Stimulation Of Glucose Production Research Articles

Islet function and stress hyperglycemia: plasma glucose and epinephrine interaction.

Catecholamines and a number of other hormones released during stress states contribute to the development of hyperglycemia by directly stimulating glucose production and interfering with tissue disposal of glucose. However, hyperglycemia stimulates the secretion of insulin and inhibits the secretion of glucagon, effects that will diminish the degree of hyperglycemia resulting from direct actions of stress hormones on glucose production and disposal. The key additional role of catecholamines in the development of stress hyperglycemia is interference with the normal feedback control of insulin and glucagon secretion by circulating glucose levels. Although pancreatic islet responses to hyperglycemia may be modulated by catecholamines, any increase of insulin secretion or suppression of glucagon secretion that does occur may be important for limiting the degree of elevation of circulating glucose that results. Thus, plasma insulin and glucagon levels during stress states will reflect the interaction between the opposing effects of hyperglycemia and catecholamines. Diabetic patients who have impaired islet responses to glucose will be particularly prone to the development of marked hyperglycemia during stress states because they may be unable to respond to the influence of hyperglycemia in counteracting adrenergic inhibition of insulin secretion and stimulation of glucagon secretion.

Role of fructose 2,6-bisphosphate in the control by glucagon of gluconeogenesis from various precursors in isolated rat hepatocytes.

Hepatocytes from overnight-starved rats were incubated with 1-20 mM-fructose, -dihydroxyacetone, -glycerol, -alanine or -lactate and -pyruvate with or without 0.1 microM-glucagon. The production of glucose and lactate was measured, as was the content of fructose 2,6-bisphosphate. The concentrations of fructose (below 5 mM) and dihydroxyacetone (above 1 mM) that gave rise to an increase in fructose 2,6-bisphosphate were those at which a glucagon effect on the production of glucose and lactate could be observed. Glycerol had no effect on fructose 2,6-bisphosphate content or on production of lactate, and glucagon did not stimulate the production of glucose from this precursor. With alanine or lactate/pyruvate as substrates, glucagon stimulated glucose production whether the concentration of fructose 2,6-bisphosphate was increased or not. The extent of inactivation of pyruvate kinase by glucagon was not affected by the presence of the various gluconeogenic precursors. The role of fructose 2,6-bisphosphate in the effect of glucagon on gluconeogenesis from precursors entering the pathway at the level of triose phosphates or pyruvate is discussed.

Renal gluconeogenesis: axial and internephron heterogeneity and the effect of parathyroid hormone.

To better understand the regulation of renal gluconeogenesis that occurs in the proximal nephron, glucose production rates from various substrates were determined in defined proximal tubule segments of the rat. Tubule segments tested were the S1 and S2 segments of superficial (SF) nephrons, the S1 segments of juxtamedullary (JM) nephrons, and the S3 segments. Glucose production (in decreasing order) was: from alpha-ketoglutarate, JM S1, SF S1, SF S2; from pyruvate, SF S2, JM S1, and SF S1; from glutamine, SF S1, JM S1; and from glutamate, SF S1 = JM S1. Little glucose was produced in the S3 segments. Glucose production from glutamate was lower than that from the other three substrates in JM S1, and glutamine was the best gluconeogenic substrate in SF S1. The effects of parathyroid hormone (PTH), a known stimulator of renal gluconeogenesis, and cAMP were examined using alpha-ketoglutarate as the substrate. Both stimulated glucose production in the S1 and S2 segments of the SF nephron. Although PTH stimulated adenylate cyclase in the S1 segments of the SF and JM nephrons, it had no effect on glucose production in the JM S1. Glucose production rose in the SF S1 and JM S1 in response to increasing concentrations of hydrogen or calcium ions, indicating that gluconeogenesis can be increased in these nephron segments. Differences may therefore be present in the cellular responses to PTH distal to cAMP formation in the nephron segments of the SF and JM nephrons. These findings show the presence of both axial and internephron heterogeneity of renal gluconeogenesis and suggest the difference in the effects of PTH on the function of SF and JM nephrons.

