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Biomedical subjects

E J Barrett

Publications and source records attributed to E J Barrett.

At least 73 records · Page 4Linked to original sources
PubMed1991

Growth hormone acutely stimulates forearm muscle protein synthesis in normal humans.

The short-term effects of growth hormone (GH) on skeletal muscle protein synthesis and degradation in normal humans are unknown. We studied seven postabsorptive healthy men (age 18-23 yr) who received GH (0.014 micrograms.kg-1.min-1) via intrabrachial artery infusion for 6 h. The effects of GH on forearm amino acid and glucose balances and on forearm amino acid kinetics [( 3H]Phe and [14C]Leu) were determined after 3 and 6 h of the GH infusion. Forearm deep vein GH rose to 35 +/- 6 ng/ml in response to GH, whereas systemic levels of GH, insulin, and insulin-like growth factor I (IGF-I) were unchanged. Forearm glucose uptake did not change during the study. After 6 h, GH suppressed forearm net release (3 vs. 6 h) of Phe (P less than 0.05), Leu (P less than 0.01), total branched-chain amino acids (P less than 0.025), and essential neutral amino acids (0.05 less than P less than 0.1). The effect on the net balance of Phe and Leu was due to an increase in the tissue uptake for Phe (71%, P less than 0.05) and Leu (37%, P less than 0.005) in the absence of any significant change in release of Phe or Leu from tissue. In the absence of any change in systemic GH, IGF-I, or insulin, these findings suggest that locally infused GH stimulates skeletal muscle protein synthesis. These findings have important physiological implications for both the role of daily GH pulses and the mechanisms through which GH can promote protein anabolism.

Adult↗
PubMed1991

Myocardial protein turnover in patients with coronary artery disease. Effect of branched chain amino acid infusion.

The regulation of protein metabolism in the human heart has not previously been studied. In 10 postabsorptive patients with coronary artery disease, heart protein synthesis and degradation were estimated simultaneously from the extraction of intravenously infused L-[ring-2,6-3H]phenylalanine (PHE) and the dilution of its specific activity across the heart at isotopic steady state. We subsequently examined the effect of branched chain amino acid (BCAA) infusion on heart protein turnover and on the myocardial balance of amino acids and branched chain ketoacids (BCKA) in these patients. In the postabsorptive state, there was a net release of phenylalanine (arterial-cardiac venous [PHE] = -1.71 +/- 0.32 nmol/ml, P less than 0.001; balance = -116 +/- 21 nmol PHE/min, P less than 0.001), reflecting protein degradation (142 +/- 40 nmol PHE/min) in excess of synthesis (24 +/- 42 nmol PHE/min) and net myocardial protein catabolism. During BCAA infusion, protein synthesis increased to equal the degradation rate (106 +/- 24 and 106 +/- 28 nmol PHE/min, respectively) and the phenylalanine balance shifted (P = 0.01) from negative to neutral (arterial-cardiac venous [PHE] = 0.07 +/- 0.36 nmol/ml; balance = 2 +/- 25 nmol PHE/min). BCAA infusion stimulated the myocardial uptake of both BCAA (P less than 0.005) and their ketoacid conjugates (P less than 0.001) in proportion to their circulating concentrations. Net uptake of the BCAA greatly exceeded that of other essential amino acids suggesting a role for BCAA and BCKA as metabolic fuels. Plasma insulin levels, cardiac double product, coronary blood flow, and myocardial oxygen consumption were unchanged. These results demonstrate that the myocardium of postabsorptive humans is in negative protein balance and indicate a primary anabolic effect of BCAA on the human heart.

Aged↗
PubMed1991

Hypoxemic stimulation of heart glycogen synthase and synthesis. Effects of insulin and diabetes mellitus.

