Hormonal control of ketogenesis. Rapid activation of hepatic ketogenic capacity in fed rats by anti-insulin serum and glucagon

Human ketone body production and utilization studied using tracer techniques: regulation by free fatty acids, insulin, catecholamines, and thyroid hormones

Ketone body concentrations fluctuate markedly during physiological and pathological conditions. Tracer techniques have been developed in recent years to study production, utilization, and the metabolic clearance rate of ketone bodies. This review describes data on the roles of insulin, catecholamines, and thyroid hormones in the regulation of ketone body kinetics. The data indicate that insulin lowers ketone body concentrations by three independent mechanisms: first, it inhibits lipolysis, and thus lowers free fatty acid availability for ketogenesis; second, it restrains ketone body production within the liver; third, it enhances peripheral ketone body utilization. To assess these effects in humans in vivo, experimental models were developed to study insulin effects with controlled concentrations of free fatty acids, insulin, glucagon, and ketone bodies. Presently available data also support an important role of catecholamines in increasing ketone body concentrations. Evidence was presented that norepinephrine increases ketogenesis not only by stimulating lipolysis, and thus releasing free fatty acids, but also by increasing intrahepatic ketogenesis. Thyroid hormone availability was associated with lipolysis and ketogenesis. Ketone body concentrations after an overnight fast were only modestly elevated in hyperthyroidism resulting from increased peripheral ketone body clearance. There was a significant correlation between serum triiodothyronine levels and the ketone body metabolic clearance rate. Thus, ketone body homeostasis in human subjects resulted from the interaction of hormones such as insulin, catecholamines, and thyroid hormones regulating lipolysis, intrahepatic ketogenesis, and peripheral ketone body utilization.

Synergistic inhibition of hepatic ketogenesis in the presence of insulin and a cAMP antagonist

The separate or combined effects of insulin and the cAMP antagonist, the Rp-diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), were examined on fatty acid-stimulated ketogenesis in hepatocytes from normal fasted rats. Addition of 0.4 mM oleic acid or 0.4 mM octanoic acid resulted in a linear increase in ketone production measured over 60 min. When oleic acid was the substrate, incubation with 1 to 30 microns Rp-cAMPS alone or 0.1 to 10 nM insulin alone caused a variable decrease in the production of ketones which did not exceed an average value of 30% in any one experiment. The simultaneous addition of Rp-cAMPS and insulin resulted in a greater than additive inhibition which reached average values between 47-60% when the theoretical combined inhibitory effect of the insulin alone plus the Rp-cAMPS alone was less than 18%. No significant effects of either insulin or Rp-cAMPS, alone or in combination, were seen when octanoic acid was the substrate. These data imply that Rp-cAMPS can potentiate insulin inhibition of hepatic ketogenesis through inhibition of a cAMP-mediated process.

Pyruvate dehydrogenase activities during the fed-to-starved transition and on re-feeding after acute or prolonged starvation

We investigated the temporal relationship between hepatic glycogen depletion and cardiac and hepatic PDH (pyruvate dehydrogenase complex) activities during the acute phase of starvation. There was a striking correlation between the decline in hepatic glycogen and PDH inactivation during the first 10 h of starvation. Re-feeding after 6 h starvation was associated with complete re-activation of PDH in liver and re-activation to approx. 75% of the fed value in heart, whereas in rats previously starved for 24-48 h re-activation was delayed in liver and diminished in heart. The results are discussed with reference to the fate of dietary carbohydrate after re-feeding.

Fatty acid-independent inhibition of hepatic ketone body production by insulin in humans

To investigate whether elevated plasma insulin or glucagon concentrations are capable of modifying hepatic ketogenesis independently of plasma free fatty acid (FFA) concentrations, ketone body production was determined by [3-14C]acetoacetate infusions in overnight-fasted normal subjects during exogenous supply of FFA (Intralipid and heparin infusion). When plasma FFA concentrations were elevated from 0.73 +/- 0.07 to 1.53 +/- 0.16 mmol/l during low insulin concentrations (approximately equal to 13 microU/ml) in group A (n = 7), total ketone body production increased from 3.6 +/- 0.6 to 8.2 +/- 1.0 mumol.kg-1.min-1 (P less than 0.001). When plasma FFA were similarly elevated during raised plasma insulin concentrations (approximately equal to 110 microU/ml) in group B (n = 5), total ketone body production was only 3.8 +/- 0.8 mumol.kg-1.min-1 (P less than 0.01 vs. group A). Low plasma FFA and low insulin concentrations resulted in total ketone body production of 0.70 +/- 0.18 mumol.kg-1.min-1 in group C (n = 7; P less than 0.01 vs. groups A and B). Elevation of plasma glucagon during Intralipid infusion in group D (n = 7) failed to affect ketogenesis, but the beta-hydroxybutyrate-to-acetoacetate concentration ratio decreased significantly (P less than 0.01). The data indicate that elevation of plasma insulin to high physiological concentrations restrains FFA-induced ketogenesis.

Whole body lipid and energy metabolism in the cancer patient

The relationship between whole-body energy and lipid kinetics in eight cancer patients was investigated after an overnight fast. Respiratory gas exchange and indirect calorimetry were used to obtain resting energy expenditure (REE) and net substrate oxidation rates. Free fatty acid (FFA) turnover, oxidation, and clearance rates were obtained after a primed-constant infusion of albumin bound 1-14C-Na palmitate. This was followed by a primed-constant, two-stage infusion of unlabeled glycerol to measure plasma glycerol turnover and clearance. The REE was 1.3 times the predicted (by the Harris-Benedict equation) basal energy expenditure. FFA and glycerol, plasma concentrations, and turnover rates were higher in these depleted but hypermetabolic cancer patients, compared to reported values for healthy normals. The ratio of FFA turnover to glycerol turnover was 3.14 +/- 0.38, which is close to the theoretical value of 3, suggesting complete hydrolysis of triglycerides and the absence of any extensive reesterification of FFA in adipose tissue. The net fat oxidation accounts for 53 +/- 5% of fat mobilized and 29 +/- 3% of the FFA turnover was converted to CO2 in the process of supplying energy in cancer patients. The results suggest that fat is efficiently mobilized and utilized as a fuel source in hypermetabolic cancer patients in the postabsorptive state.