Regulation of gluconeogenesis by norepinephrine, vasopressin, and angiotensin II: A comparative study in the absence and presence of extracellular Ca 2+

In hepatocytes isolated from fasted rats, vasopressin and angiotensin II stimulate the rate of gluconeogenesis from lactate or pyruvate in a Ca 2+-dependent manner similar to that previously reported for norepinephrine. Actions of the peptide hormones on gluconeogenesis from glycerol or sorbitol, reduced substrates that require oxidation before they enter the gluconeogenic pathway at triosephosphate, also resemble those of norepinephrine. Stimulation of glucose production from these substrates is observed only in the presence of extracellular Ca 2+. Actions of the peptide hormones on gluconeogenesis from dihydroxyacetone or fructose, the oxidized counterparts of glycerol and sorbitol, respectively, do not resemble those of norepinephrine. While norepinephrine enhances rates of glucose production from dihydroxyacetone or fructose in the absence of extracellular Ca 2+, vasopressin and angiotensin II are ineffective either in the absence or presence of extracellular Ca 2+. When the oxidation-reduction state in hepatocytes metabolizing dihydroxyacetone is altered by adding an equimolar concentration of ethanol (to provide cytosolic reducing equivalents), the results are similar to those obtained when cells are incubated with the reduced counterpart of dihydroxyacetone, glycerol, i.e., the peptide hormones cause an apparent increase in the rate of glucose production in a Ca 2+-dependent manner. If, on the other hand, hepatocytes are incubated with glycerol or sorbitol and an equimolar concentration of pyruvate (to provide a cytosolic hydrogen acceptor), the peptide hormones, unlike norepinephrine, are ineffective in stimulating gluconeogenesis in the absence of extracellular Ca 2+. These results indicate that whereas many of the actions of vasopressin and angiotensin II are similar to those of α 1-adrenergic agents, there are major differences in the manner in which the hormones act at various sites to regulate gluconeogenesis.

Effects of medium-chain triglyceride feeding on glucose homeostasis in the newborn rat

The mechanism of the profound hypoglycemia that develops in newborn rats during a fast of 16-h beginning at birth has been investigated. This fasting hypoglycemia was completely reversed by giving oral medium-chain fatty acids (MCT). The rise in blood glucose induced by MCT feeding was not secondary to a decreased uptake of glucose by peripheral tissues because [6-3H]glucose turnover rate was increased in MCT-fed neonates. Several lines of evidence strongly suggest that MCT feeding was associated with a stimulation of hepatic gluconeogenesis. 1) The rate of [6-3H]glucose turnover was enhanced after MCT feeding. 2) A fivefold increase in the conversion of labeled lactate into glucose was observed in vivo after MCT feeding. 3) The rise in blood glucose induced by MCT feeding was totally suppressed by an inhibitor of gluconeogenesis (3-mercaptopicolinate). Despite their utilization for glucose synthesis, blood levels of lactate, alanine, and pyruvate were increased two- to threefold after MCT feeding. When MCT feeding was given in association with dichloroacetate, an activator of pyruvate dehydrogenase (PDH), no increase in blood lactate, alanine, and pyruvate was observed and the rise in glycemia was prevented. This suggested that hyperketonemia due to MCT feeding could decrease the oxidation of 3-carbon glucose precursors in peripheral tissues, secondary to an inhibition of PDH, and thus enhanced their release in blood. These data indicate that MCT feeding stimulates glucose production in the newborn rat, both by increasing the availability of gluconeogenic precursors and by a direct effect on hepatic gluconeogenesis.

The effect of hyperglucagonaemia on blood glucose concentrations and on insulin requirements during fasting in insulin dependent diabetes mellitus.

The effect of sustained hyperglucagonaemia on blood glucose concentrations and on insulin requirements was evaluated in 6 fasting insulin dependent diabetic subjects whose blood glucose concentrations were being controlled with a closed loop insulin infusion system. Subjects were iv infused initially with either saline or glucagon and subsequently with the other infusate. All determinations were performed following the period during which transient increases in glucagon stimulated glucose production have been reported to occur. Plasma glucagon concentrations were significantly higher during the glucagon study period (491 +/- 65 vs 70 +/- 13 pg/ml +/- SD, P less than 0.001) as were blood glucose concentrations (104 +/- 2 vs 84 +/- 7 mg/ml +/- SD, P less than 0.001) and insulin requirements (3.5 to 36.5 vs 0 to 2.3 mU/kg/h, P less than 0.05). Sustained hyperglucagonaemia continues to have an effect on glucose homeostasis for at least 2 h following the initiation of a continuous infusion.

