Leucine kinetics and the effects of hyperinsulinemia in patients with Cushing's syndrome

As muscle wasting and resistance to insulin-mediated glucose utilization are features of Cushing's syndrome (CS), we examined glucose and amino acid metabolism in six patients with CS and six normal subjects before and during euglycemic hyperinsulinemic clamp studies (plasma insulin concentrations, approximately 0.36, approximately 0.65, and approximately 10.05 mmol/L). The two groups had similar body mass index values. In the postabsorptive state, leucine and alpha-ketoisocaproate (KIC) rates of appearance (Ra), KIC oxidation, and nonoxidized leucine-carbon flux, an index of leucine entering protein (Leu----P), were comparable in CS patients [2.38 +/- 0.14 (+/- SE), 0.22 +/- 0.04, and 2.16 +/- 0.12 mumol/kg.min) and in normal subjects (2.73 +/- 0.25, 0.17 +/- 0.02, and 2.59 +/- 0.22 mumol/kg.min). During the euglycemic clamp studies the leucine and KIC Ra values, KIC oxidation, and Leu----P decreased to a similar extent in both groups. In contrast, insulin-mediated glucose utilization was impaired in the CS patients at each clamp step (P less than 0.05). In summary, postabsorptive whole body leucine metabolism is normal in patients with CS and is normally suppressed by hyperinsulinemia, indicating a dissociation in insulin sensitivity with respect to glucose and amino acid metabolism.     

Augmentation of protein degradation by L-triiodothyronine in uremia

To ascertain if excessive protein catabolism is a feature of uremia, we determined leucine flux and nitrogen balance in 11 stable chronic dialysis patients and in 7 normal subjects. Leucine flux was determined during primed constant infusion of 2H3 and 15N leucine. Nitrogen balance was determined by measurement of nitrogen in the food, dialysate, and urine, and in the dialysis patients by correcting for the changing urea nitrogen pool. To assess if thyroid hormone adversely affects protein metabolism, the above-mentioned studies were done once in the basal state and once after a 7-day course of L-triiodothyronine (T3) treatment. Leucine carbon flux (mumol/kg/min) was 1.22 +/- 0.05 in the controls and 1.40 +/- 0.09 in the renal patients in the basal state (P = NS). Following T3 treatment, leucine carbon flux was increased to 1.40 +/- 0.05 and 1.72 +/- 0.09, respectively, in the controls and the renal patients (P less than .05). Fractional increment of the leucine carbon flux was 14% +/- 3% in the controls and 23% +/- 9% in the renal patients (P less than .05). The leucine nitrogen flux (mumol/kg/min) was 2.10 +/- 0.15 in the controls and 2.54 +/- 0.23 in the renal patients in the basal state (P = NS), and increased to 2.48 +/- 0.14 and 3.44 +/- 0.22, respectively, in controls and renal patients after T3 administration (P less than .05). Fractional increment of leucine nitrogen flux was 19.5% +/- 4.3% in the controls and 36.4% +/- 5.0% in the renal patients (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Failure of carbohydrate to spare leucine oxidation in obese subjects

Leucine kinetics were studied in six obese subjects (W/H2 = 39 +/- 4) and six normal subjects (W/H2 = 21 +/- 3) before and after an oral load of 150 g glucose. An intravenous infusion of 1(-13)C leucine was given to the fasting subjects for 450 min: a steady state of plasma leucine enrichment was established 90 min after the start of the infusion, and the glucose load was given 220 min after the start of the infusion. Compared with the lean controls the obese subjects showed a greater area under the curve of blood glucose after the glucose load (P less than 0.025) and higher insulin and glucagon levels both before and after the meal (P less than 0.05), thus indicating the well-known insulin insensitivity of obese (but not diabetic) subjects with respect to glucose metabolism. After the glucose load the lean subjects showed a significant and sustained decrease in leucine oxidation (from 20.0 +/- 2.2 to 13.3 +/- 1.5 mumol/kg LBM/h: P less than 0.01). This response is similar to that observed when insulin-dependent diabetic subjects are given insulin. However the obese subjects showed no decrease in leucine oxidation after the glucose meal (20.3 +/- 1.9 before, and 21.2 +/- 3.6 after). This indicates that obese subjects show insensitivity to the action of insulin with respect to protein metabolism as well as carbohydrate metabolism.     