Studies of gut and hepatic metabolism in conscious rabbits

The present study was designed to develop the techniques for chronic catheterization of the hepatic and portal venous circulation in conscious rabbits and to apply these techniques to a study of hepatic metabolism in this species. Experiments were made after an 18-h fast and for 4 h after the initial feeding. Measurements of arteriovenous differences of substrates were combined with measurements of hepatic and gastrointestinal blood flow. Hepatic glucose production was suppressed by 60% at 1 h and had returned to control levels by 4 h. The hepatic uptake of lactate declined slightly at 1 h and had returned to control level 2 h after the meal. There was a marked and rapid fall in hepatic ketone body output after refeeding. Although amino acid concentrations displayed a transient increase 1 h after the meal, only the arterial concentration of branched-chain amino acids remained significantly elevated for 4 h. The total hepatic uptake of the gluconeogenic amino acids (alanine, serine, threonine) remained constant. Refeeding resulted in a doubling of arterial insulin concentrations at 1 h followed by a progressive decline over the next 3 h. It is concluded that in rabbits fed a mixed meal partial suppression of hepatic glucose output is mainly due to a decline in glycogenolysis rather than a decrease in gluconeogenesis, shortly after refeeding the liver is able to virtually shut off its ketone body production, the major gluconeogenic precursors (lactate, alanine, glycine, serine, and threonine) may contribute to approximately 40% of the glucose release.(ABSTRACT TRUNCATED AT 250 WORDS)

Very low density lipoprotein metabolism in diabetes mellitus

The concentration of VLDL and their major lipid, triglyceride, are regulated at many levels from the initial availability of the substrates needed for their synthesis all the way to the function of the enzymes and receptors involved in their removal from plasma. It should be clear from this review that in diabetes mellitus metabolic derangements resulting from the absolute lack of insulin or from resistance to the actions of insulin can affect VLDL triglyceride metabolism at any or all of these regulatory points. The outcome of this interplay between diabetes and VLDL metabolism is the common occurrence of elevated plasma VLDL and triglyceride concentrations in individuals with both Type 1 and Type 2 diabetes mellitus. Mildly elevated plasma levels of triglycerides are nearly universal in diabetics; more significant hypertriglyceridemia can be the consequence of either metabolic decompensation or the concomitant inheritance of a familial pattern of hyperlipoproteinemia. The combination of the latter two situations usually presents with as severe hypertriglyceridemia. Although deregulation can occur at many points, the most common abnormality associated with hypertriglyceridemia in human diabetes appears to be overproduction of VLDL triglycerides. Increased rates of synthesis of VLDL apoB may also be a common consequence of diabetes. The basis for this belief is the accumulated data from kinetic studies in humans and in experimental models of diabetes in rats. Although the latter may also demonstrate defects in catabolism when insulin deficiency is severe, catabolic abnormalities appear to be uncommon as the primary force in the development of hypertriglyceridemia in humans. Finally, despite the complexity of the systems regulating VLDL metabolism and the many metabolic abnormalities that may be present in diabetic subjects, it appears that reduction of the hyperglycemia by means of dietary or pharmacologic interventions is associated with normalization of the rates of synthesis and catabolism of the VLDL and their triglycerides. In view of the probable atherogenecity of VLDL, particularly in individuals with diabetes, such intervention must be aimed at both plasma glucose and lipid concentrations.

Hyperglucagonemia and insulin-mediated glucose metabolism

The effect of chronic physiologic hyperglucagonemia on basal and insulin-mediated glucose metabolism was evaluated in normal subjects, using the euglycemic insulin clamp technique (+50, +100, and +500 microU/ml). After glucagon infusion fasting glucose increased from 76 +/- 4 to 93 +/- 2 mg/dl and hepatic glucose production (HGP) rose from 1.96 +/- 0.08 to 2.25 +/- 0.08 mg/kg X min (P less than 0.001). Basal glucose oxidation after glucagon increased (P less than 0.05) and correlated inversely with decreased free fatty acid concentrations (r = -0.94; P less than 0.01) and decreased lipid oxidation (r = -0.75; P less than 0.01). Suppression of HGP and stimulation of total glucose disposal were impaired at each insulin step after glucagon (P less than 0.05-0.01). The reduction in insulin-mediated glucose uptake was entirely due to diminished non-oxidative glucose utilization. Glucagon infusion also caused a decrease in basal lipid oxidation and an enhanced ability of insulin to inhibit lipid oxidation and augment lipid synthesis. These results suggest that hyperglucagonemia may contribute to the disturbances in glucose and lipid metabolism in some diabetic patients.