With radiotracer and 13C nuclear magnetic resonance (13C-NMR) methods, we studied the time course of glycogen resynthesis after three 90-s episodes of hypoxemia in both control and diabetic rats in vivo. Glycogen synthesis was measured in the presence and absence of infused insulin and compared with the changes in glycogen synthase (GS) and phosphorylase activities. We observed in 13C-NMR spectra the expected mobilization of glycogen during hypoxia in vivo. In control rats with or without exogenous insulin, this was followed by a rapid resynthesis of glycogen during a 40-min recovery period. A marked activation of GS was observed by 10 min (glucose-6-phosphate-independent form of GS [GSl] = 0.65 mumol.min-1.g-1 or 92% of total GS), and activation persisted up to 40 min in both groups. Glycogen synthesis during the recovery period averaged 0.51 and 0.45 mumol.min-1.g-1 in the saline- and insulin-treated rats, respectively. In the diabetic rats by 10 min after hypoxemia, GSl increased only modestly in both saline-treated (0.16 mumol.min-1.g-1) and insulin-treated (0.21 mumol.min-1.g-1) rats, and activation persisted up to 40 min only with insulin treatment. Glycogen synthesis was slower in the diabetic rats given insulin (0.28 mumol.min-1.g-1) and essentially absent in the saline-treated rats (0.03 mumol.min-1.g-1) compared with controls. We conclude that recovery from hypoxemia is accompanied by a marked activation of GSl and rapid rates of glycogen synthesis in nondiabetic rats, and diabetes markedly blunts this response. Acute insulin infusion only partially overcomes this block.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Effect of infused branched-chain amino acids on muscle and whole-body amino acid metabolism in man.

1. Using the forearm balance method, together with systemic infusions of L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine, we examined the effects of infused branched-chain amino acids on whole-body and skeletal muscle amino acid kinetics in 10 postabsorptive normal subjects; 10 control subjects received only saline. 2. Infusion of branched-chain amino acids caused a four-fold rise in arterial branched-chain amino acid levels and a two-fold rise in branched-chain keto acids; significant declines were observed in circulating levels of most other amino acids, including phenylalanine, which fell by 34%. Plasma insulin levels were unchanged from basal levels (8 +/- 1 mu-units/ml). 3. Whole-body phenylalanine flux, an index of proteolysis, was significantly suppressed by branched-chain amino acid infusion (P less than 0.002), and forearm phenylalanine production was also inhibited (P less than 0.03). With branched-chain amino acid infusion total leucine flux rose, with marked increments in both oxidative and non-oxidative leucine disposal (P less than 0.001). Proteolysis, as measured by endogenous leucine production, showed a modest 12% decrease, although this was not significant when compared with saline controls. The net forearm balance of leucine and other branched-chain amino acids changed from a basal net output to a marked net uptake (P less than 0.001) during branched-chain amino acid infusion, with significant stimulation of local leucine disposal. Despite the rise in whole-body non-oxidative leucine disposal, and in forearm leucine uptake and disposal, forearm phenylalanine disposal, an index of muscle protein synthesis, was not stimulated by infusion of branched-chain amino acids. 4. The results suggest that in normal man branched-chain amino acid infusion suppresses skeletal muscle proteolysis independently of any rise of plasma insulin. Muscle branched-chain amino acid uptake rose dramatically in the absence of any apparent increase in muscle protein synthesis, as measured by phenylalanine disposal, or in branched-chain keto acid release. Thus, an increase in muscle branched-chain amino acid concentrations and/or local branched-chain amino acid oxidation must account for the increased disposal of branched-chain amino acids.

Adolescent↗
PubMed1990

Measurement of myocardial glycogen synthesis in diabetic and fasted rats.