Effects of thyroid hormone on the gluconeogenic capacity of lipemic BHE rats.

Gluconeogenesis and ketogenesis from a variety of substrates by isolated hepatocytes from thyroxine-treated and control 40-hr starved BHE rats were determined. Thyroid hormone treatment stimulated glucose production from lactate and inhibited glucose production from alanine, lactate plus NH4Cl, or glutamate. beta-Hydroxybutyrate production by cells incubated with glycerol was decreased in the cells from thyroxine-treated rats. Glycerol had a greater antiketogenic effect in the thyroxine-treated rats than in the control rats. Acetoacetate production from 10:1 lactate:pyruvate, lactate plus NH4Cl, lactate plus lysine, and lactate plus ethanol was also less in the hepatocytes from the thyroxine-treated rats. Acetoacetate production from alanine was increased by thyroxine treatment. From these results it can be suggested that the metabolic abnormalities previously reported in the BHE rats can, in part, be attributable to abnormalities in endogenous thyroid hormone action and in part due to some as yet unknown regulatory process within the liver cell.

Nonoxidative glucose metabolism a prerequisite for formation of dilute urine.

To examine links between norepinephrine- (NE) stimulated sodium transport and gluconeogenesis, we perfused isolated rat kidneys with 6% albumin, containing various combinations of glucose, alanine, pyruvate. and lactate and inhibitors of gluconeogenesis (0.1 mM mercaptopicolinate, MP) or glucose metabolism (0.2-0.5 mM 2-deoxyglucose, DG). Inulin clearance, fractional potassium reabsorption, total sodium reabsorption, and free water clearance were higher in kidneys perfused with 5 mM glucose plus 2 mM alanine than in kidneys perfused with either 10 mM lactate or 5 mM pyruvate. NE, added after 40 min of perfusion, decreased fractional sodium and potassium excretion in all experiments. In lactate- and/or pyruvate-perfused kidneys NE decreased fractional water excretion with little increase in free water clearance; free water formation was lowest in kidneys perfused with DG or MP. Glucose (5 mM) reversed the inhibitory effect of MP on free water clearance. In glucose-perfused kidneys NE did not decrease fractional water excretion, whereas free water clearance increased threefold. NE stimulated glucose production from pyruvate 2.4-fold and from lactate 1.6-fold. MP inhibited gluconeogenesis both in the basal state and after NE. We conclude that the formation of dilute urine requires nonoxidative glucose metabolism to maintain low water permeability in the diluting segment and a high peritubular glucose concentration that is ensured by gluconeogenesis in adjacent proximal tubules.

Evaluation of the role of acetaldehyde in the actions of ethanol on gluconeogenesis by comparison with the effects of crotonol and crotonaldehyde.

Liver cells from fasted rats oxidized ethanol and crotonol at identical rates. Ethanol and crotonol increased the cytosolic lactate/pyruvate ratio to the same extent, however only ethanol increased the mitochondrial B‐hydroxybutyrate/acetoacetate ratio. The rate of oxidation of crotonaldehyde by liver cells was 30% to 50% of the rate of oxidation of acetaldehyde. Cyanam‐ide, which is especially inhibitory towards the low Km mitochondrial aldehyde dehydrogenase, inhibited the oxidation of acetaldehyde to a greater extent than it inhibited the oxidation of crotonaldehyde. Acetaldehyde, but not crotonaldehyde, increased the B‐hydrox‐ybutyrate/acetoacetate ratio; both aldehydes produced some increase in the lactate/pyruvate ratio. Consequently, differences between the effects of ethanol and crotonol on the mitochondrial redox state may relate to differences between the metabolism of their respective aldehydes.Intact mitochondria oxidized crotonaldehyde at about 10% the rate found with acetaldehyde. In deox‐ychoiate‐disrupted mitochondria, in the presence of external NAD+, crotonaldehyde was oxidized at rates less than one‐fourth the rate found with acetaldehyde. Cyanamide inhibited the oxidation of acetaldehyde to a greater extent than the oxidation of crotonaldehyde. These results suggest that crotonaldehyde is a poor substrate for the low Km mitochondrial aldehyde dehydrogenase.In isolated hepatocytes, acetaldehyde stimulated glucose production from pyruvate, but inhibited gluconeogenesis from glycerol, xylitol, and sorbitol. Crotonaldehyde had no effect on glucose production from these substrates, indicating that the effects of acetaldehyde were due to the metabolism of acetaldehyde in the mitochondria. Ethanol stimulated glucose production from pyruvate, whereas crotonol was without effect. The stimulation by ethanol, and the lack of effect by crotonol, appears to be due to changes in the mitochondrial redox state produced as a consequence of the further oxidation of acetaldehyde, but not crotonaldehyde. Crotonol and ethanol inhibited glucose production from glycerol, xylitol, and sorbitol, suggesting that changes in the cytosolic redox state play the major role in the effect of the alcohols on glucose production from these substrates. However, the inhibition by ethanol was consistently 10% to 15% greater than the inhibition by crotonol, suggesting that acetaldehyde contributes to the effects of ethanol. These differences between ethanol and crotonol are consistent with a role for acetaldehyde metabolism in the actions of ethanol. Crotonol may be a useful aid in determining the mechanism whereby ethanol alters henatic functions.