Splanchnic bed utilization of enteral alpha-ketoisocaproate in humans

The branched-chain ketoacids (BCKAs) are used as dietary supplements to spare essential amino acid nitrogen, yet little is known about their absorption and utilization in the body. To study the fate of enterally delivered alpha-ketoisocaproate (KIC), seven healthy adults were infused in the postabsorptive state with [1-(13)C]KIC and [phenyl-2H5]phenylalanine intravenously (NGI) and with [5,5,5-2H3]KIC by nasogastric tube (NG). After 3.5 hours, the routes of tracer infusion were switched for an additional 3.5 hours. Each subject received a second infusion study on a different day with the order of tracer infusion reversed. KIC and phenylalanine kinetics and first-pass uptake and disposal of the enteral tracer by the splanchnic bed were calculated from the tracer enrichments measured in plasma KIC, leucine, and phenylalanine and breath CO2. Phenylalanine flux was 39.5 +/- 1.2 micromol/kg/h during the i.v. infusion periods. KIC flux was 33.1 +/- 1.8 and 30.4 +/- 1.4 micromol/kg/h measured with 13C- and 2H3-KIC, respectively, and these values were significantly different. The fraction of enterally delivered tracer sequestered by the splanchnic bed on the first pass was 30.9% +/- 2.0%, 30.0% +/- 1.4%, and 30.7% +/- 2.7% for 13C-KIC, 2H3-KIC, and 2H5-phenylalanine, respectively. The fraction of infused 13C-KIC tracer recovered as 13CO2 was 27.1% +/- 1.2% and 24.0% +/- 0.9% during i.v. and NG infusion, respectively. From these data, the fraction of ng KIC tracer extracted and oxidized on the first pass was calculated to be 5.1% +/- 1.1%. This fraction was greater than that previously reported for leucine extraction and oxidation (2%), but it was still only a small fraction of the overall extraction (5/30 = 16%). Because the only two fates of the KIC tracer extracted by the splanchnic bed are oxidation or transamination to leucine, the majority (84%) of the KIC tracer was extracted and converted to leucine. These results demonstrate that KIC delivered enterally to postabsorptive humans is rapidly extracted and predominantly converted to leucine by the splanchnic bed. This leucine appears to be available for use by both the splanchnic bed and the whole body.     

Differential effects of insulin resistance on leucine and glucose kinetics in obesity

The effects of insulin resistance on glucose and amino acid metabolism were studied in obese nondiabetic women (body mass index [BMI], (32.8 +/- 2) and in lean controls. Glucose disposal rate, hepatic glucose production, and leucine carbon flux and oxidation were simultaneously measured during the postabsorptive state and during euglycemic hyperinsulinemia, by means of primed, constant infusions of D-[6,6-2H2]glucose and L-[1-13C]leucine. Each subject participated in two insulin clamp studies on separate days, at infusion rates of 10 and 40 mU (m2.min)-1, producing plasma insulin levels of 20 to 25 and 70 to 80 microU/mL, respectively. Fat-free mass (FFM) was calculated from underwater weighing measurements. Insulin-mediated glucose disposal rate was significantly slower in the obese group: 2.05 +/- 0.05 versus 3.84 +/- 0.18 mg (kg.min)-1 in controls during the 10-mU insulin clamp, and 3.80 +/- 0.23 versus 9.16 +/- 0.47 mg (kg.min)-1 during the 40-mU clamp. The insulin-induced decrease in plasma levels of branched chain amino acids was also significantly blunted in the obese group. Baseline leucine flux was similar in lean and obese subjects (78 +/- 3 and 71 +/- 2 mumol (kg.h)-1, respectively), and its decline in response to insulin infusion was also comparable (8% and 10% during the 10-mU/m2 clamp, and of 17% and 18% during the 40-mU/m2 clamp in lean and obese, respectively). Basal leucine carbon oxidation (from [13C]leucine and [13C]alpha ketoisocaproate [alpha-KIC] plasma enrichments) was also similar in lean and obese, and did not change significantly with insulin infusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucose turnover and recycling in diabetes secondary to total pancreatectomy: effect of glucagon infusion