Diabetes and fasting provoke an increase in heart glycogen content, despite a decline in the amount of active glycogen synthase present. To determine if the activity of glycogen synthase i is still rate limiting for glycogen synthesis, we used 13C-nuclear magnetic resonance to measure the in vivo rate of glycogen synthesis and compared this with the activity of glycogen synthase and phosphorylase measured in tissue extracts using physiological concentrations of substrates and activators. In the basal state the activity of glycogen synthase i was depressed in the diabetic and fasted hearts (P less than 0.01). The rate of heart glycogen synthesis was measured during a 50-min infusion of D-[1-13C]-glucose (10 mg/min) and insulin (1 U/min) and averaged 0.32 +/- 0.04 mumol.min-1.g wet wt-1 in controls and was diminished in both the diabetic (0.18 +/- 0.04 mumol.min-1.g wet wt-1) and fasted (0.16 +/- 0.03 mumol.min-1.g wet wt-1) and fasted (0.16 +/- 0.03 mumol.min-1.g wet wt-1) rats (P less than 0.05 for each). During the glucose and insulin infusion the average activity of glycogen synthase i was greater in control than diabetic or fasted hearts (P less than 0.01 for each) and approximated the rates of net glycogen synthesis in each group. In contrast, there were no significant differences in phosphorylase alpha activity, measured in tissue extracts, among the three groups. Furthermore, although this phosphorylase alpha activity greatly exceeded synthase activity, it did not appear to be expressed in vivo. We conclude that in normal, diabetic, and fasted rats, glycogen synthase is rate limiting for glycogen synthesis.

Animals↗
PubMed1990

Effect of starvation on human muscle protein metabolism and its response to insulin.

Although starvation is known to impair insulin-stimulated glucose disposal, whether it also induces resistance to insulin's antiproteolytic action on muscle is unknown. To assess the effect of fasting on muscle protein turnover in the basal state and in response to insulin, we measured forearm amino acid kinetics, using [3H]phenylalanine (Phe) and [14C]leucine (Leu) infused systemically, in eight healthy subjects after 12 (postabsorptive) and 60 h of fasting. After a 150-min basal period, forearm local insulin concentration was selectively raised by approximately 25 muU/ml for 150 min by intra-arterial insulin infusion (0.02 mU.kg-1. min-1). The 60-h fast increased urine nitrogen loss and whole body Leu flux and oxidation (by 50-75%, all P less than 0.02). Post-absorptively, forearm muscle exhibited a net release of Phe and Leu, which increased two- to threefold after the 60-h fast (P less than 0.05); this effect was mediated exclusively by accelerated local rates of amino acid appearance (Ra), with no reduction in rates of disposal (Rd). Local hyperinsulinemia in the postabsorptive condition caused a twofold increase in forearm glucose uptake (P less than 0.01) and completely suppressed the net forearm output of Phe and Leu (P less than 0.02). After the 60-h fast, forearm glucose disposal was depressed basally and showed no response to insulin; in contrast, insulin totally abolished the accelerated net forearm release of Phe and Leu. The action of insulin to reverse the augmented net release of Phe and Leu was mediated exclusively by approximately 40% suppression of Ra (P less than 0.02) rather than a stimulation of Rd. We conclude that in short-term fasted humans 1) muscle amino acid output accelerates due to increased proteolysis rather than reduced protein synthesis, and 2) despite its catabolic state and a marked impairment in insulin-mediated glucose disposal, muscle remains sensitive to insulin's antiproteolytic action.

Adult↗
PubMed1990

In vivo measurement of myocardial protein turnover using an indicator dilution technique.

We applied a nondestructive tracer technique, previously developed for measuring skeletal muscle protein turnover, to the measurement of myocardial protein turnover in vivo. During a continuous infusion of L-[ring-2,6-3H]phenylalanine to anesthetized, overnight-fasted dogs, we measured the uptake of radiolabeled phenylalanine from plasma and the release of unlabeled phenylalanine from myocardial proteolysis using arterial and coronary sinus catheterization and analytic methods previously applied to skeletal muscle. Using these measurements, together with a model of myocardial protein synthesis that assumes rapid equilibration of tracer specific activity between myocardial phenylalanyl-tRNA and circulating phenylalanine, we estimated the rates of heart protein synthesis and degradation. The rate of heart protein synthesis was also estimated directly from the incorporation of labeled phenylalanine into tissue protein. The use of [3H]phenylalanine was compared with L-[1-14C]leucine in the measurement of heart protein turnover in dogs given simultaneous infusion of both tracers. Leucine uptake and release by the myocardium exceeded that of phenylalanine by 3.1 +/- 0.4- and 1.7 +/- 0.3-fold, respectively, consistent with leucine's 2.4-fold greater abundance in heart protein and its metabolism via other pathways. Phenylalanine is the preferred tracer for use with this method because of its limited metabolic fate in muscle. One theoretical limitation to the method, slow equilibration of circulating labeled phenylalanine with myocardial phenylalanyl-tRNA, was resolved by comparison of these specific activities after a 30-minute infusion of labeled phenylalanine in the rat. A second, empirical limitation involves precision in the measurement of the small decrements in phenylalanine specific activity that occur with each pass of blood through the coronary circulation. This was addressed by improving the precision of both the measurements of phenylalanine concentration and phenylalanine specific activity using high-performance liquid chromatography. We conclude that the in vivo measurement of phenylalanine tracer exchange across the myocardium permits the nondestructive estimation of heart protein turnover in the intact animal.