Stimulation of Gluconeogenesis by Glucagon and Norepinephrine in the Perfused Chicken Liver

The effects of glucagon and norepinephrine on gluconeogenesis by perfused chicken liver were studied with fluorimetric monitoring of the redox states of the intracellular pyridine nucleotides. Glucagon stimulated glucose production from precursors entering the pathway both above and below the level of triose phosphates without causing a detectable effect on the redox states of pyridine nucleotides. Glucagon stimulation was not abolished by subsequent infusion of octanoate or ethanol. The presence of a pyruvate/lactate mixture plus NH4Cl resulted in a maximum efficacy of glucagon. Glucose production from lactate and fructose was stimulated by norepinephrine. Norepinephrine always caused a change towards increased reduction of pyridine nucleotides with an increase in the beta-hydroxybutyrate/acetoacetate ratio, but displayed no stimulation of glucose and lactate production from pyruvate. As a result of octanoate infusion with lactate, the changes induced by norepinephrine were reversed. The responses to norepinephrine and phenylephrine were decreased markedly in liver perfused with a calcium-free medium and/or with phentolamine. Infusion of calcium into the calcium-deficient liver caused an abrupt elevation of glucose production together with a reduction of pyridine nucleotides, and the original response to norepinephrine was recovered. The results demonstrate that the effects of glucagon and norepinephrine on gluconeogenesis are not identical, and that norepinephrine stimulation is mediated through an alpha-adrenergic and calcium-dependent mechanism in which redox changes of mitochondrial pyridine nucleotides are involved.

Effects of Medium-Chain Triglycerides (M.C.T)in the starved newborn rat

Newborn rats,starved from birth develop between 12 and 16h a profound hypoglycemia due to a deficient gluconeogenesis.The lack of lipid stores results in a low rate of hepatic fatty acid oxidation which has been proposed as the rate limiting factor in gluconeogenesis.When 13h old starved rats are fed with M.C.T,blood glucose increases,in 3h,from 1.30 ± 0.09mM to 4.23 ± 0.23mM.This results from a stimulation of glucose production (63H glucose) from 3.9 ± 0.5mg/min/kg to 11.1 ± 0.6mg/min/kg due to a 5-fold increased gluconeogenesis.Despite their utilization for glucose synthesis, blood levels of lactate,alanine and pyruvate are increased 2 to 3-fold after M.C.T feeding. When M.C.T feeding is given in association with dichloroacetate (an activator of pyruvate dehydrogenase-(P-D.H)) there is no increase in blood lactate,alanine and pyruvate levels and the increase in glycemia is prevented.This suggests that hyperketonemia due to M.C.T feeding could decrease the oxidation of 3 carbons glucose precursors in peripheral tissues secondary to an inhibition of P.D.H and thus enhances their release in blood. These studies demonstrate that M.C.T feeding stimulates glucose production in the newborn rat,both,by increasing the availability of gluconeogenic precursors and by a direct effect on hepatic gluconeogenesis.