This study was designed to evaluate whether chronic deficiency of pancreatic glucagon in patients with diabetes secondary to total pancreatectomy (PX) is responsible for the commonly observed increase in blood concentrations of gluconeogenic precursors (alanine, lactate, and pyruvate). Seven PX patients were studied on two different occasions: 1) after an overnight insulin infusion (0.15 mU/kg.min) and 2) after an overnight insulin/glucagon infusion (2 ng/kg.min). Five type 1 diabetic individuals were also studied after a similar overnight insulin infusion. In the morning of each study day, [6-3H]glucose and [1-14C]glucose were rapidly injected for determination of total glucose turnover rate [( 6-3H]glucose) and glucose recycling (difference between [6-3H]glucose and [1-14C]glucose turnover rate). Basal concentrations of hormones, glucose, and intermediary metabolites were measured. After overnight insulin infusion, plasma glucose concentration (3.8 +/- 0.4 vs. 6.8 +/- 1.4 mmol/L), turnover rate (8.4 +/- 1.0 vs. 13.7 +/- 1.9 mumol/kg.min), and percent glucose recycling (5.6 +/- 3.9% vs. 19.0 +/- 3.8%) were significantly lower in PX patients than in type 1 diabetic individuals (P less than 0.05-0.01). On the contrary, blood alanine (459 +/- 93 vs. 263 +/- 28 mumol/L), lactate (1157 +/- 109 vs. 818 +/- 116 mumol/L), and pyruvate (71 +/- 8 vs. 42 +/- 3 mumol/L) were significantly higher than those values in type 1 diabetic patients (P less than 0.05-0.01). Insulin/glucagon infusion increased plasma glucose concentration (8.7 +/- 1.5 mmol/L), total turnover (18.1 +/- 1.7 mumol/kg.min), and percent recycling (20.4 +/- 6.6%) to values similar to those in type 1 diabetic subjects. The change in glucose metabolism was associated with a significant drop in blood concentrations of alanine (179 +/- 24 mumol/L), lactate (611 +/- 25 mumol/L), and pyruvate (30 +/- 3 mumol/L; all P less than 0.05-0.01 vs. insulin infusion alone). In PX patients, the glucose turnover rate was inversely correlated with blood concentrations of both alanine (r = 0.67) and lactate (r = 0.71; P less than 0.01). In conclusion, chronic deficiency of pancreatic glucagon in PX patients 1) is associated with a decreased rate of glucose turnover, 2) causes a marked impairment in glucose recycling (an index of the activity of hepatic gluconeogenesis), and 3) increases blood concentrations of alanine, lactate, and pyruvate. All abnormalities are reversed by glucagon.     

Diurnal variation in glucose and leucine metabolism in non-insulin-dependent diabetes

Glucose and leucine metabolism were investigated in 5 poorly controlled non-insulin-dependent diabetics (NIDDM) following an i.v. injection of 3-[3H]glucose and 1-[14C]leucine in the morning and evening. In the morning glucose concentration (11.2 +/- 0.8 mmol/l) (mean +/- SEM) and production rate (14.2 +/- 1.3 mumol/min/kg) were significantly greater (P less than 0.001, P less than 0.05) and glucose metabolic clearance rate (MCR) (1.3 +/- 0.2 ml/min/kg) significantly lower (P less than 0.05) than in a group of control subjects. Glucose concentration was lower in the evening (P less than 0.05) as a result of a decrease in glucose production rate (P less than 0.05). Leucine concentration and production rate were not significantly different from normal but leucine oxidation rate was increased (P less than 0.05). There was no diurnal variation in leucine metabolism. Since leucine production is a measure of protein breakdown, the higher morning glucose production rate was not due to an increased supply of gluconeogenic precursors from protein catabolism.     

Leucine and alpha-ketoisocaproate metabolism and interconversions in fed and fasted sheep