Plasma clearances of branched-chain amino acids in control subjects and in patients with cirrhosis

In an attempt to clarify the pathogenesis of the decreased branched-chain amino acid (BCAA) plasma concentrations in cirrhosis, the plasma clearances were measured in 7 patients with cirrhosis and in 7 age- and sex-matched control subjects. BCAA were given as prime-continuous infusions. The plasma clearances of valine, isoleucine, and leucine, calculated as infusion rate divided by steady state concentration, were low normal in cirrhotics despite hyperinsulinaemia, but different BCAA had different clearances (P less than 0.01). The endogenous basal appearance rates of BCAA, estimated by the basal concentrations multiplied by the plasma clearances, were lower in cirrhotics (P less than 0.025). The apparent theoretical volumes of distribution of BCAA, assessed by the ratio between the clearance and the concentration decay constant after infusion stop, were on average 67% of the total body weight, and were neither different among the three BCAA, nor between the two groups. The urea nitrogen synthesis rate did not increase significantly, suggesting that most of the infused BCAA nitrogen was taken up in peripheral tissues. The decreased concentration of BCAA in cirrhotics (394 +/- 81 mumol/l (mean +/- SD) in the present series vs 510 +/- 68 in controls; P less than 0.025) is not attributable to changes in plasma clearance. The most likely explanation is decreased afflux of BCAA into plasma.

Differences in insulin sensitivity between normal men and women

An incremental intravenous low-dose insulin infusion has been used to examine differences in insulin sensitivity between normal young men and women. Fasting blood glucose concentration did not differ significantly at the start of the infusion but women had significantly higher plasma insulin and C-peptide concentrations. Similar changes in blood glucose occurred during insulin infusion but insulin concentrations were higher in women. Blood total ketone bodies and alanine were lower in women over the four hours of infusion. Significant differences were found between normal men and women for the effect of insulin upon blood glucose concentration.

Succinylation and inactivation of 3-hydroxy-3-methylglutaryl-CoA synthase by succinyl-CoA and its possible relevance to the control of ketogenesis

Succinyl-CoA (3-carboxypropionyl-CoA) inactivates ox liver mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (EC 4.1.3.5) in a time-dependent manner, which is partially prevented by the presence of substrates of the enzyme. The inactivation is due to the enzyme catalysing its own succinylation. Complete inactivation corresponds to about 0.5 mol of succinyl group bound/mol of enzyme dimer. The succinyl-enzyme linkage appears to be a thioester bond and is probably formed with the active-site cysteine residue that is normally acetylated by acetyl-CoA. Succinyl-CoA binds to 3-hydroxy-3-methylglutaryl-CoA synthase with a binding constant of 340 microM and succinylation occurs with a rate constant of 0.57 min-1. Succinyl-enzyme breaks down with a half-life of about 40 min (k = 0.017 min-1) at 30 degrees C and pH 7 and is destabilized by the presence of acetyl-CoA and succinyl-CoA. A control mechanism is postulated in which flux through the 3-hydroxy-3-methylglutaryl-CoA cycle of ketogenesis is regulated according to the extent of succinylation of 3-hydroxy-3-methylglutaryl-CoA synthase.

Role of basal glucagon levels in the regulation of splanchnic glucose output and ketogenesis in insulin-deficient humans

The aim of the present study was to investigate the influence of hepatic glycogen depletion and increased lipolysis on the response of splanchnic glucose output and ketogenesis to combined glucagon and insulin deficiency in normal man. Healthy subjects were studied after a 60-h fast and compared with a control group studied after an overnight fast. Net splanchnic exchange of glucose, gluconeogenic precursors, free fatty acids (FFA) and ketone acids were measured in the basal state and during intravenous infusion of somatostatin (9 micrograms/min) for 90-140 min (overnight fasted subjects) or for 5 h (60-h fasted subjects). During the infusion of somatostatin, euglycemia was maintained by a variable intravenous infusion of glucose. Prior to somatostatin infusion, after an overnight (12-14 h) fast, splanchnic uptake of glucose precursors (alanine, lactate, pyruvate, glycerol) could account for 26% of splanchnic glucose output (SGO) indicating primarily glycogenolysis. Somatostatin infusion resulted in a 50% reduction in both insulin and glucagon concentrations and a transient decline in SGO which returned to baseline values by 86 +/- 11 min at which point the glucose infusion was no longer necessary to maintain euglycemia. Arterial concentrations of FFA and beta-OH-butyrate and splanchnic beta-OH-butyrate production rose 2.5-fold, 6-fold and 7.5-fold, respectively, in response to somatostatin infusion. In the 60-h fasted state, basal SGO (0.29 +/- 0.03 mmol/min) was 60% lower than after an overnight fast and basal splanchnic uptake of glucose precursors could account for 85% of SGO, indicating primarily gluconeogenesis. Somatostatin administration suppressed the arterial glucagon and insulin concentrations to values comparable to those observed during the infusion in the overnight fasted state. SGO fell promptly in response to the somatostatin infusion and in contrast to the overnight fasted state, remained inhibited by 50-100% for 5 h. Infusion of glucose was consequently necessary to maintain euglycemia throughout the 5-h infusion of somatostatin. Splanchnic uptake of gluconeogenic precursors was unchanged during somatostatin despite the sustained suppression of SGO. Basal arterial concentration and splanchnic exchange of beta-OH-butyrate were respectively 22-fold and 6- to 7-fold elevated and basal FFA concentration was 70% increased as compared to the corresponding values in the overnight fasted state.(ABSTRACT TRUNCATED AT 400 WORDS)

Pancreatic A cell response to arginine in the hibernating hedgehog (Erinaceus europaeus)

Pancreatic A cell response to arginine was measured in hedgehogs during the periods of lethargy and arousal and then during activity. Spontaneous plasma glucagon concentrations were lower during lethargy than during activity, and they increased during arousal. Arginine administration induced a slight, but significant delayed increase in plasma glucagon concentration in the lethargic hedgehog (body temperature: 6 degrees). During arousal, in vitro glucagon secretion was temperature dependent suggesting that body rewarming might, in itself, be an important stimulating factor of the A cells. In the presence of arginine, the glucagon output of the pancreas of lethargic hedgehogs was high at low temperatures. It decreased to a nadir at 19 degrees and increased up to 37 degrees. However, the basal or arginine-stimulated glucagon secretion of animals in lethargy was higher than that of animals in activity. These characteristics suggested the presence of a particular pool of cold-adapted enzymes in the A cells of lethargic hedgehogs.

