Citations

Elective surgery was performed after overnight fasting, a routine that may affect the metabolic response to surgery. We investigated the effects of insulin and glucose infusions before and during surgery on postoperative substrate utilization and insulin sensitivity. Seven patients were given insulin and glucose infusions 3 h before and during surgery (insulin group), and a control group of six patients underwent surgery after fasting overnight. Insulin sensitivity and glucose kinetics (d-[6,6-2H2]glucose) were measured before and immediately after surgery using a hyperinsulinemic, normoglycemic clamp. Glucose infusion rates and whole body glucose disposal decreased after surgery in the control group (−40 and −29%, respectively), whereas no significant change was found in the insulin group (+16 and +25%). Endogenous glucose production remained unchanged in both groups. Postoperative changes in cortisol, glucagon, fat oxidation, and free fatty acids were attenuated in the insulin group (vs. control). We conclude that perioperative insulin and glucose infusions minimize the endocrine stress response and normalize postoperative insulin sensitivity and substrate utilization.

A decline in muscle mass and contractile function are prominent features of the sarcopenia of old age. Because myosin heavy chain is an important contractile protein, it was hypothesized that synthesis of this protein decreases in sarcopenia. The fractional synthesis rate of myosin heavy chain was measured simultaneously with rates of mixed muscle and sarcoplasmic proteins from the increment of [13C]leucine in these proteins purified from serial needle biopsy samples taken from 24 subjects (age: from 20 to 92 yr) during a primed continuous infusion ofl-[1-13C]leucine. A decline in synthesis rate of mixed muscle protein ( P < 0.01) and whole body protein ( P < 0.01) was observed from young to middle age with no further change with advancing age. An age-related decline of myosin heavy-chain synthesis rate was also observed ( P < 0.01), with progressive decline occurring from young, through middle, to old age. However, sarcoplasmic protein synthesis did not decline with age. Myosin heavy-chain synthesis rate was correlated with measures of muscle strength ( P < 0.05), circulating insulin-like growth factor I ( P < 0.01), and dehydroepiandrosterone sulfate ( P < 0.05) in men and women and free testosterone levels in men ( P < 0.01). A decline in the synthesis rate of myosin heavy chain implies a decreased ability to remodel this important muscle contractile protein and likely contributes to the declining muscle mass and contractile function in the elderly.

Mixed muscle protein synthetic rate has been measured in humans. These measurements represent the average of synthetic rates of all muscle proteins with variable rates. We determined to what extent the synthesis rate of mixed muscle protein in humans reflects that of myosin heavy chain (MHC), the main contractile protein responsible for the conversion of ATP to mechanical energy as muscle contraction. Fractional synthetic rates of MHC and mixed muscle protein were measured from the increment of [13C]leucine in these proteins in vastus lateralis biopsy samples taken at 5 and 10 h during a primed continuous infusion of L-[1-13C]leucine in 10 young healthy subjects. Calculations were done by use of plasma [13C]ketoisocaproate (KIC) and muscle tissue fluid [13C]leucine as surrogate measures of leucyl-tRNA. Fractional synthetic rate of MHC with plasma KIC (0.0299 +/- 0.0043%/h) and tissue fluid leucine (0.0443 +/- 0.0056%/h) were only 72 +/- 3% of that of mixed muscle protein (0.0408 +/- 0.0032 and 0.0603 +/- 0.0059%/h, respectively, with KIC and tissue fluid leucine). Contribution of MHC (7 +/- 1 mg.kg-1.h-1) to synthetic rates of whole body mixed muscle protein (36 +/- 5 mg.kg-1.h-1) and whole body protein (127 +/- 4 mg.kg-1.h-1) is only 18 +/- 1 and 5 +/- 1%, respectively. This relatively low contribution of MHC to whole body and mixed muscle protein synthesis warrants direct measurement of synthesis rate of MHC in conditions involving abnormalities of muscle contractile function.