Fed and three-day-fasted sheep were infused with [1-14C] alpha-ketoisocaproate (KIC), L-[1-14C] leucine, and [14C] bicarbonate for determination of their whole-body turnovers, interconversions, and oxidation. Protein synthesis (PS), protein degradation (PD), net tissue metabolism, unidirectional utilization, and production rates also were estimated for the portal-drained viscera, liver, and hindquarters. KIC and leucine arterial concentrations (6.5 and 95 mumol X L-1) both increased with fasting. KIC turnover (9 mumol X min-1) also increased but leucine turnover (108 mumol X min-1) decreased. About 40% of KIC and 15% of leucine were oxidized, but they contributed less than 1% of whole-body CO2 production. The portal-drained viscera released KIC and leucine into the blood only in fed sheep. Hepatic net utilization of KIC and leucine (approximately 2 and 12 mumol X min-1) changed only little with fasting; thus, total splanchnic tissues utilized both in fasted sheep. Net metabolism by the hindquarters (representative of skeletal muscle) was always opposite to splanchnic metabolism. Thus, muscle must produce both KIC and leucine during fasting. In fed sheep whole-body PS, expressed as mumol X min-1 of leucine, was 92 +/- 6 and PD was 71 +/- 5. After fasting, PS decreased by 27%. Calculated liver protein metabolism was unaffected by the fast; PS (fixed and plasma) remained at about 25 and PD at about 15 mumol X min-1. However, protein metabolism by the hindquarters was sensitive to fasting; PS decreased from 30 +/- 4 in fed sheep to 20 +/- 3 mumol X min-1 after fasting and PD increased from 27 +/- 2 to 35 +/- 6 mumol X min-1. Thus, hepatic PS was maintained at the expense of muscle. If the total muscle mass of the body is considered, muscle PS contributed more than one half of whole-body PS.     

The hyperinsulinemic amino acid clamp increases whole-body protein synthesis in young subjects

We propose that hyperinsulinemia stimulates protein synthesis when postabsorptive plasma amino acid (AA) concentrations are maintained. During a euglycemic hyperinsulinemic clamp, many AA, notably the branched-chain amino acids (BCAA), decline markedly. Therefore, we tested whether individual plasma AA could be maintained within the range of postabsorptive concentrations to assess the effects of insulin, infused at 40 mU/m(2) x min on whole-body protein and glucose metabolism, using [1-(13)C]-leucine and [3-(3)H]-glucose methodology. Validation studies of background [(13)C] enrichment and breath (13)CO(2) recovery factors were performed in a subset of 6 subjects. In 10 healthy, young men, infusion rates of an AA solution were based on fluorometric determinations of total BCAA every 5 minutes. All 21 plasma AA remained in the target range; 15, including the BCAA, alanine, and glycine were within 13% of baseline, and only 6 (Thr, His, Arg, Asn, Cit, Tyr) varied more (18% to 42%). Notably, both leucine flux and nonoxidative leucine R(d) (protein synthesis) increased with insulin (2.36 +/- 0.06 to 2.81 +/- 0.10 and 1.79 +/- 0.05 to 2.18 +/- 0.10 micromol/kg fat-free mass (FFM) x min, respectively; P <.0005) while leucine oxidation only tended to increase (P =.05) and endogenous leucine R(a) (protein breakdown) decreased by 18% (2.36 +/- 0.06 to 1.94 +/- 0.09 micromol/kg FFM x min; P <.0005), resulting in a marked elevation of net protein synthesis (-0.57 +/- 0.02 to 0.24 +/- 0.02 micromol/kg FFM x min; P <.0000001). Thus, in vivo protein anabolism was induced when maintaining postabsorptive plasma amino acid concentrations during hyperinsulinemia through a suppression of whole-body protein breakdown, no significant change in oxidation and an elevation of synthesis compared with postabsorptive conditions.     

Acute cortisol excess results in unimpaired insulin action on lipolysis and branched chain amino acids, but not on glucose kinetics and C-peptide concentrations in man

To assess whether acute cortisol excess impairs insulin action on lipolysis, plasma amino acids, endogenous insulin secretion, and glucose kinetics, nine normal subjects were studied after acute cortisol excess (80 mg hydrocortisone by mouth) and after placebo. Insulin sensitivity was assessed 6 hours after hydrocortisone using the glucose clamp technique (insulin infusion of 20 mU/m2 X minute for 120 minutes, plasma insulin levels of approximately equal to 50 mU/L). Hyperinsulinemia suppressed plasma free fatty acids (FFA) similarly by 75 and 76%, respectively. Most plasma amino acid concentrations were increased after hydrocortisone; however, the insulin-induced decrease of branched chain amino acids, serine, threonine, and tyrosine was unimpaired after hydrocortisone. Plasma C-peptide concentrations were less suppressed during hyperinsulinemia after hydrocortisone than after placebo (by 0.15 +/- 0.03 v 0.25 +/- 0.02 nmol/L, P less than 0.01), suggesting diminished insulin-induced suppression of insulin secretion. The glucose infusion rates required to maintain euglycemia were 35% lower (P less than 0.01) after hydrocortisone due to decreased insulin effects on metabolic clearance rate of glucose and diminished suppression of hepatic glucose production (0.4 +/- 0.1 v -0.1 +/- 0.1 mg/kg X minute, p less than 0.05, 3-3H-glucose infusion method). The data demonstrate that acute elevation of plasma cortisol to levels near those observed in severe stress results in insulin resistance of peripheral and hepatic glucose metabolism but in unimpaired insulin effects on plasma FFA and branched chain amino acids, suggesting that cortisol's lipolytic and proteolytic effects are antagonized by elevated plasma insulin levels.     