Effect of cyanamide on the metabolism of ethanol and acetaldehyde and on gluconeogenesis by isolated rat hepatocytes

Previous experiments demonstrated that acetaldehyde stimulated glucose production from pyruvate, whereas gluconeogenesis from glycerol, xylitol and sorbitol was inhibited [A.I. Cederbaum and E. Dicker, Archs Biochem. Biophys. 197, 415 (1979)]. To determine the mechanism whereby acetaldehyde affects glucose production from these precursors, and to evaluate the role of acetaldehyde in the actions of ethanol, experiments with cyanamide were carried out. The oxidation of acetaldehyde by isolated rat liver cells was inhibited by cyanamide after a brief incubation period. Associated with this inhibition of acetaldehyde oxidation was an inhibition of ethanol oxidation by cyanamide and an increase in the amount of acetaldehyde which arose during the oxidation of ethanol. Ethanol oxidation was decreased because of the ineffective removal of acetaldehyde in the presence of cyanamide. Cyanamide had no effect on hepatic oxygen uptake. The increase in the β-hydroxybutyrate/acetoacetate ratio produced by acetaldehyde was completely prevented by cyanamide, whereas the slight increase in the lactate/pyruvate ratio was not prevented by cyanamide. Cyanamide partially reversed the ethanol-induced increase in the lactate/pyruvate ratio, but it completely prevented the ethanol-induced increase in the β-hydroxybutyrate/acetoacetate ratio. The ethanol-induced change in the mitochondrial redox state may, therefore, be due primarily to the mitochondrial oxidation of the acetaldehyde which arises during the oxidation of ethanol. The inhibitory effects of acetaldehyde on gluconeogenesis from glycerol, xylitol and sorbitol, as well as the stimulation of acetaldehyde of glucose production from pyruvate, were completely prevented by cyanamide. These results indicate that the effects of acetaldehyde on gluconeogenesis represent metabolic effects, rather than direct effects of acetaldehyde. Changes in the cellular NADH/NAD − ratio as a consequence of acetaldehyde metabolism are postulated to be responsible for these actions of acetaldehyde. Ethanol stimulated glucose production from pyruvate, while inhibiting gluconeogenesis from glycerol, xylitol and sorbitol. Cyanamide, which prevented the effects of acetaldehyde on gluconeogenesis, also prevented the effects of ethanol on gluconeogenesis. This prevention by cyanamide may be suggestive for a role for acetaldehyde in the actions of ethanol on gluconeogenesis. The possibility cannot be ruled out, however, that the prevention of the effects of ethanol by cyanamide may be due to the partial inhibition of ethanol oxidation by cyanamide. These results indicate that cyanamide is an effective inhibitor of acetaldehyde oxidation by isolated liver cells and therefore can be used to determine the mechanism whereby acetaldehyde affects metabolic function. Depending on the reaction under investigation, acetaldehyde can have direct or indirect effects on cellular metabolism.

Importance of glucagon in mediating epinephrine-induced hyperglycemia in alloxan-diabetic dogs.

In normal dogs epinephrine stimulates glucose production (Ra) independently of glucagon. To investigate the role of this interaction in diabetes, epinephrine (0.1 micrograms . kg-1 . min-1) was infused for 90 min in five alloxan-diabetic dogs in the presence or absence of somatostatin (0.1 micrograms . kg-1 . min-1). In response to epinephrine, glycemia rose by 40% reflecting a near maximal (122%) increase in Ra. Plasma glucagon (IRG) rose to 953 pg/ml, whereas insulin (IRI) increased minimally. When somatostatin was infused with epinephrine to prevent the rise of IRG and IRI, there was only a marginal increase of glucose concentration (12%) and production (38%). The effect of somatostatin was reversed by infusing glucagon (10 ng . kg-1 . min-1) together with epinephrine and somatostatin into five additional alloxan-diabetic dogs. Increments in IRG, glycemia, and Ra were fully reestablished. A 100% FFA increase was observed in all three groups, indicating that the lipolytic effect of epinephrine was independent of glucagon. In conclusion, in diabetic dogs, in contrast to normal dogs, epinephrine induced a marked and prolonged increase in glucose concentration and production mostly through a stimulation of IRG secretion.

Altered ability of the liver to produce glucose following a period of glucagon deficiency.