The control of fatty acid metabolism in liver cells from fed and starved sheep

Isolated liver cells prepared from starved sheep converted palmitate into ketone bodies at twice the rate seen with cells from fed animals. Carnitine stimulated palmitate oxidation only in liver cells from fed sheep, and completely abolished the difference between fed and starved animals in palmitate oxidation. The rates of palmitate oxidation to CO2 and of octanoate oxidation to ketone bodies and CO2 were not affected by starvation or carnitine. Neither starvation nor carnitine altered the ratio of 3-hydroxybutyrate to acetoacetate or the rate of esterification of [1-14C]palmitate. Propionate, lactate, pyruvate and fructose inhibited ketogenesis from palmitate in cells from fed sheep. Starvation or the addition of carnitine decreased the antiketogenic effectiveness of gluconeogenic precursors. Propionate was the most potent inhibitor of ketogenesis, 0.8 mM producing 50% inhibition. Propionate, lactate, fructose and glycerol increased palmitate esterification under all conditions examined. Lactate, pyruvate and fructose stimulated oxidation of palmitate and octanoate to CO2. Starvation and the addition of gluconeogenic precursors stimulated apparent palmitate utilization by cells. Propionate, lactate and pyruvate decreased cellular long-chain acylcarnitine concentrations. Propionate decreased cell contents of CoA and acyl-CoA. It is suggested that propionate may control hepatic ketogenesis by acting at some point in the beta-oxidation sequence. The results are discussed in relation to the differences in the regulation of hepatic fatty acid metabolism between sheep and rats.

Glucose and ketone body kinetics in diabetic ketoacidosis

The hyperglycaemia and hyperketonaemia of diabetic ketoacidosis are initiated primarily by overproduction of these substrates; subsequent maintenance of hyperglycaemia occurs, in large part, due to impaired utilization of glucose, whereas overproduction of ketone bodies continues to be the major mechanism for maintenance of hyperketonaemia. Insulin deficiency results in increased rates of lipolysis and provides increased substrate (free fatty acids) for ketogenesis. Hyperglucagonaemia can augment ketogenesis further in the setting of insulin deficiency. It is likely that other counter-insulin hormones (growth hormone, catecholamines) also contribute to the pathogenesis of DKA, though their role is less well defined. Insulin corrects DKA largely via suppression of lipolysis (and thus ketone body production); insulin suppresses glucose production at lower levels than it does ketone body production.

Oleate metabolism in isolated hepatocytes from lean and obese Zucker rats. Influence of a high fat diet and in vitro response to glucagon

The uptake and metabolism of [1-14C]oleate (0.3 mmol/L) were studied in isolated hepatocytes from lean and obese Zucker rats fed either a control (low-fat) diet or a high-fat diet. With the control diet, [1-14C]oleate uptake was increased by 70% in the obese rats, and fat-feeding decreased this uptake to values comparable to that of their lean littermates. Interestingly, the hepatocyte mean surface area was increased in the obese mutants by 21% with the control diet and by 30% with the high-fat diet. The possible reasons for the differences in oleate uptake are discussed. With the control diet, cells from the obese rats showed a four-fold rise in [1-14C]oleate esterification, while ketogenesis (beta-hydroxybutyrate + acetoacetate production) as well as the radioactive acid-soluble products were greatly depressed. Production of CO2 was very low and similar in both groups of animals. Adaptation to the high-fat diet in the obese rats resulted in a reversal between esterification and oxidation of oleate: the latter became the major metabolic pathway as in the lean rats. The ketogenic capacity was greatly if not completely restored. In the lean animals, glucagon stimulated ketogenesis both in the presence or absence of oleate and decreased [1-14C]oleate esterification. In the obese rats, the hormone exerted a significant ketogenic effect only if oleate was present and did not influence its esterification. The data demonstrate the following abnormalities in the hepatocytes of obese Zucker rats: (1) an enlargement of cell size, (2) an increased oleate uptake, (3) a virtual absence of a ketogenic response to exogenous oleate, and (4) a markedly increased esterification of the latter. The metabolic defects, but not the cell size, appear to be largely corrected by an adaptation to a high-fat diet. The hepatic response to glucagon was decreased in the obese rats at the level of endogenous ketogenesis.

Effects of free fatty acid availability, glucagon excess, and insulin deficiency on ketone body production in postabsorptive man

