 27.2 micromol leucine/kg/h), d 4 (302 +/- 22.1 micromol leucine/kg/h), and d 7 (321 +/- 21.2 micromol leucine/kg/h), and neither was leucine breakdown (272 +/- 21.9 micromol leucine/kg/h on d 0, 225 +/- 21.5 micromol leucine/kg/h on d 2, 231 +/- 21 micromol leucine/kg/h on d 4, and 242 +/- 17.6 micromol leucine/kg/h on d 7). Weight gain rates were significantly lower during the first week of dexamethasone treatment compared with the week before treatment or the second and third week. We conclude that during insulin and corticosteroid administration in very-low-birth-weight infants, no changes were observed in leucine kinetics in contrast to previous studies. The decrease in weight gain was not reversed.

Effects of growth hormone on steroid-induced increase in ability of urea synthesis and urea enzyme mRNA levels

Growth hormone (GH) reduces the catabolic side effects of steroid treatment due to its effects on tissue protein synthesis/degradation. Little attention is focused on hepatic amino acid degradation and urea synthesis. Five groups of rats were given 1) placebo, 2) prednisolone, 3) placebo, pair fed to the steroid group, 4) GH, and 5) prednisolone and GH. After 7 days, the in vivo capacity of urea N synthesis (CUNS) was determined by saturating alanine infusion, in parallel with measurements of liver mRNA levels of urea cycle enzymes, N contents of organs, N balance, and hormones. Prednisolone increased CUNS (micromol . min-1 . 100 g-1, mean +/- SE) from 9.1 +/- 1.0 (pair-fed controls) to 13.2 +/- 0.8 (P < 0.05), decreased basal blood alpha-amino N concentration from 4.2 +/- 0.5 to 3.1 +/- 0.3 mmol/l (P < 0.05), increased mRNA levels of the rate- and flux-limiting urea cycle enzymes by 20 and 65%, respectively (P < 0. 05), and decreased muscle N contents and N balance. In contrast, GH decreased CUNS from 6.1 +/- 0.9 (free-fed controls) to 4.2 +/- 0.5 (P < 0.05), decreased basal blood alpha-amino N concentration from 3. 8 +/- 0.3 to 3.2 +/- 0.2, decreased mRNA levels of the rate- and flux-limiting urea cycle enzymes to 60 and 40%, respectively (P < 0. 05), and increased organ N contents and N balance. Coadministration of GH abolished all steroid effects. We found that prednisolone increases the ability of amino N conversion into urea N and urea cycle gene expression. GH had the opposite effects and counteracted the N-wasting side effects of prednisolone.

Post-transplant diabetes mellitus. The role of immunosuppression

Immunosuppressive agents increase the risk of death due to coronary disease or stroke by their ability to cause 3 different adverse effects: dyslipidaemia, hypertension and hyperglycaemia. Post-transplant diabetes mellitus has emerged as a major adverse effect of immunosuppressants. As recipients of organ transplants survive longer, the secondary complications of diabetes mellitus have assumed greater importance. There is a need for a precise definition of post-transplant diabetes mellitus to facilitate inter-centre comparison and to study the natural history of post-transplant diabetes mellitus. We recommend broad criteria to define hyperglycaemia, as a fasting blood glucose level of > 400 mg/dl at any point or > 200 mg/dl for 2 weeks, or a need for insulin treatment for at least 2 weeks. We also recommend serial measurements of HbA1c. Cyclosporin and tacrolimus cause post-transplant diabetes mellitus by a number of mechanisms, including decreased insulin secretion, increased insulin resistance or a direct toxic effect on the beta cell. For corticosteroids, the induction of insulin resistance seems to be the predominant factor. However, few studies have examined the mechanism of diabetogenicity at the molecular level. This may hold the key for pharmacological manipulation of current immunosuppressive regimens which may result in decreased metabolic complications. Corticosteroid sparing regimens have been shown to reduce the metabolic complications of immunosuppressants including post-transplant diabetes mellitus. However, their use should be balanced against the increased incidence of transplant rejections. Post-transplant diabetes mellitus may be organ-specific irrespective of the immunosuppressant used. Tacrolimus causes a high incidence of post-transplant diabetes mellitus in recipients of kidney transplants (upto 20% in some reports); the diabetogenicity of cyclosporin-based regimens is comparable with that of tacrolimus-based regimens in recipients of liver transplants. A few clinical studies in which attempts were made to discontinue cyclosporin resulted in an unacceptable loss of the transplant. In the case of tacrolimus, complete withdrawal of immunosuppression may be possible in selected patients with liver transplants. However, post-transplant recipients who may benefit from this approach are difficult to identify. In some early series, patients received doses of tacrolimus that were approximately 2 to 3 times higher than those currently used, which may have resulted in a higher incidence of post-transplant diabetes mellitus. More recently, it has been shown that tacrolimus was successful in salvaging whole pancreatic grafts which were maintained on cyclosporin. Tacrolimus-based immunosuppression as primary therapy was also used with remarkable success in solitary whole pancreas transplants. Strategies to reduce the metabolic complications of immunosuppressants should be pursued aggressively as this will directly lead to a decrease in long term cardiovascular adverse effects.