Animals↗
PubMed1990

Simultaneous synthesis and degradation of rat liver glycogen. An in vivo nuclear magnetic resonance spectroscopic study.

Using 13C nuclear magnetic resonance spectroscopic methods we examined in vivo the synthesis of liver glycogen during the infusion of D-[1-13C]glucose and the turnover of labeled glycogen during subsequent infusion of D-[1-13C]glucose. In fasted rats the processes of glycogen synthesis and degradation were observed to occur simultaneously with the rate of synthesis much greater than degradation leading to net glycogen synthesis. In fed rats, incorporation of infused D-[1-13C]glucose occurred briskly; however, over 2 h there was no net glycogen accumulated. Degradation of labeled glycogen was greater in the fed versus the fasted rats (P less than 0.001), and the lack of net glycogen synthesis in fed rats was due to degradation and synthesis occurring at similar rates throughout the infusion period. There was no indication that suppression of phosphorylase a or subsequent activation of glycogen synthase was involved in modulation of the flux of tracer into liver glycogen. We conclude that in both fed and fasted rats, glycogen synthase and phosphorylase are active simultaneously and the levels of liver glycogen reached during refeeding are determined by the balance between ongoing synthetic and degradative processes.

Animals↗
PubMed1989

Physiologic hyperinsulinemia stimulates lactate extraction by heart muscle in the conscious dog.

The effect of physiologic hyperinsulinemia on the net balance of lactate, glucose, and free fatty acids across the heart was studied in eight normal postabsorptive conscious dogs. After obtaining basal measurements of myocardial substrate balance, arterial plasma insulin was increased from 8 +/- 1 to 68 +/- 14 microU/mL while blood glucose was maintained constant (64 +/- 1 mg/dL) using the hyperinsulinemic euglycemic clamp. Myocardial lactate uptake increased nearly fourfold, from 5.8 +/- 1.8 to 22.4 +/- 2.9 mumol/min (P less than .005). Despite a small increase in arterial lactate concentration from 0.46 +/- 0.08 to 0.79 +/- 0.11 mmol/L (P less than .02), the lactate extraction fraction increased from 23% +/- 7% to 54% +/- 2% (P less than .001) indicating an increased efficiency of lactate extraction. Euglycemic hyperinsulinemia led to a comparable increase in myocardial glucose uptake (6.7 +/- 2.3 to 18.2 +/- 3.7 mumol/min, P less than .05). Arterial free fatty acid concentrations fell from 1.06 +/- 0.13 to 0.35 +/- 0.06 mmol/L (P less than .001) with a concomitant decline in the myocardial uptake of free fatty acids from 18.5 +/- 5.3 to 5.8 +/- 2.9 mumol/min (P less than .05). These results indicate that physiologic hyperinsulinemia increases lactate as well as glucose uptake in normal heart muscle.

Animals↗
PubMed1989

The power of policy: the relationship of smoking policy to adolescent smoking.

