Eugene J. Barrett

Effect of fatty acids on glucose production and utilization in man.

Since the initial proposal of the glucose fatty acid cycle, considerable controversy has arisen concerning its physiologic significance in vivo. In the present study, we examined the effect of acute, physiologic elevations of FFA concentrations on glucose production and uptake in normal subjects under three controlled experimental conditions. In group A, plasma insulin levels were raised and maintained at approximately 100 microU/ml above base line by an insulin infusion, while holding plasma glucose at the fasting level by a variable glucose infusion. In group B, plasma glucose concentration was raised by 125 mg/100 ml and plasma insulin was clamped at approximately 50 microU/ml by a combined infusion of somatostatin and insulin. In group C, plasma glucose was raised by 200 mg/100 ml above the fasting level, while insulin secretion was inhibited with somatostatin and peripheral glucagon levels were replaced with a glucagon infusion (1 ng/min X kg). Each protocol was repeated in the same subject in combination with a lipid-heparin infusion designed to raise plasma FFA levels by 1.5-2.0 mumol/ml. With euglycemic hyperinsulinemia (study A), lipid infusion caused a significant inhibition of total glucose uptake (6.3 +/- 1.3 vs. 7.4 +/- 0.6 mg/min X kg, P less than 0.02). Endogenous glucose production (estimated by the [3-3H]glucose technique) was completely suppressed both with and without lipid infusion. With hyperglycemic hyperinsulinemia (study B), lipid infusion also induced a marked impairment in glucose utilization (6.2 +/- 1.1 vs. 9.8 +/- 1.9 mg/min X kg, P less than 0.05); endogenous glucose production was again completely inhibited despite the increase in FFA concentrations. Under both conditions (A and B), the percentage inhibition of glucose uptake by FFA was positively correlated with the total rate of glucose uptake (r = 0.69, P less than 0.01). In contrast, when hyperglycemia was associated with relative insulinopenia and hyperglucagonemia (study C), thus simulating a diabetic state, lipid infusion had no effect on glucose uptake (2.9 +/- 0.2 vs. 2.6 +/- 0.2 mg/min X kg) but markedly stimulated endogenous glucose production (1.4 +/- 0.5 vs. 0.5 +/- 0.4 mg/min X kg, P less than 0.005). Under the same conditions as study C, a glycerol infusion producing plasma glycerol levels similar to those achieved with lipid-heparin, enhanced endogenous glucose production (1.5 +/- 0.5 vs. 0.7 +/- 0.6 mg/min X kg, P less than 0.05). We conclude that, in the well-insulinized state raised FFA levels effectively compete with glucose for uptake by peripheral tissues, regardless of the presence of hyperglycemia. When insulin is deficient, on the other hand, elevated rates of lipolysis may contribute to hyperglycemia not by competition for fuel utilization, but through an enhancement of endogenous glucose output.

Acute effects of insulin-like growth factor I on glucose and amino acid metabolism in the awake fasted rat. Comparison with insulin.

To elucidate the acute metabolic actions of insulin-like growth factor I (IGF-I), we administered a primed (250 micrograms/kg), continuous (5 micrograms/kg.min) infusion of human recombinant (Thr 59) IGF-I or saline to awake, chronically catheterized 24-h fasted rats for 90 min. IGF-I was also infused while maintaining euglycemia (glucose clamp technique) and its effects were compared to those of insulin. IGF-I infusion caused a twofold rise in IGF-I levels and a 75-85% decrease in plasma insulin. When IGF-I alone was given, plasma glucose fell by 30-40 mg/dl (P less than 0.005) due to a transient twofold increase (P less than 0.05) in glucose uptake; hepatic glucose production and plasma FFA levels remained unchanged. IGF-I infusion with maintenance of euglycemia produced a sustained rise in glucose uptake and a marked stimulation of [3-3H]glucose incorporation into tissue glycogen, but still failed to suppress glucose production and FFA levels. IGF-I also produced a generalized 30-40% reduction in plasma amino acids, regardless of whether or not hypoglycemia was prevented. This was associated with a decrease in leucine flux and a decline in the incorporation of [1-14C]leucine into muscle and liver protein (P less than 0.05). When insulin was infused in a dosage that mimicked the rise in glucose uptake seen with IGF-I, nearly identical changes in amino acid metabolism occurred. However, insulin suppressed glucose production by 65% and FFA levels by 40% (P less than 0.001). Furthermore, insulin was less effective than IGF-I in promoting glycogen synthesis. We conclude that (a) IGF-I produces hypoglycemia by selectively enhancing glucose uptake; (b) IGF-I is relatively ineffective in suppressing hepatic glucose production or FFA levels; and (c) IGF-I, like insulin, lowers circulating amino acids by reducing protein breakdown rather than by stimulating protein synthesis. Thus, IGF-Is metabolic actions in fasted rats are readily distinguished from insulin.