The present studies were undertaken to assess the relative effects of free fatty acid (FFA) availability, glucagon excess, and insulin deficiency on ketone body (KB) production in man. To determine whether an increase in FFA availability would augment KB production in the absence of insulin deficiency and glucagon excess, plasma insulin and glucagon were maintained at basal concentrations by infusion of somatostatin and exogenous insulin and glucagon, and plasma FFA were increased from 0.32 +/- 0.06 to 1.4 +/- 0.1 mM by a 2.5-h-infusion of a triglyceride emulsion plus heparin. KB production increased fivefold from 2.2 +/- 0.4 to 11.4 +/- 1.2 mumol . kg-1 . min-1, P less than 0.001. To determine whether insulin deficiency would further augment KB production, analogous experiments were performed but the replacement infusion of insulin was stopped. Despite a greater increase in plasma FFA (from 0.26 +/- 0.04 to 1.95 +/- 0.3 mM), KB production increased (from 1.5 +/- 0.3 to 11.1 +/- 1.8 mumol . kg-1 . min-1) to the same extent as in the absence of insulin deficiency. To determine whether hyperglucagonemia would augment KB production beyond that accompanying an increase in plasma FFA and, if so, whether this required insulin deficiency, similar experiments were performed in which the glucagon infusion rate was increased to produce plasma glucagon concentrations of 450-550 pg/ml with and without maintenance of the basal insulin infusion. When basal plasma insulin concentrations were maintained, hyperglucagonemia did not further increase KB production; however, when the basal insulin infusion was discontinued, hyperglucagonemia increased KB production significantly, whereas no change was observed in saline control experiments. These studies indicate that, in man, FFA availability is a major determinant of rates of KB production; insulin does not appear to influence ketogenesis rates by a direct hepatic effect, and glucagon can further augment KB production when FFA concentrations are increased but only in the setting of insulin deficiency.

Effect of somatostatin-induced suppression of postprandial insulin response upon the hypertriglyceridemia associated with a high carbohydrate diet

In an attempt to define the relationship between plasma insulin and triglyceride concentrations, we have studied the effect of suppression of the postprandial insulin response upon the secretion and plasma concentration of very low density lipoprotein (VLDL)-triglycerides. Eight nondiabetic subjects with a wide range of fasting plasma triglyceride levels (100-358 mg/dl) were studied during three dietary periods: base line, high carbohydrate (80% calories), and high carbohydrate (80% calories) with a daily intravenous infusion of somatostatin (SRIF) (1.3 micrograms/min) between 800 and 2,100 h. The significant increase in postprandial insulin response observed during high carbohydrate vs. base line was completely abolished during high carbohydrate-SRIF. However, plasma triglyceride levels rose in all subjects during each high carbohydrate period (with/without SRIF) vs. base line and the mean values reached during each period were the same (476 +/- 165 vs. 482 +/- 152 mg/dl, respectively). The secretion of VLDL-triglyceride into plasma was higher in four subjects, the same in two subjects, and lower in one subject during high carbohydrate-SRIF vs. high carbohydrate alone. The mean production rate of VLDL-triglyceride (mg/kg per h) was 25.6 +/- 4.9 during the high carbohydrate and 40.9 +/- 28.1 during the high carbohydrate-SRIF periods. These values were not significantly different. Postprandial glucose levels were slightly increased during high carbohydrate-SRIF, but overnight glucose concentrations were not affected. Plasma FFA levels were not different during the two high carbohydrate periods. Plasma glucagon levels did not appear to affect the results either. This study indicates that postprandial hyperinsulinemia during a high carbohydrate diet is not necessary for induction of hypertriglyceridemia.

Very low density lipoprotein metabolism in non-ketotic diabetes mellitus: effect of dietary restriction

We have measured the turnover of very low density lipoprotein (VLDL) triglyceride as well as plasma glucose, insulin and non-esterified fatty acid levels in nine mildly obese non-ketotic, insulinopenic diabetic subjects before and during an energy restricted diet. During the baseline period, subjects were hypertriglyceridaemic, hyperglycaemic and insulinopenic. During dietary restriction (mean weight loss: 2.3 +/- 0.4 kg) plasma triglyceride fell from 8.4 +/- 3.0 to 3.4 +/- 0.89 mmol/l (mean +/- SEM: p less than 0.05), and plasma glucose fell from 13.9 +/- 1.7 to 9.8 +/- 1.4 mmol/l (p less than 0.01). Neither fasting plasma insulin nor the insulin response to an oral glucose load changed. Plasma non-esterified fatty acid concentrations remained constant as well. During the baseline period, the transport rate of VLDL-triglyceride in the diabetic subjects was more than twice that in an age-weighted matched control group (27.4 +/- 2.9 versus 12.1 +/- 0.8 mg/kg ideal body weight per h). The fractional catabolic rates were similar in the two groups (0.20 +/- 0.05 versus 0.21 +/- 0.02/h). During energy restriction of the diabetic subjects, the VLDL-triglyceride transport rate fell to 17.4 +/- 2.9 mg/kg ideal body weight per h (p less than 0.05 versus baseline) while the fractional catabolic rate remained constant at 0.21 +/- 0.06/h (NS versus baseline). These data indicate that the major abnormality in triglyceride metabolism in these non-ketotic, insulinopenic diabetic patients was over-production of VLDL-triglyceride.

Failure of glucagon to stimulate ketone body production during acute insulin deficiency or insulin replacement in man

To assess the role of glucagon and insulin in the acute regulation of ketone body kinetics in man, somatostatin was administered with various combinations of these hormones by replacement infusions in groups of six to seven normal subjects. Somatostatin-induced insulin and glucagon deficiency produced a threefold increase in total ketone body concentrations within 2 h. This increase was the combined result of enhanced production (71%), and decreased metabolic clearance (32%), as determined by 14C-acetoacetate infusions. An associated elevation of non-esterified fatty acids (66%) and glycerol levels occurred. Glucagon replacement (2 ng . kg-1 . min-1) during insulin deficiency failed to enhance ketogenesis or lipolysis but lowered the beta-hydroxybutyrate/acetoacetate concentration ratios. Hyperglycaemia, observed during glucagon administration and insulin deficiency, did not diminish ketone body production or lipolysis. In contrast, insulin replacement (150 microunits . kg-1 . min-1) diminished lipolysis, lowered ketone production, and elevated the metabolic clearance rate of ketone bodies. Glucagon infusions (2 and 4 ng . kg-1 . min-1) during somatostatin and insulin replacement did not accelerate ketone body production or raise non-esterified fatty acid levels, but produced a dose-dependent elevation of blood glucose levels. The results suggest that glucagon is not an important ketogenic hormone under the conditions studied.