This study examined the effects of smoking policy on 4,807 adolescents in 23 schools over a two-county area in California. Amounts and prevalence rates of adolescent smoking were measured with a self-report survey and a biochemical measure; school smoking policy was measured with two independent surveys of school staff. Policy effects were evaluated with multiple and logistic regression analyses controlling for school-level socioeconomic status and environmental support for teaching and administration. Of the 23 schools, 100 percent had a formal written and regularly enforced policy component restricting student smoking on school grounds, 94 percent restricted students leaving school grounds, 65 percent restricted smoking near school grounds, and 57 percent had a smoking prevention education plan. Schools with policies having all four versus less than four components, high versus low emphasis on prevention, and a low versus high emphasis on cessation reported lower amounts of smoking in the last week and in the last 24 hours. Punitive consequences of policy violation had no effect. Results were compared to school staff observations of adolescent smoking, and school archival records of student smoking violations in the last year. Results suggest that school smoking policy is associated with decreased amounts of smoking in adolescents.

Adolescent↗
PubMed1988

13C NMR studies of glycogen turnover in the perfused rat liver.

To assess whether hepatic glycogen is actively turning over under conditions which promote net glycogen synthesis we perfused livers from 24-h fasted rats with 20 mM D-[1-13C]glucose, 10 mM L-[3-13C]alanine, 10 mM L-[3-13C]lactate, and 1 microM insulin for 90 min followed by a 75-min "chase" period with perfusate of the same composition containing either 13C-enriched or unlabeled substrates. The peak height of the C-1 resonance of the glucosyl subunits in glycogen was monitored, in real time, using 13C NMR techniques. During the initial 90 min the peak height of the C-1 resonance of glycogen increased at almost a constant rate reflecting a near linear increase in net glycogen synthesis, which persisted for a further 75 min if 13C-enriched substrates were present during the "chase" period. However, when the perfusate was switched to the unenriched substrates, the peak height of the C-1 resonance of glycogen declined in a nearly linear manner reflecting active glycogenolysis during a time of net glycogen synthesis. By comparing the slopes of the curve describing the time course of the net [1-13C] glucose incorporation into glycogen with the rate of net loss of 13C label from the C-1 resonance of glycogen during the "chase" period we estimated the relative rate of glycogen breakdown to be 60% of the net glycogen synthetic rate. Whether this same phenomenon occurs to such an appreciable extent in vivo remains to be determined.

Alanine↗
PubMed1988

NMR measurements of in vivo myocardial glycogen metabolism.

Using 13C and 1H NMR we measured the rate of glycogen synthesis (0.23 +/- 0.10 mumol/min gram wet weight tissue (gww) in rat heart in vivo during an intravenous infusion of D-[1-13C]glucose and insulin. Glycogen was observed within 10 min of starting and increased linearly throughout a 50-min infusion. This compared closely with the average activity of glycogen synthase I (0.22 +/- 0.03 mumol/min gww) measured at physiologic concentrations of UDP-glucose (92 microM) and glucose-6-phosphate (110 microM). When unlabeled glycogen replaced D-[1-13C]glucose in the infusate after 50 min the D-[1-13C]glycogen signal remained stable for another 60 min, indicating that no turnover of the newly synthesized glycogen had occurred. Despite this phosphorylase a activity in heart extracts from rats given a 1 h glucose and insulin infusion (3.8 +/- 2.4 mumol/min gww) greatly exceeded the total synthase activity and if active in vivo should promote glycogenolysis. We conclude that during glucose and insulin infusion in the rat: (a) the absolute rate of myocardial glycogen synthesis can be measured in vivo by NMR; (b) glycogen synthase I can account for the observed rates of heart glycogen synthesis; (c) there is no futile cycling of glucose in and out of heart glycogen; and (d) the activity of phosphorylase a measured in tissue extracts is not reflected in vivo. These studies raise the question whether significant regulation of phosphorylase a activity in vivo is mediated by factors in addition to its phosphorylation state.

Algorithms↗
PubMed1988

The disposal of an oral glucose load in patients with non-insulin-dependent diabetes.