Utilization for protein synthesis in individual rat organs of extracellular 2-ketoisocaproate relative to utilization of extracellular leucine

Rats were given constant intravenous infusions of [3H]-leucine plus [1-14C]-2-ketoisocaproate (KIC). Specific activities of plasma leucine and plasma KIC reached plateaus by two to three hours. 3H specific activity of KIC was 85% +/- 2% of that in leucine. 14C specific activity of leucine was 36% +/- 2% of that in KIC. The 14C/3H ratios in leucine and KIC were constant from the earliest sampling time (one hour) at 0.65 +/- 0.03 and 2.20 +/- 0.07, respectively. In various tissues, 14C/3H in free leucine and in tissue protein were approximately equal, but in most organs these ratios were significantly greater than the ratio 14C/3H in plasma leucine. From these data we estimate that the fraction of leucine incorporated into protein in individual organs derived from extracellular KIC rather than extracellular leucine varies from zero (in liver and bone marrow) to 35% to 45% (in brain and heart), and comprises 12% in the body as a whole.     

Glucose and amino acid metabolism in aging man: differential effects of insulin

Insulin is a major regulator of glucose and body protein homeostasis, both of which demonstrate age-related changes. To clarify insulin's role in these age-related changes and to compare age-related glucose and protein homeostatic responses, insulin-mediated aspects of glucose and amino acid metabolism were simultaneously examined in healthy postabsorptive young (n = 5, mean age, 25 years) and elderly (n = 5, mean age, 76 years) men. Primed constant infusions of L-[1-13C]leucine and L-[15N]alanine were administered during a basal period (0 to 180 minutes) and during four separate single rate euglycemic insulin infusions (180 to 360 minutes). Steady state insulin concentrations were 16 +/- 1, 29 +/- 3, 75 +/- 5, and 2407 +/- 56 microU/mL in the young and 23 +/- 4, 37 +/- 8, 96 +/- 11 and 3,357 +/- 249 microU/mL in the elderly at the different insulin infusion rates of 6, 10, 30, and 400 mU mU.m-2.min-1, respectively. For the 6 and 10 mU insulin infusion rates, a primed, constant infusion of [6,6 - 2H2]glucose permitted quantitation of hepatic glucose production. Glucose disposal rates adjusted for lean body mass (LBM) were lower in the elderly than in the young at the 6, 10, and 30 mU insulin infusion rates and similar in the two age groups in the 400 mU studies. Insulin dose-dependent reductions occurred in eight of ten plasma amino acids and were not influenced by age. There was an insulin dose-dependent reduction in plasma leucine flux which was similar in both age groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo differences between the turnover rates of leucine and leucine's ketoacid in stable cirrhosis.

Based on urinary nitrogen excretion, previous studies have indicated increased protein breakdown rates in cirrhosis. However, studies using [1-13C]-leucine infusion methodology have found normal protein breakdown rates. Because abnormal partitioning between extracellular and intracellular leucine exists in cirrhosis, plasma enrichment of leucine's keto acid (KIC), a marker of intracellular leucine, may more accurately reflect protein metabolism than plasma [1-13C]leucine enrichment. Therefore, protein breakdown and oxidation were calculated using both [1-13C]leucine and [1-13C]KIC and compared with urinary nitrogen excretion in seven cirrhotics and seven matched controls after an overnight fast. The ratio of KIC and leucine plasma enrichment was decreased (P less than 0.001) in cirrhosis because of lower KIC enrichment (P less than 0.006). Cirrhotics had increased rates of protein breakdown (P less than 0.006) and protein oxidation (P less than 0.05) based on KIC (P less than 0.006) but not leucine enrichment. In controls, protein oxidation calculated from urinary nitrogen excretion did not differ from KIC results (0.88 +/- 0.08 vs. 0.83 +/- 0.06) but was higher than the leucine method (0.88 +/- 0.08 vs. 0.73 +/- 0.05; P less than 0.01). However, in cirrhotics protein oxidation based on urinary nitrogen was lower than the KIC methodology (P less than 0.01). Therefore, cirrhotics have accelerated rates of protein breakdown and oxidation associated with increased extrarenal nitrogen loss. Furthermore, these results suggest abnormal leucine transport across cell membranes.     