Muscle wasting and weakness occur frequently in patients with chronic renal failure. The mechanism(s) by which these abnormalities occur is unclear. We hypothesized that such findings were due to defective muscle protein synthesis. We measured synthetic rates of mixed muscle proteins, myosin heavy chain, and mitochondrial proteins in serial muscle biopsy samples during a continuous infusion ofl[1-13C]leucine from 12 patients with chronic renal failure and 10 healthy control subjects under identical study conditions. Patients with chronic renal failure have significantly lower synthetic rates of mixed muscle proteins and myosin heavy chain (27 and 37% reductions, respectively, P < 0.05 and P < 0.02). Significant declines in the synthetic rates of muscle mitochondrial protein (27%) ( P < 0.05), muscle cytochrome c-oxidase activity (42%) ( P < 0.007), and citrate synthase (27%) ( P < 0.007) were also observed in patients with chronic renal failure. The synthetic rates of muscle proteins and activity of mitochondrial enzymes were negatively correlated to the severity of renal failure. These results indicate that in chronic renal failure there is a decrease in the synthesis of muscle contractile and mitochondrial proteins and a decrease in muscle mitochondrial oxidative enzymes. Reduced synthetic rate of several muscle proteins is the likely biochemical basis of muscle loss and muscle weakness in people with chronic renal failure.

To investigate the underlying molecular mechanisms for surgery-induced insulin resistance in skeletal muscle, six otherwise healthy patients undergoing total hip replacement were studied before, during, and after surgery. Patients were studied under basal conditions and during physiological hyperinsulinemia (60 μU/ml). Biopsies of vastus lateralis muscle were used to measure GLUT-4 translocation, glucose transport, and glycogen synthase activities. Surgery reduced insulin-stimulated glucose disposal ( P < 0.05) without altering the insulin-stimulated increase in glucose oxidation or suppression of endogenous glucose production. Preoperatively, insulin infusion increased plasma membrane GLUT-4 in all six subjects ( P < 0.05), whereas insulin-stimulated GLUT-4 translocation only occurred in three patients postoperatively (not significant). Moreover, nonoxidative glucose disposal rates and basal levels of glycogen synthase activities in muscle were reduced postoperatively ( P< 0.05). These findings demonstrate that peripheral insulin resistance develops immediately postoperatively and that this condition might be associated with perturbations in insulin-stimulated GLUT-4 translocation as well as nonoxidative glucose disposal, presumably at the level of glycogen synthesis.

Phenylalanine (Phe) kinetics are increasingly used in studies of amino acid kinetics, because the metabolic fate of Phe is limited to incorporation into protein (protein synthesis, Sp) and catabolism via hydroxylation ( Q pt) to tyrosine (Tyr). Besides an infusion of labeled Phe to measure Phe flux ( Q p), a priming dose of Tyr and an independent Tyr tracer are used to measure Tyr flux ( Q t) and Q pt. Alternatively, Q t, Q pt, and Sp can be approximated by using equations, based on Phe and Tyr concentrations in body proteins, that eliminate the need for a Tyr tracer. To evaluate the accuracy of this approach, data were obtained from 12 type I diabetic patients and 24 nondiabetic control subjects who were studied with the full complement of tracers both with and without insulin infusion. Sp approximations closely matched measured values in both groups (mean difference <2%, all values <5%), but the agreement was poor for Q pt (error range = −8 to +43%) and Q t (error range −22 to +41%). Insulin status had no effect on these comparisons. The lower approximation error for Sp vs. Q pt is due to the small contribution (∼10%) of Q pt to Q p. Approximation error for Q pt( r > 0.99) can be explained by variability in the ratio of Tyr to Phe coming from protein breakdown, ( Q t − Q pt)/ Q p. Ideally, all fluxes should be directly measured, but these data suggest that whole body Sp can be approximated with an acceptably small margin of error. However, the same equations do not yield reliably accurate values for Q pt or Q t.

Accumulation of collagen produces organ dysfunction in many pathological conditions. We measured the fractional synthesis rate (FSR) of dermal collagen in five human volunteers from the increment of [13C]proline in detergent-soluble dermal collagen hydroxylated to hydroxyproline during a continuous infusion ofl-[1-13C]proline. In these and eight other volunteers, we measured [13C]proline enrichment in skin aminoacyl-tRNA, skin tissue fluid amino acid, and plasma. The prolyl-[13C]tRNA enrichment was one-half that in tissue fluid proline and more than threefold less than in plasma. The FSR of dermal collagen was 0.076 ± 0.063%/h (mean ± SD), similar to previously reported rates for skeletal muscle contractile proteins and substantially slower than hepatically derived circulating proteins such as albumin or fibrinogen. We conclude that the FSR of human dermal collagen resembles that of other human proteins considered to display slow turnover. The current method for its measurement may be used to determine the regulation of collagen synthesis in other organs and disease states.