Following glucose ingestion, tissue glucose uptake is enhanced and endogenous glucose production is inhibited, thus contributing to the maintenance of normal glucose tolerance. To examine whether these responses are disturbed in diabetes, glucose kinetics after oral glucose administration were studied in 12 non-insulin-dependent diabetic and 10 age- and weight-matched control subjects. A double tracer approach was used, whereby the endogenous glucose pool was labeled with 3-3H-glucose and the oral load with 1-14C-glucose. The two glucose tracers were separated in plasma by a two-step chromatographic procedure, and the two sets of isotopic data were analyzed according to a two-compartment model for the glucose system. Basally, glucose production was slightly higher in diabetics than in controls (2.51 +/- 0.24 v 2.28 +/- 0.11 mg/kg.min, NS) even though the former had higher plasma glucose (189 +/- 19 v 93 +/- 2 mg/dL, P less than .001) and insulin (23 +/- 4 v 12 +/- 1 microU/mL, P less than .05) concentrations. Following the ingestion of 1 g/kg of glucose, oral glucose appeared in the peripheral circulation in similar time-course and amount in the two groups (75 +/- 2% of the load over 3.5 hours in the diabetics v 76 +/- 3% in controls). Endogenous glucose production was promptly inhibited in diabetic and normal subjects alike, but the mean residual hepatic glucose production after glucose ingestion was significantly greater in the diabetic group (17 +/- 2 v 10 +/- 3 g/3.5 h, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗
PubMed1988

Metabolic effects of low-dose insulin therapy on glucose metabolism in diabetic ketoacidosis.

The effect of low-dose insulin treatment (5-10 U/h) on hepatic glucose production (HGP) and peripheral glucose disposal was determined in 5 insulin-dependent diabetes mellitus (IDDM) subjects who were admitted with diabetic ketoacidosis (DKA; plasma glucose 598 +/- 50 mg/dl, blood pH 7.20 +/- 0.06, plasma bicarbonate 12 +/- 2 meq/L). Basal hepatic glucose production (4.3 +/- 0.5 mg.kg-1.min-1) in the DKA patients was 1.5- to 2-fold greater (P less than .01) than in controls (2.1 +/- 0.1 mg.kg-1.min-1) and nonketotic IDDM subjects (2.9 +/- 0.3 mg.kg-1.min-1), whereas tissue glucose disposal was significantly reduced (1.7 +/- 0.1 vs. 2.1 +/- 0.1 mg.kg-1.min-1, P less than .05). After the institution of insulin therapy (1 mU.kg-1.min-1), the plasma glucose concentration fell at the rate of 60 +/- 5 mg.dl-1.h-1 to reach a value of 220 +/- 10 mg/dl, which was maintained constant for 2 h (insulin-clamp technique). Blood pH (7.21 +/- 0.06 to 7.35 +/- 0.05) and plasma bicarbonate (12 +/- 3 to 18 +/- 2 meq/L) both increased during insulin therapy (P less than .01). The decline in plasma glucose concentration during insulin therapy primarily resulted from a suppression of HGP (from 4.3 +/- 0.5 to 1.7 +/- 0.2 mg.kg-1.min-1, P less than .01) and to a lesser extent from the stimulation of tissue glucose disposal (1.7 +/- 0.2 to 2.6 +/- 0.3 mg.kg-1.min-1, P less than .01). At this time, urine glucose excretion decreased from 2.6 +/- 0.2 to 0.6 +/- 0.1 mg.kg-1.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hydroxybutyric Acid↗
PubMed1988

Splanchnic amino acid and glucose metabolism during amino acid infusion in dogs.

With the organ-balance technique, we studied amino acid and glucose metabolism by hepatic and extrahepatic splanchnic tissues in awake dogs in the postabsorptive state and during a 3-h intravenous amino acid infusion. Dogs received a high (1.4 g/kg body wt, n = 5) or low (0.7 g/kg body wt, n = 8) dose of amino acids. In four of the latter dogs, the dose was delivered into a mesenteric vein. During the basal period there was a net removal of gluconeogenic amino acids (particularly alanine), but not branched-chain amino acids, and a net production of glucose by the liver in all dogs. During this time there was a net removal of glucose and production of alanine by the extrahepatic splanchnic tissues. During either high- or low-dose amino acid infusion, net hepatic glucose release increased; despite this, arterial plasma glucose declined due to an increase in tissue glucose uptake at extrasplanchnic sites. The net amount of glucogenic amino acids removed by the liver during high-dose (9.1 +/- 1.0 mmol.kg-1.3 h-1) and low-dose (4.8 +/- 0.6 mmol.kg-1.3 h-1) infusion equaled or exceeded the infused load of these amino acids. In addition, the liver contributed to the net disposal of branched-chain amino acids during high-dose (536 +/- 147 mumol.kg-1.3 h-1) and low-dose (341 +/- 70 mumol.kg-1.3 h-1) infusion. During high-dose infusion, extrahepatic splanchnic tissues participated in the net removal of branched-chain amino acids (436 +/- 162 mumol.kg-1.3 h-1) but not glucogenic amino acids, and net alanine production continued (410 +/- 91 mumol.kg-1.3 h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1988