Whole body leucine, phenylalanine, and tyrosine kinetics in end-stage liver disease before and after hepatic transplantation

The kinetics of leucine, phenylalanine, and tyrosine metabolism following orthotopic human liver transplantation in end-stage liver disease in hospitalized patients were evaluated and compared to controls. The investigation was carried out by protein turnover studies using 13C leucine, D5-phenylalanine, and [U-14C] tyrosine by continuous infusion and employing a stochastic model in 32 patients with end-stage liver disease, 17 of whom went on to receive an hepatic allograft, and 7 controls without significant liver disease who underwent elective abdominal surgery. Mean tyrosine flux in the liver disease group was 3,242 +/- 811 (n = 32) v 2,899 +/- 688 mumol/h in controls (n = 7) (P less than .001), while the tyrosine oxidation was 328 +/- 179 v 422 +/- 185 mumol/h (P less than .001). Tyrosine clearance in pretransplant patients was 719 +/- 345 (n = 17) v 1,193 +/- 568 mL/min (P less than .005) in posttransplant patients (n = 17) with virtually no overlap. There was a significant correlation between serum albumin levels and the tyrosine clearance (r = .60, P less than .05), but correlations with other conventional liver function tests were of a low order. Leucine and phenylalanine kinetics in liver disease patients did not show any significant differences from controls. Leucine and tyrosine fluxes in controls did exhibit a significant correlation (r = .70, P less than .05), but no correlation was observed in patients with liver disease. These findings indicate that the kinetics of the amino acid tyrosine are substantially altered by end-stage liver disease, with the most profound effect on tyrosine clearance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of increasing doses of recombinant human insulin-like growth factor-I on glucose, lipid, and leucine metabolism in man.

The metabolic effects of recombinant human insulin-like growth factor-I (IGF-I) were assessed in five groups of normal male overnight-fasted volunteers receiving infusions of either 0, 5, 7.5, 15, or 30 micrograms/kg.h IGF-I during 8 h, resulting in total plasma IGF-I concentrations 127 +/- 7, 247 +/- 30, 389 +/- 39, 573 +/- 62, 620 +/- 105 ng/ml, respectively. Glucose consumption (euglycemic glucose clamp) increased dose dependently during IGF-I infusion (P < 0.001) up to 6.7 +/- 1.3 mg/kg. min in the 30 micrograms/kg.h group. Plasma triglyceride concentrations decreased with increasing doses of IGF-I (P < 0.03); the fall was 43% in the 30 micrograms/kg.h group. Plasma free fatty acid concentrations decreased during 7.5, 15, and 30 micrograms/kg.h IGF-I by 23%, 34%, and 48%, respectively. IGF-I lowered plasma beta-hydroxybutyrate concentrations in a dose-dependent manner (P < 0.025). Plasma concentrations of leucine and alpha-ketoisocaproate decreased dose dependently (P < 0.001 and P < 0.015). Whole body leucine flux (1-13C-leucine infusion technique) decreased with increasing doses of IGF-I by 41% during 30 micrograms/kg.h, indicating decreased whole body protein breakdown. Leucine oxidation into 13CO2 decreased with increasing doses of IGF-I (P < 0.045) by 57% in the 30 micrograms/kg.h group, suggesting inhibition of irreversible loss of leucine. Plasma C-peptide and insulin concentrations decreased dose dependently (P < 0.005 and P < 0.02), indicating diminished insulin secretion. Thus, acute elevation of plasma IGF-I concentrations in man results in metabolic effects which are qualitatively similar to those described previously of insulin.     