An estimate of total body muscle mass with dual-energy X-ray absorptiometry (DXA; appendicular muscle mass divided by 0.75) was compared with 24-h urinary creatinine excretion in 59 healthy men and women [20–30 yr (younger), 45–59 yr (middle age), and 60–79 yr (older)] who stayed in a clinical research center for 5 days. Total body water (2H2O dilution), fat (underwater weighing), bone mineral (DXA), and total body protein mass (based on a 4-compartment model) were also measured. Muscle mass estimates by DXA and creatinine were highly correlated ( r = 0.80). However, stepwise multiple regression indicated that a significant amount of additional between-subject variability in DXA-based muscle mass estimates could be explained by total body water. Creatinine excretion, knee extensor strength, and total body protein mass all decreased with age, suggesting a decline in muscle cell mass with aging. However, DXA-based muscle mass and measures of nonfat body mass (i.e., lean body mass by2H2O and fat-free body mass by underwater weighing) did not change with age. These results indicate that DXA and urinary creatinine excretion give different results regarding the decline in total body muscle mass with aging. The factor(s) responsible for the apparent underestimate of age-related sarcopenia by DXA remain to be fully defined, but changes in body water may be an important contributor.

In human protein turnover studies with isotopically labeled leucine (Leu) as a tracer, plasma ketoisocaproate (KIC) enrichment is extensively used as a surrogate measure of intracellular leucine enrichment. To test how accurately arterial ketoisocaproate (A-KIC) represents leucine isotopic enrichment in the hepatic (HV) and femoral veins (FV), which drain liver and muscle beds, we measured Leu and KIC enrichments in samples collected from HV, FV, and femoral artery (A) in 24 control and 6 type I diabetic subjects after a primed, continuous infusion ofl-[1-13C,15N]-Leu. Studies were performed during insulin deprivation or insulin replacement in the diabetic group, whereas the effect of normal saline or three different doses of insulin infusion (0.25, 0.50, and 1 mU ⋅ kg−1⋅ min−1) were assessed in healthy controls. The ratios of baseline isotopic enrichments of A-KIC to HV Leu and FV Leu were 0.93 ± 0.01 and 0.94 ± 0.02, respectively, in normal subjects and 1.07 ± 0.04 and 1.05 ± 0.03, respectively, in diabetic subjects ( P < 0.01, diabetic vs. normal subjects). Insulin did not change A-KIC-to-HV Leu ratios in either group, but the A-KIC-to-FV Leu ratio decreased during insulin infusion in normal subjects ( P < 0.05). In conclusion, A-KIC represents a reliable surrogate measure of HV Leu enrichment at different levels of circulating insulin in humans. The present data support the use of A-KIC as a surrogate precursor pool for hepatic protein synthesis.

A strong association has been reported between atherosclerotic diseases and fibrinogen levels, and a decreased whole body protein synthesis has also been reported with aging. We investigated the effect of age on fractional synthesis rates (FSR) of fibrinogen and albumin in 12 human subjects of young (20–30 yr), middle (45–60 yr), and old (65–79 yr) age by use ofl-[1-13C]leucine andl-[15N]phenylalanine as tracers. An age-related decline in FSR of fibrinogen ( P < 0.01) was observed with use of both tracers, with the maximal decrease (average 37% with α-[13C]ketoisocaproate as the precursor) occurring by middle age and with no further changes thereafter. In contrast, plasma concentrations of fibrinogen increased with age ( P < 0.002). There was no age-related change in synthesis rate and concentrations of albumin. An age-related decline in fibrinogen FSR, but not FSR of albumin, indicates a differential effect of age on synthesis rate of these two liver proteins. This study also demonstrated that the increased circulating levels of fibrinogen represent a slower rate of disposal of fibrinogen rather than an increased production rate.