Effect of chronic diabetes on myocardial fuel metabolism and insulin sensitivity.

To assess the effect of chronic insulin-deficient diabetes on myocardial fuel substrate metabolism in vivo, we measured the myocardial balance of glucose, free fatty acids (FFAs), and amino acids in nine postabsorptive conscious dogs 4-6 wk after treatment with streptozocin. The acute effect of insulin on the myocardial balance of these same substrates was measured in six dogs by use of the euglycemic insulin clamp technique. To further examine the effect of insulin on heart amino acid balance, we studied three additional dogs given a constant infusion of amino acids during the insulin clamp to blunt the insulin-induced hypoaminoacidemia. In these dogs, the fasting plasma glucose concentration was markedly elevated (258 +/- 3 mg/dl). In the basal period, there was no significant glucose uptake by the heart [arterial vs. coronary sinus concentration difference (delta) = 1.0 +/- 2.0 mg/dl]; furthermore, physiologic hyperinsulinemia did not stimulate glucose uptake (delta = 2.0 +/- 2.5 mg/dl). Postabsorptively, arterial FFAs were elevated (1550 +/- 320 microM) in diabetic animals, and there was a significant net extraction of FFAs by the heart (net uptake 26 +/- 9 mumol/min; extraction ratio 30 +/- 8%). During the insulin clamp, arterial FFAs declined (645 +/- 240 microM), as did heart FFA uptake (11 +/- 6 mumol/min), and the net extraction ratio for FFAs was unchanged (30 +/- 7%). Similarly, the arterial branched-chain amino acid (BCAA) concentration was elevated in the postabsorptive state, and there was a significant myocardial uptake of these amino acids and of alanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Alanine↗
PubMed1987

An isotopic method for measurement of muscle protein synthesis and degradation in vivo.

In eight anaesthetized post-absorptive dogs we measured the concentration and specific radioactivity of phenylalanine and leucine in arterial and femoral-venous plasma, together with hindlimb flow during a continuous infusion of L-[ring-2,6-3H]phenylalanine and [1-14C]leucine. The femoral-venous plasma concentration was greater than arterial for both phenylalanine and leucine (P less than 0.05 for each). Despite net amino acid release there was a significant removal of both labelled phenylalanine and labelled leucine. Consequently, a significant dilution of specific radioactivity was observed between artery and vein for both radio-tracers. The uptake of leucine from the arterial circulation by the hindlimb exceeded by 2.6-fold that of phenylalanine; the measured molar ratio of leucine to phenylalanine in hindlimb muscle protein averaged 2.4 +/- 0.1. Since phenylalanine is neither synthesized nor degraded by muscle tissue, the measured removal of tracer and the dilution of tracer specific radioactivity across the hindlimb can be used to estimate rates of phenylalanine incorporation into, and release from, tissue protein. The estimated rate of protein synthesis by hindlimb averaged 644 +/- 250 nmol of phenylalanine/min. This was exceeded by the rate of tissue protein degradation (987 +/- 285 nmol of phenylalanine/min). The present results demonstrate that the dilution of the specific radioactivity of labelled phenylalanine can be readily measured across dog hindlimb. This measurement, coupled with an estimate of tissue blood flow, can provide a readily measured, non-destructive, method for estimation of protein turnover in specific muscle beds in vivo. Measurements can be made repeatedly over time in a single experiment, allowing the study of factors which regulate protein turnover. The method developed here in dogs can be readily extended to clinical studies.

Amino Acids↗