Substrate metabolism during modest hyperinsulinemia in response to isolated hyperketonemia in insulin-dependent diabetic subjects

To assess the metabolic effects of moderate hyperketonemia, six young male type 1 diabetic patients received a 200-minute intravenous (IV) infusion of (1) 0.9 mmol 3-hydroxybutyrate (3-OHB)/kg/h, and (2) saline. To ensure comparable metabolic conditions, a low-dose hyperinsulinemic euglycemic glucose clamp was performed from 5 hours before and throughout 3-OHB/saline infusions. The forearm technique was employed to estimate substrate fluxes in muscle. Infusion of 3-OHB caused: (1) increases (P less than .05) in circulating levels of 3-OHB (from 112 +/- 73 mumol/L to 825 +/- 111 mumol/L) and forearm arteriovenous differences of 3-OHB (from 19 +/- 10 mumol/L to 145 +/- 46 mumol/L), as well as an eightfold increase of plasma acetoacetate. (2) Decreased (P less than .05) levels of nonesterified fatty acids (NEFA; from 466 +/- 85 mumol/L to 201 +/- 14 mumol/L) and glycerol (from 39 +/- 7 mumol/L to 11 +/- 4 mumol/L) and decreased (P less than .05) arteriovenous differences of glycerol (from -16 +/- 8 mumol/L to -3 +/- 2 mumol/L). (3) Increased (P less than .05) levels of serum growth hormone (GH; from 4.1 +/- 1.5 micrograms/L to 15.9 +/- 8.0 micrograms/L). No change was recorded in circulating concentrations of free insulin, glucagon, glucose, lactate, or alanine. Nor were arteriovenous balances of these intermediary metabolites, isotopically determined glucose turnover or amounts of exogenously administered glucose affected. In conclusion, in type 1 diabetic man, the main regulatory effect of isolated hyperketonemia appears to be a direct negative feedback inhibition of lipolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of insulin and plasma amino acid concentration on leucine metabolism in cirrhosis

Clinically stable patients with cirrhosis demonstrate insulin resistance with regard to glucose metabolism. However, much less is known about the two major factors, insulin and plasma amino acid concentration, that regulate protein metabolism in cirrhotic patients. To examine this question, we performed paired euglycemic insulin clamp studies in combination with 14C-leucine and indirect calorimetry. In the first study insulin alone was infused, and the plasma amino acid concentration was allowed to decline. During the second study a balanced amino acid solution was infused with insulin to increase the total plasma amino acid concentration approximately twofold. Insulin-mediated glucose disposal (4.68 vs. 6.45 mg/kg-min, p less than 0.01) was significantly impaired by 30% in cirrhotic patients during both insulin clamp studies. In the postabsorptive state, cirrhotic patients manifested low plasma leucine (76 vs. 102 mumol/L) and alpha-ketoisocaproate (19 vs. 30 mumol/L) concentrations, but all parameters of leucine turnover were normal. When insulin alone was infused, the endogenous leucine flux (an index of protein degradation) declined similarly in cirrhotic patients (30.8 mumol/m2-min) and control (26.9) subjects, and this was accompanied by a similar decrease in plasma leucine concentration (31% vs. 33%). The decline in circulating leucine concentration was accompanied by a parallel decline in leucine oxidation (5.1 vs. 4.6 mumol/m2-min) and nonoxidative (28.9 vs. 26.0 mumol/m2-min) leucine disposal, which were of similar magnitude in cirrhotic patients and control subjects, respectively. In both cirrhotic patients and control subjects, combined hyperinsulinemia/hyperaminoacidemia elicited a similar stimulation of nonoxidative leucine disposal (an index of protein synthesis) and leucine oxidation while causing a greater suppression of endogenous leucine flux than observed with insulin alone. Thus the suppressive effect of insulin on protein degradation and the stimulatory effect of insulin/amino acid infusion on protein synthesis are not impaired in cirrhotic patients, demonstrating a clear-cut dissociation between the effects of insulin on protein and glucose metabolism.     