Use of [13C]bicarbonate infusion for measurement of CO2 production

To determine whether infusion of 13C-labeled bicarbonate can be used to measure rates of CO2 production (VCO2), seven healthy adults received 6-h primed continuous intravenous infusions of NaH13CO3 and L-[1-14C]leucine in the post-absorptive state while VCO2 was measured by indirect calorimetry. Indirect calorimetry and the use of specific activity and rate of 14CO2 expired yielded identical values of VCO2: 8.97 +/- 0.82 and 8.80 +/- 0.83 mmol/min, respectively (P = NS). The concentration of NaH13CO3 in the infusates and the 13C enrichment in breath CO2 were determined using gas chromatography-isotope ratio mass spectrometry. The rate of appearance of CO2 measured using the NaH13CO3 infusion rate and the steady-state breath 13CO2 enrichments was 11.41 +/- 1.56 mmol/min, which was higher (P < 0.001) than that determined by either of the other two methods. When corrected for the recovery of labeled CO2 during the infusion of NaH13CO3 by use of published values, rate of appearance of CO2 was 9.24 +/- 0.78 mmol/min, which did not differ from VCO2 determined using the other two methods. We conclude that infusion of NaH13CO3 can be used to determine VCO2. This method should be useful to study the oxidation of substrates in populations such as ventilator-dependent neonates, in whom indirect calorimetry is laborious and inaccurate.

Effect of cortisol on energy expenditure and amino acid metabolism in humans

Hydrocortisone was infused overnight into nine normal healthy adults on three occasions at 0, 80, and 200 micrograms.kg-1.h-1, producing plasma cortisol concentrations of 10.6 +/- 1.2, 34.0 +/- 2.0, and 64.9 +/- 4.3 micrograms/dl, respectively. L-[1-13C]leucine, L-[phenyl-2H5]phenylalanine, and L-[2-15N]glutamine were infused during the last 7 h of hypercortisolemia to measure amino acid kinetics. During the last 3.5 h, somatostatin, glucagon, and insulin were infused to reduce the cortisol-induced elevation in plasma insulin to basal. Hypercortisolemia increased plasma glucose, free fatty acid (FFA), and insulin concentrations. Institution of the somatostatin clamp returned insulin to basal but increased glucose and FFA. Acute hypercortisolemia increased protein breakdown 5-20%, as measured by increases in leucine and phenylalanine appearance rates. Normalizing insulin during hypercortisolemia did not alter phenylalanine flux but enhanced leucine appearance rate, the latter result indicating that insulin was affecting leucine metabolism during hypercortisolemia. The fraction of the leucine flux that was oxidized was not significantly increased with hypercortisolemia, but disposal by the nonoxidative route of leucine uptake for protein synthesis was increased. Hypercortisolemia increased cycling of amino acids by increasing protein breakdown and synthesis, but the increase in this process could have increased resting energy expenditure (REE) only 1-2%. Hypercortisolemia increased glutamine flux in a dose-dependent fashion through an increase in de novo synthesis, which presumably reflects increased release from skeletal muscle. Hypercortisolemia increased REE 9-15% at the 80 and 200 micrograms.kg-1.h-1 infusion rates. Respiratory quotient did not rise with cortisol infusion but tended to decrease, suggesting that the increase in REE was fueled by increased oxidation of fat. These data demonstrate that hypercortisolemia increases metabolic rate and may be in part responsible for the hypermetabolic state in injury.