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

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

Growth Hormone Stimulates Skeletal Muscle Protein Synthesis and Antagonizes Insulin's Antiproteolytic Action in Humans

We examined the effects of a combined, local intra-arterial infusion of growth hormone (GH) and insulin on forearm glucose and protein metabolism in seven normal adults. GH was infused into the brachial artery for 6 h with a dose that, in a previous study, stimulated muscle protein synthesis (phenylalanine Rd) without affecting systemic GH, insulin, or insulinlike growth factor I concentrations. For the last 3 h of the GH infusion, insulin was coinfused with a dose that, in the absence of infused GH, suppressed forearm muscle proteolysis by 30–40% without affecting systemic insulin levels. Measurements of forearm glucose, amino acid balance, and [3H]phenylalanine and [14C]leucine kinetics were made at 3 and 6 h of the infusion. Glucose uptake by forearm tissues in response to GH and insulin did not change significantly between 3 and 6 h. By 6 h, the combined infusion of GH and insulin promoted a significantly more positive net balance of phenylalanine, leucine, isoleucine, and valine (all P < 0.05). The change in net phenylalanine balance was due to a significant increase in phenylalanine Rd (51%, P < 0.05) with no observable change in phenylalanine Ra. For leucine, a stimulation of leucine Rd (50%, P < 0.05) also accounted for the change in leucine net balance, with no suppression of leucine Ra. The stimulation of Rd, in the absence of an observed effect on Ra, suggests that GH blunts the action of insulin to suppress proteolysis in addition to blunting insulins action on Rd.

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.

Insulin Resistance in Diabetic Ketoacidosis

The effect of “low-dose” (6–10 U/h) insulin treatment on the rate of decline of plasma glucose concentration was determined in 15 diabetic subjects admitted in ketoacidosis (plasma glucose = 948 ± 79 mg/dl) and in six normal volunteers rendered hyperglycemic by a combined infusion of somatostatin and glucose (plasma glucose = 653 ± 28 mg/dl). The fractional glucose turnover and the half-time of the fall in plasma glucose during insulin treatment were both 10-fold reduced (P < 0.001) in the diabetics as compared with the controls. In the ketoacidotic subjects, the mean glucose clearance during insulin treatment was only 8% of that in the controls (P < 0.001). In the normal subjects, tissue glucose clearance during insulin treatment of the hyperglycemia (5.8 ± 0.7 ml/min · kg) was similar to that measured in the same subjects using a standard technique to quantitate insulin sensitivity (euglycemic insulin clamp). In the ketoacidotic patients, a history of prior insulin therapy, but not the degree of hyperglycemia at the time of admission, was associated with a more rapid rate of decline of plasma glucose in response to insulin treatment. We conclude that marked insulin resistance is present in virtually all diabetics in ketoacidosis.

Hepatic and extrahepatic splanchnic glucose metabolism in the postabsorptive and glucose fed dog

In awake dogs we measured the glucose balance across the liver and extrahepatic splanchnic tissues in the postabsorptive state and during two hours of IV infusion of glucose or for three hours following ingestion of oral glucose and during four hours of sequential intraportal followed by oral glucose. The IV glucose infusion rate was adjusted to maintain a steady state glucose concentration of either euglycemic levels (insulin clamp, group 1, N = 4), 125 mg/100 mL above the postabsorptive glucose concentration (+125 mg glucose clamp, group 2, N = 3) or 200 mg/100 mL above basal glucose levels (+200 mg glucose clamp, group 3, N = 7). Oral glucose was given at a dose of either 1.5 g/kg (group 4, N = 7) or 2.5 g/kg (group 5, N = 12). In dogs that received IV glucose, basal gut glucose uptake (0.5 +/- 0.1 mg/min X kg) was stimulated by hyperglycemia (1.5 +/- 0.5 and 1.4 +/- 0.1 mg/min X kg for group 2 and 3, respectively, P less than 0.05). In these same animals basal hepatic glucose output (-2.7 +/- 0.3 mg/min X kg) was promptly suppressed and net hepatic glucose uptake occurred (2.8 +/- 0.2 and 2.4 +/- 0.5 mg/min X kg in group 2 and 3 respectively). Euglycemic hyperinsulinemia (group 1) suppressed postabsorptive hepatic glucose release but did not enhance glucose removal by either the liver or gut tissues. After oral glucose gut tissues released absorbed glucose into portal blood. Over three hours following the glucose meal 74% and 59% of the ingested glucose was absorbed in group 4 and 5, respectively. As with IV glucose, postabsorptive hepatic glucose production was suppressed and over the first two hours after feeding the liver took up glucose (3.4 +/- 1.0 and 3.1 +/- 0.7 mg/min X kg groups 4 and 5, respectively) at a rate similar to that seen with IV glucose. To further examine the effect of the route of glucose administration on liver glucose handling, hepatic glucose balance was measured serially over four hours in three dogs that received IV glucose into a mesenteric vein to produce portal hyperglycemia (+125 mg/dL portal glucose clamp N = 3). Oral glucose (2.5 mg/kg) was given at two hours, and the rate of the mesenteric glucose infusion adjusted to maintain portal glycemia constant. The hepatic glucose balance averaged 5.5 mg/min X kg over the 0 to 2 hour period and 4.2 +/- 1.0 mg/min X kg over the 2 to 4 hour time.(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatic and peripheral insulin resistance following streptozotocin-induced insulin deficiency in the dog