In vitro reversal of the fasting state of liver metabolism in the rat. Reevaluation of the roles of insulin and glucose

Studies were conducted to determine whether the direction of hepatic carbohydrate and lipid metabolism in the rat could be switched simultaneously from a "fasted" to a "fed" profile in vitro. When incubated for 2 h under appropriate conditions hepatocytes from fasted animals could be induced to synthesize glycogen at in vivo rates. There was concomitant marked elevation of the tissue malonyl-coenzyme A level, acceleration of fatty acid synthesis, and suppression of fatty acid oxidation and ketogenesis. In agreement with reports from some laboratories, but contrary to popular belief, glucose was not taken up efficiently by the cells and was thus a poor substrate for eigher glycogen synthesis or lipogenesis. The best precursor for glycogen formation was fructose, whereas lactate (pyruvate) was most efficient in lipogenesis. In both case the addition of glucose to the gluconeogenic substrates was stimulatory, the highest rates being obtained with the further inclusion of glutamine. Insulin was neither necessary for, nor did it stimulate, glycogen deposition or fatty acid synthesis under favorable substrate conditions. Glucagon at physiological concentrations inhibited both glycogen formation and fatty acid synthesis. Insulin readily reversed the effects of glucagon in the submaximal range of its concentration curve. The following conclusions were drawn. First, the fasted-to-fed transition of hepatic carbohydrate and lipid metabolism can be accomplished in vitro over a time frame similar to that operative in vivo. Second, reversal appears to be a substrate-driven phenomenon, in that insulin is not required. Third, unless an unidentified factor (present in protal blood during feeding) facilitates the uptake of glucose by liver it seems unlikely that glucose is the immediate precursor for liver glycogen or fat synthesis in vivo. A likely candidate for the primary substrate in both processes is lactate, which is rapidly formed from glucose by the small intestine and peripheral tissues. Fructose and amino acids may also contribute. Fourth, the requirement for insulin in the reversal of the fasting state of liver metabolism in vivo can best be explained by its ability to offset the catabolic actions of glucagon.

Effects of mild hyperinsulinemia on the metabolic response to exercise

To assess the effects of mild hyperinsulinemia on the metabolic adaptations to exercise, five normal male subjects were studied during 2 cycles of 30 min rest followed by 60 min mild exercise. During one cycle, insulin was infused at a rate of 0.33 mU/kg/min. During both cycles, plasma glucose concentration was kept constant by a glucose-controlled glucose infusion system. Studies with and without insulin were performed in random order, with 30 min between studies. During the insulin infusion, plasma non-esterified fatty acids fell during rest and failed to rise with exercise, indicating a limited availability of this substrate to working muscle. Insulin infusion also inhibited the expected rise in glycerol and 3-hydroxybutyrate normally observed during exercise. Plasma lactate concentrations at the completion of exercise with insulin infusion were higher than after exercise without insulin infusion. Greater metabolic dependence on carbohydrate metabolism is suggested by an increased respiratory quotient during insulin infusion. Insulin infusion had no significant effect on the amount of glucose which needed to be infused for maintaining a constant plasma glucose during rest, but there was a large and significant difference in the need for infused glucose during exercise with and without insulin infusion. The results indicate that even mild hyperinsulinism interferes with normal metabolic responses to exercise, and suggest that fall in insulin concentration seen with exercise is an important regulatory process, not merely a secondary consequences of a declining plasma glucose level.

Acute hormonal effects on carnitine metabolism in thin and obese subjects: responses to somatostatin, glucagon, and insulin

Plasma free carnitine and acylcarnitines were determined in man during acutely induced insulin deficiency. A 5-hr infusion of somatostatin at 6 microgram/min in 10 thin subjects produced profound, sustained hypoinsulinemia and led to rapid increases in plasma free fatty acids and ketoacids (peak increments of 0.67 mM each). Simultaneously, plasma free carnitine decreased, while plasma long-chain and short-chain acylcarnitines increased significantly. When hyperglucagonemia was created by inclusion of glucagon with the somatostatin, the hyperketonemia was reversed after 2 hr and the increase in acylcarnitine abolished. However, the decrease in free carnitine was accentuated. The antiketogenic effect of adding glucagon was due to an eventual breakthrough of the somatostatin blockade on insulin secretion, the latter gradually returning toward preinfusion levels. Inclusion of exogenous insulin with the somatostatin-glucagon infusion immediately lowered free fatty acids and ketoacids. Acylcarnitines also declined promptly, while the accelerated fall in free carnitine produced by glucagon was blunted by the addition of insulin. Qualitatively and quantitatively comparable results were seen in seven obese subjects. This study suggests: (1) the increase in plasma acylcarnitines previously described in fasting and diabetic ketosis is largely due to insulin deficiency; (2) the corresponding decrease in plasma free carnitine is attributable both to insulin deficiency and glucagon excess; and (3) the resistance of obese subjects to ketosis is unlikely to be due to deficits in carnitine or carnitine acyltransferases.