It is known that the effectiveness of a physiologic increment in glucagon to stimulate glucose production wanes with time even when counterregulatory insulin secretion is prevented. The aim of the present study was to establish whether the converse is true: does glucagon become a more effective stimulus of glucose production following a period of acute hypoglucagonemia? To determine this, somatostatin was infused along with basal replacement amounts of glucagon (0.65 ng/mg x min) and insulin (228 microU/kg x min) into five postabsorptive conscious dogs. After 1.5 h of basal hormone replacement, the glucagon infusion was terminated and a selective fall in the plasma glucagon level (75 +/- 6 to 30 +/- 4 pg/ml) occurred. This resulted in a drop in tracer (3H-glucose)-determined glucose production from 3.0 +/- 0.4 to 1.5 +/- 0.3 mg/kg x min. The plasma insulin level remained unchanged at 10 +/- 1 microU/ml throughout the experiment. Euglycemia (110 +/- 4 mg/dl) was maintained by exogenous glucose infusion. After 3 h of glucagon lack, restoration of the glucagon infusion returned the IRG level to control values (78 +/- 6 pg/ml) but restored glucose output only partially (42 +/- 9%), necessitating continued glucose infusion to preserve euglycemia. Repetition of the experiment in dogs whose pancreatic glucoregulatory feedback loops were broken surgically (two-stage pancreatectomy) rather than pharmacologically resulted in similar findings. It is concluded that glucagon deficiency of 3-h duration leads to a decrease, rather than an increase, in hepatic sensitivity to glucagon.

Glucagon structure-function relationships using isolated rat hepatocytes

The ability of glucagon and several of its semi-synthetic analogues to stimulate glucose production in isolated rat hepatocytes was measured and compared for relative potencies. The order of decreasing biological activities of glucagon in this assay was as follows: glucagon > [HArg 12]-glucagon > [des-Asn 28, Thr 29][homoserinehydrazide 27]-glucagon approx. equal to [des-His 1]-glucagon > [des-Asn 28, Thr 29] [homoserinelactone 27]-glucagon > [des-Asn 28, Thr 29]-[ n-butylhomoserineamide 27]-glucagon > glucagon 1−21. Qualititatively, these results are similar to those obtained previously in the hepatic plasma membrane adenylate cylase assay. Minor exceptions were noted for the hydrazide derivative nd the partial agonist [des-His 1]-glucagon, both of which were slightl y more potent relative to glucagon in the glycogenolytic assay than in the adenylate cyclase assay. The assay provides important insight into glucagon structure-function relationships.

Metabolic events associated with the preparation of the fetus for independent life.

The metabolic changes late in fetal development that are essential for neonatal survival are discussed. In many species gluconeogenesis develops before birth but provides substrate for intracellular biosynthesis and not for glucose production because of low activities of glucose 6-phosphate translocase. At the time of glycogen deposition in species with a relatively mature brain at birth the translocase develops and glucagon and adrenaline can stimulate glucose production and synthesis to elevate blood glucose concentrations both pre- and postnatally. The other metabolic fuel accumulated before birth, fat, can also be mobilized prenatally and in fetuses that are relatively mature at birth it may be used as an alternative fuel. The fetal rat brain can oxidize fatty acids and the brain of fetuses such as that of the guinea-pig and man can oxidize ketone bodies before birth. The timing and degree of oxidation of ketone bodies relates to the timing of myelination and protects the brain against hypoglycaemia. These late changes in development are associated with a sharp increase in plasma cortisol and adrenaline concentrations and a high fetal insulin concentration.

Effects of somatostatin and adrenergic blockade on glucagon, insulin and glucose in exercising sheep.

This study was conducted to characterize the mechanisms of hyperglycaemia in exercising sheep. Sheep were run on a treadmill for 45 min (5.5 km h-1, 8% incline) during adrenergic blockade (propranolol or phentolamine mesylate infusions) and during suppression of the rise in glucagon by infusion of somatostatin (SRIF). Propranolol did not alter the glucagon, insulin or glucose responses, except it tended to increase the metabolic clearance of glucose, presumably as a result of blocking the beta-adrenergic inhibition of glucose uptake. Phentolamine mesylate administration was associated with a suppression of the rise in glucagon concentrations, a reversal of alpha-adrenergic inhibition of insulin release and a reduction in glucose appearance during exercise. SRIF prevented the rise in glucagon and reduced insulin concentrations to below resting values. Propranolol and phentolamine mesylate did not alter the glucagon, insulin or glucose response to SRIF. However, SRIF prevented the insulin rise that occurred during phentolamine administration. The increment in glucose appearance produced in response to exercise was the same for SRIF, plus phentolamine mesylate and phentolamine mesylate in the first 25 min of exercise, but was significantly less than in the controls. During the last 20 min of exercise, glucose appearance was not significantly different from the control for any of the groups. The depression by SRIF and alpha-adrenergic blockade of the increment in glucose appearance due to exercise was associated with an impairment of the glucagon response. It appears, therefore, that glucagon may stimulate glucose production early in exercise in sheep directly, as well as by having a permissive effect.