Insulin resistance and insulin deficiency are both present in many patients with diabetes mellitus. We tested the hypothesis that insulin resistance can evolve from a primary lesion of the beta-cell secretory function. Insulin-mediated glucose uptake (insulin clamp), endogenous glucose production, and glucose-stimulated insulin secretion (hyperglycemic clamp) were measured in awake dogs before and four to six weeks after streptozotocin-induced diabetes mellitus. Streptozotocin (30 mg/kg) resulted in a significant rise in the mean fasting plasma glucose concentration from 104 +/- 2 mg/100 mL to 200 +/- 34 mg/100 mL, (P less than 0.05), and a slight decrease in the mean fasting plasma insulin concentration (from 21 +/- 2 microU/mL to 15 +/- 2 microU/mL). Under conditions of steady-state hyperglycemia (+75 mg/100 mL hyperglycemic clamp, insulin secretion was reduced by 75% in the streptozotocin-treated dogs (P less than 0.025), and the total amount of glucose metabolized decreased from 13.56 +/- 1.04 to 4.74 +/- 0.70 mg/min X kg (P less than 0.001). In the postabsorptive state, endogenous glucose production was slightly, although not significantly, higher in the diabetic dogs (3.05 +/- 0.46 v 2.51 +/- 0.22 mg/min . kg), while the glucose clearance rate was 35% lower (P less than 0.001). When the plasma insulin concentration was increased to approximately 45 microU/mL (insulin clamp) while holding plasma glucose constant at the respective fasting levels (99 +/- 1 and 186 +/- 30 mg/100 mL), endogenous glucose production was completely suppressed in control dogs but suppressed by only 51% (1.46 +/- 0.37 mg/min . kg, P less than 0.025) in diabetic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Regulation of myocardial amino acid balance in the conscious dog.

The effects in vivo of physiologic increases in insulin and amino acids on myocardial amino acid balance were evaluated in conscious dogs. Arterial and coronary sinus concentrations of amino acids and coronary blood flow were measured during a 30-min basal and a 100-min experimental period employing three protocols: euglycemic insulin clamp (plasma insulin equaled 70 +/- 11 microU/ml, n = 6); euglycemic insulin clamp during amino acid infusion (plasma insulin equaled 89 +/- 12 microU/ml, n = 6); and suppression of insulin with somatostatin during amino acid infusion (plasma insulin equaled 15 +/- 4 microU/ml, n = 6). Basally, only leucine and isoleucine were removed significantly by myocardium (net branched chain amino acid [BCAA] uptake equaled 0.5 +/- 0.2 mumol/min), while glycine, alanine, and glutamine were released. Glutamine demonstrated the highest net myocardial production (1.6 +/- 0.2 mumol/min). No net exchange was seen for valine, phenylalanine, tyrosine, cysteine, methionine, glutamate, asparagine, serine, threonine, taurine, and aspartate. In group I, hyperinsulinemia caused a decline of all plasma amino acids except alanine; alanine balance switched from release to an uptake of 0.6 +/- 0.4 mumol/min (P less than 0.05), while the myocardial balance of other amino acids was unchanged. In group II, amino acid concentrations rose, and were accompanied by a marked rise in myocardial BCAA uptake (0.4 +/- 0.1-2.6 +/- 0.3 mumol/min, P less than 0.001). Uptake of alanine was again stimulated (0.9 +/- 0.3 mumol/min, P less than 0.01), while glutamine production was unchanged (1.3 +/- 0.4 vs. 1.6 +/- 0.3 mumol/min). In group III, there was a 4-5-fold increase in the plasma concentration of the infused amino acids, accompanied by marked stimulation in uptake of only BCAA (6.8 +/- 0.7 mumol/min). Myocardial glutamine production was unchanged (1.9 +/- 0.4-1.3 +/- 0.7 mumol/min). Within the three experimental groups there were highly significant linear correlations between myocardial uptake and arterial concentration of leucine, isoleucine, valine, and total BCAA (r = 0.98, 0.98, 0.92, and 0.97, respectively); P less than 0.001 for each). In vivo, BCAA are the principal amino acids taken up by the myocardium basally and during amino acid infusion. Plasma BCAA concentration and not insulin determines the rate of myocardial BCAA uptake. Insulin stimulates myocardial alanine uptake. Neither insulin nor amino acid infusion alters myocardial glutamine release.