Endogenous hyperglycemia restores insulin release impaired by somatostatin analogue

These studies assessed the ability of des-Asn5-[D-Trp8-D-Ser13]-somatostatin (d-ATS-SS) to selectively inhibit insulin release and produce a hyperglycemia sufficient to compensate for the original impairment. d-ATS-SS at 0.017 micrograms/min inhibited basal insulin output (delta = -38 +/- 6%, P less than 0.005) and increased basal pancreatic glucagon output (delta - +21 +/- 6%, P less than 0.05, n = 5). d-ATS-SS at 0.17 micrograms/min markedly inhibited insulin output (delta = -84 +/- 4%, P less than 0.0005) and slightly inhibited glucagon output (delta = -14 +/- 6%, P less than 0.05, n = 5). d-ATS-SS at 0.055 micrograms/min decreased basal and stimulated insulin release but not basal nor stimulated glucagon release. By 3.5 of analogue infusion, plasma glucose had risen by 116 +/- 13 mg/dl, and base-line insulin levels and the insulin responses to both isoproterenol and arginine, but not glucose, increased toward control values. We conclude that d-ATS-SS produces selective insulinopenia resulting in hyperglycemia which in turn compensates for the original impairment. Thus, the hyperglycemia observed in other states of selective insulin deficiency (e.g., noninsulin-dependent diabetes mellitus) may compensate for defects in beta-cell function.

The effect of glucagon treatment and starvation of virgin and lactating rats on the rates of oxidation of octanoyl-L-carnitine and octanoate by isolated liver mitochondria

1. Oxygen-consumption rates owing to oxidation of octanoate or octanoylcarnitine by isolated mitochondria from livers of fed, starved and glucagon-treated virgin or 12-day-lactating animals were measured under State-3 and State-4 conditions, in the presence or absence of l-malate and inhibitors of tricarboxylic acid-cycle activity (malonate and fluorocitrate). 2. Mitochondria from fed lactating animals had a slightly lower rate of octanoylcarnitine oxidation than did those of fed virgin animals, whereas the rates of octanoate oxidation were unaffected. 3. Starvation of virgin animals for 24h or 48h resulted in a large (70-100%) increase in mitochondrial octanoylcarnitine oxidation; rates of octanoate oxidation were either unaffected (24 and 48h starvation in the absence of malonate and fluorocitrate) or diminished by 30% (48h starvation in the presence of inhibitors). In lactating animals, 24h starvation resulted in a smaller increase in the rate of octanoylcarnitine oxidation than that obtained for mitochondria from virgin rats. 4. Glucagon treatment (by intra-abdominal injection) of fed virgin and lactating rats increased the rate of mitochondrial oxidation of both octanoylcarnitine and octanoate. Injection of glucagon into 48h-starved virgin rats did not increase further the already elevated rate of octanoylcarnitine oxidation, but reversed the inhibition of octanoate beta-oxidation observed for these mitochondria in the presence of malonate and fluorocitrate. 5. It is suggested that glucagon activates octanoylcarnitine oxidation by increasing the activity of the carnitine/acylcarnitine transport system [Parvin & Pande (1979) J. Biol. Chem.254, 5423-5429] and that the increase in octanoate oxidation by mitochondria from glucagon-treated animals is caused by the increased rate of ATP synthesis in these mitochondria. 6. The results are discussed in relation to the increased capacity of the liver to oxidize long-chain fatty acids and carnitine esters of medium-chain fatty acids under conditions characterized by increased ketogenesis.

Blood plasma magnesium, potassium, glucose, and immunoreactive insulin changes in cows moved abruptly from barn feeding to early spring pasture

Cations and immunoreactive insulin in plasma were measured in 35 lactating cows moved abruptly to early spring pasture. After change of cows from grass-clover hay to fescue-bluegrass pasture containing 22 to 31 g potassium/kg dry matter, immunoreactive insulin of 5 Holstein cows increased 30% in 5 days and averaged 45% above prepasture concentrations for 40 days. Magnesium averaged 44% below prepasture content of plasma during this period and was correlated negatively with potassium -.17 and immunoreactive insulin -.37. Thirty Herford cows were changed from corn silage and grass-clover hay to wheat-rye pasture containing 3.06% potassium in the dry matter. Each day on pasture, 10 cows each were fed 2.3 kg cornmeal, 10 were given 30 g magnesium oxide by capsule, and 10 were given no supplement. After unsupplemented cows were moved to pasture, immunoreactive insulin rose 51% in 8 days and plasma magnesium fell 24%. Both supplements reduced immunoreactive insulin, but magnesium was maintained higher by magnesium oxide than by cornmeal. Injection of two Holstein cows with insulin (2 IU/kg body weight) reduced plasma concentrations of both potassium and magnesium 20% below that of two cows injected with only physiological saline. Whether elevated plasma insulin may accelerate development of hypomagnesemia in cattle on spring pasture with relatively high potassium content has not been established.

Hormonal and metabolic characteristics of genetically obese Zucker and dietary obese Sprague-Dawley rats

The endocrine-metabolic plasma pattern and the capacity of isolated perfused livers to produce triglycerides and ketone bodies have been studied in genetically and diet-acquired obese rats (Zucker and Sprague-Dawley obese rats), and in control groups of the same strains. An increased plasma insulin/glucagon molar ratio with hyperinsulinaemia and hypoglucagonaemia was associated with hypertriglyceridaemia, normal ketonaemia, elevated free fatty acids and normal or slight hyperglycaemia in obese rats. During oleate perfusion, the livers of Zucker and Sprague-Dawley obese rats showed an increase in triglyceride output and liver triglyceride content. The ketone body output as well as the mitocondrial carnitine palmitoyl transferase activity were normal or slightly decreased. In our rat population, a positive correlation between the insulin/glucagon molar ratio and triglyceride output has been found.