Hyperglycaemic effect of xylazine

In cats, xylazine, an analogue of clonidine, produced hyperglycaemia when injected intravenously. The effect was obtained in unanaesthetized cats and in pentobarbitone sodium anaesthesia. The hyperglycaemia was not a central effect, nor due to adrenaline release from the adrenals, nor to a direct action of xylazine on the liver. It resulted from a fall in plasma insulin produced by an action of xylazine on the pancreas, inhibiting insulin secretion without affecting glucagon secretion. The increase in the glucagon/insulin ratio, by stimulating glucose production in the liver, was probably responsible for the xylazine‐induced hyperglycaemia.

Early kinetics of glucagon action in isolated hepatocytes at the mitochondrial level.

The temporal relationship between the effect of glucagon on respiratory functions and the changes in metabolites related to gluconeogenesis has been studied. Mitochondria prepared from hepatocytes after incubation with glucagon for 1 min already displayed a maximal stimulation of state-3 respiration. The increase in succinate dehydrogenase activity was almost fully expressed 3 min after glucagon. With respect to the utilization of pyruvate, 2-oxoglutarate and glutamate, glucagon produced a significant effect within 1 min. The rate of this decrease was linear for about 3 min slowing down thereafter. The stimulation of glucose production from lactate became significant within 1 min and remained constant up to 15 min. The influence of glucagon on the mitochondrial redox state also was an early event. It was maximally shifted to the more reduced state within 2 min and declined within 15 min. Under the conditions employed no effect of glucagon on urea synthesis or branched-chain amino acid release up to 15 min incubation time was discernible. Glucagon influenced the respiratory parameters virtually independent of Ca2+, in contrast to its action on intermediary metabolism. As to the hormone specificity, no enhancement of state-3 respiration and succinate dehydrogenase activity was caused by phenylephrine or isoproterenol. From the time course studies presented, it appears that the mitochondrial effects of glucagon might be causally interrelated with the regulation of gluconeogenesis. Moreover, our results indicate that the stimulation of state-3 respiration represents the earliest, specific action of glucagon at the mitochondrial level.

The hormonal stimulation of ureogenesis in isolated hepatocytes through increases in mitochondrial ATP production

Isolated hepatocytes incubated with 2 m m ornithine-10 m m glutamine as substrates and challenged with either glucagon, epinephrine, or phenylephrine exhibited stimulated rates of urea production, and mitochondria isolated from these cells displayed an increased rate of energy-dependent citrulline formation. There was no change in the total carbamyl phosphate synthetase I activity, nor mitochondrial content of the positive effector N-acetyl glutamate after acute hormonal treatment. The time of onset of ureogenesis and its sensitivity to glucagon were compared with stimulation of glucose production from lactate-pyruvate. No apparent differences in time of onset or sensitivity of the responses were observed indicating both pathways may be stimulated by a common mechanism. Mitochondria prepared from cells treated with catecholamines exhibited increased rates of State 3 respiration and increased uncoupler-dependent ATPase activity, in addition to the increased rates of citrulline formation. There was also an elevated intramitochondrial content of ATP and an increased ATP ADP ratio. The catecholamine-induced stimulation of ureogenesis was mediated by an α-adrenergic cyclic AMP independent mechanism. The addition of the α-adrenergic antagonist, dihydroergotamine, blocked both the epinephrine-induced stimulation of ureogenesis and also the stimulated functions in the isolated mitochondria. dl-Propranolol, a β-antagonist, inhibited the rise in cyclic AMP due to epinephrine, but had no effect on any of the other reactions measured. The effects of catecholamines on citrulline formation and urea production are correlated with the increased capacity of the mitochondria to generate ATP. It is suggested that both glucagon and catecholamines, acting via independent mechanisms, stimulate electron transport and the activity of the ATP-forming enzyme complex. The consequent elevated intramitochondrial ATP levels and ATP ADP ratio enhance the rate of citrulline formation and hence ureogenesis.