Splanchnic metabolism of amino acids in healthy subjects: effect of 60 hours of fasting

Brief starvation is accompanied by decreased circulating levels of most amino acids, which has been attributed to an increased splanchnic uptake of amino acids, primarily alanine, for gluconeogenesis. However, quantitative data on splanchnic exchange of amino acids and gluconeogenic precursors is lacking. Consequently, arterial concentrations and splanchnic exchange of whole blood amino acids, ketone bodies, glucose, and gluconeogenic precursors were measured in 16 prolonged fasted (60 to 64 hours) and 15 overnight fasted (12 to 14 hours) healthy, nonobese subjects. After the 60-hour fast net splanchnic glucose production decreased by 41% to 0.31 +/- 0.02 mumol/L (P less than .001), whereas the splanchnic uptake of gluconeogenic precursors increased and could account for the total glucose output. Net splanchnic uptake of taurine, threonine, serine, glycine, lysine, histidine, and arginine rose significantly in response to fasting (P less than .05 to .01) due to increased splanchnic fractional extraction. Although the splanchnic fractional extraction of alanine was augmented by 40% (P less than .001), net splanchnic uptake was not influenced by fasting. Total net splanchnic uptake of amino acids increased by 68%, from 231 +/- 44 mumol/min in the postabsorptive state to 388 +/- 63 mumol/min (mean +/- SEM) (P less than .05) in the 60-hour fasted state. However, only one half of this rise was accounted for by gluconeogenic amino acids.     

Catabolism of branched-chain amino acids by diaphragm muscles of fasted and diabetic rats

In vitro catabolism of branched-chain amino acids, leucine and valine, was investigated using diaphragm muscles from normal, streptozotocin-diabetic and overnight fasted rats. Oxidation and transamination of [1-14C] branched-chain amino acids were both stimulated to a similar extent by diabetes or fasting, when diaphragms were incubated with glucose. Transamination of leucine and valine was increased when diaphragms were incubated with pyruvate; stimulation of transamination was greatest in diaphragms from diabetic rats. Leucine and valine oxidation by control diaphragms was inhibited by pyruvate while it was unchanged or slightly stimulated in diaphragms from fasted or diabetic rats. Thus diaphragms from diabetic rats oxidized two to threefold more branched-chain amino acids than controls when they were incubated with pyruvate. The specific radioactivity of extracellular alpha-ketoisocaproate (KIC; the product of leucine transamination) produced by diaphragms incubated with [14C]leucine was similar for all groups (fed, fasted, or diabetic) in the presence or absence of pyruvate. Oxidation of [1-14C]KIC by diaphragms from fasted or diabetic rats, incubated with glucose, was the same or less than KIC oxidation by control diaphragms. Incubation with pyruvate inhibited KIC oxidation by control diaphragms to a significantly greater degree than that by diaphragms from diabetic or fasted rats. These data suggest the following Flux through branched-chain amino acid transaminase is limited by the availability of amino group acceptors in diaphragms from normal and overnight fasted rats, and to a greater extent in diaphragms from diabetic rats. Flux through the transaminase may be a major determinant of accelerated branched-chain amino acid oxidation by diaphragms in fasting and diabetes. In diaphragms of fasted and diabetic rats, flux through the branched-chain alpha-ketoacid dehydrogenase complex is resistant to inhibition by pyruvate, which is normally observed in controls.     

Short-term effects of an intravenous infusion of a nutrient solution containing amino acids, glucose and insulin on leucine turnover and amino acid metabolism in patients with liver failure

Tolerance to an intravenous nutrient infusion containing amino acids, glucose and insulin was assessed in 12 adult patients with clinical and biochemical evidence of liver failure. In six individuals, fasting plasma amino acid patterns and turnover rates were measured by a constant intravenous infusion of [1-14C]leucine tracer. In the remaining patients the same measurements were made after a 10 h intravenous infusion of a 'standard' commercial mixture of amino acids (3.4 g/h), dextrose (20 g/h) and insulin (1.6 units/h). On the second day, conditions were crossed over and measurements were repeated. With the exception of glycine, the infusion of the amino acid-containing solution failed to significantly increase plasma amino acid content. Plasma leucine flux measurements indicated that the explanation involved a reduction in input of amino acids derived from the breakdown of body proteins rather than an increased uptake of amino acids into protein; i.e., the rate of liberation of leucine from protein decreased by an average of 2.4 mmol/h (P less than 0.05) whilst intake only added 1.6 mmol/h. No consistent changes in encephalopathy were observed. In conclusion, the infusion of the amino acid solution together with glucose and insulin resulted in improved nutritional intake and had no deleterious short-term effects on encephalopathy. However, judging from the plasma amino acid concentrations, the composition of the commercial mixture could be improved by reducing the glycine content.