The effect of diazoxide-induced hormonal secretion on plasma triglyceride concentration in the rat

The relationship between non-esterified fatty acid (NEFA) mobilization and hepatic conversion to plasma triglycerides (TG), as modulated by diazoxide-induced effects upon endogenous catecholamine, glucagon, and insulin secretion, was examined in vivo in the rat. Thyrotropin (TSH)-induced NEFA mobilization provided the control study.--In all control experiments, TSH (1.5 IU/100 g) induced a 110% increase in NEFA availability, which was associated with a subsequent 52% increase in plasma TG concentration and a 73% increase in plasma ketone bodies. Following diazoxide administration (30 mg/kg), endogenous secretion of both catecholamines and glucagon was observed, resulting in a comparable 100% increase in NEFA availability, with the appropriate ketonaemic response. However, in contrast to the control TSH study, plasma triglyceride concentration did not increase. This suppression was secondary, at least in part, to a direct 40% inhibition of hepatic secretion of triglycerides.--Although plasma NEFA concentration is an important determinant of plasma triglyceride levels, the concurrent concentration of endogenous catecholamines, glucagon, and insulin modulate the hepatic conversion of NEFA to triglycerides in vivo.

The ketotic effects of glucocorticoid and growth hormone in man

The ketotic effects of both glucocorticoid and growth hormone were assessed in normal man. Experimental protocols, previously shown to induce marked ketosis in diabetic man, were utilized to explore the metabolic effects of these two stress hormones in subjects with normal insulin secretory capacity. Glucocorticoid was administered orally as 1 mg of dexamethasone at 24 and 8 h prior to study. Growth hormone was administered subcutaneously at a dosage of 1 mg, 12 h prior to study. During the 90-min study of the ketotic activity of these hormones, plasma nonesterified fatty acids were acutely increased by heparin administration to support hepatic ketogenesis. This technique permitted an assessment of the ketotic activity of glucocorticoid and growth hormone independent of their lipolytic activity. The results of this study demonstrate that glucocorticoid may cause minimal hyperketonemia in spite of hyperinsulinemia in normal man. However, this effect is accompanied by a glucocorticoid-induced instability in basal ketone body and nonesterified fatty acid concentration. In contrast, no effect of growth hormone on plasma ketone body concentration or insulin levels was observed. These results in normal man contrast to the marked ketosis previously induced by these two stress hormones in diabetic man.

The role of glucagon in the regulation of plasma lipids

The role of glucagon in regulating plasma lipid concentrations (nonesterified fatty acids, ketone bodies, and triglycerides) is reviewed. The effects of glucagon-induced insulin secretion upon this lipid regulation are discussed that may resolve conflicting reports in the literature are resolved. In addition, the unresolved problem concerning the pharmacologic versus physiologic effects of glucagon is stressed. Glucagon's role in stimulating lipolysis at the adipocyte serves two important functions. First, it provides plasma nonesterified fatty acids for energy metabolism and secondly, it ensures substrate for hepatic ketogenesis. In vitro, glucagon's lipolytic activity has been consistently observed, but in vivo, this activity has sometimes been obscured by the effects of glucagon-induced insulin secretion. Frequently, a biphasic response has been reported in which a direct lipolytic response is followed by a glucagon-induced insulin suppression of plasma nonesterified fatty acid concentration. When the glucagon-induced insulin secretion has been controlled by various in vivo techniques, glucagon's lipolytic activity in vivo has frequently been demonstrable. In the 1960s, in vitro liver perfusion experiments demonstrated that glucagon enhanced hepatic ketogenesis independent of glucagon's lipolytic activity. However, this direct effect of glucagon on the hepatocyte was not universally accepted because of conflicting reports in the literature. Failure to observe an in vitro ketogenic effect of the hormone in some studies may have been due to suboptimal experimental conditions. Certain factors are now known to influence the ketogenic response, such as the concentration of fatty acids in the media and the nutritional status of the animal. Under optimal in vitro conditions with liver preparations from fed animals, the ketogenic response to physiologic concentrations of glucagon has been demonstrated. However, further study is necessary to define the quantitative ketogenic role of the hormone. In spite of this early in vitro work, glucagon was not definitely shown to be ketogenic in vivo (independent of fatty acid availability) both in the rat and in diabetic man until 1975. Since these observations, several reports have confirmed the ketogenic action of glucagon in vivo by direct hepatic catheterization experiments. Glucagon's role in decreasing hepatic triglyceride synthesis and secretion in vitro has been repeatedly shown but the mechanism is unresolved. This lipid regulatory action of glucagon has been more difficult to demonstrate in vivo because of the many variables that affect triglyceride synthesis. Under specific experimental conditions, however, glucagon has been shown to decrease plasma triglyceride concentration in man at both physiologic and pharmacologic concentrations. Hepatic catheterization experiments have also confirmed this effect in man. The regulation of lipids by glucagon fits well into its role as a stress hormone...

Regulation of hepatic free fatty acid metabolism by glucagon and insulin

The roles of glucagon and insulin in the direct short-term regulation of hepatic free fatty acid (FFA) metabolism were studied in hepatocytes isolated from fed, fasted, and streptozotocin-induced diabetic rats. In fed animals, the principal metabolic product of palmitate metabolism was triglyceride, whereas ketones were the major product in fasted and diabetic animals. Glucagon at physiological concentrations increased ketogenesis and decreased triglyceride synthesis from palmitate in hepatocytes from fed rats at FFA concentrations 1.0 mM or less. Insulin had no effect on FFA metabolism when present as the sole hormone, but could antagonize the actions of submaximal concentrations of glucagon. The metabolism of palmitate in fasted or diabetic hepatocytes was unaffected by either hormone. Ketogenesis from octanoate was also unaffected by hormone addition in all cell types. These data are consistent with a locus of hormonal regulation at a step prior to beta-oxidation of fatty acid. Glucagon and insulin may modulate FFA metabolism by both intrahepatic and extrahepatic mechanisms.