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J H Exton

Publications and source records attributed to J H Exton.

At least 199 records · Page 11Linked to original sources
PubMed1981

Inhibitory effect of epinephrine on insulin-stimulated glucose uptake by rat skeletal muscle.

The effect of epinephrine on basal and insulin-stimulated glucose uptake in perfused hindlimbs of fed rats was studied. Insulin increased glucose uptake in a dose-dependent manner from a basal value of 1.5+/-0.3 up to a maximum value of 5.3+/-0.9 mumol/min per 100 g with 6 nM (1 m U/ml). Epinephrine at 10 nM and 0.1 muM also increased glucose uptake to 2.6+/-0.1 and 3.1+/-0.1 mumol/min per 100 g, respectively. These same concentrations of epinephrine, however, suppressed the insulin-stimulated glucose uptake to 3.2+/-0.3 mumol/min per 100 g. Both the stimulatory and inhibitory effects of epinephrine on glucose uptake were completely reversed by propranolol, but were not significantly altered by phentolamine. Uptake of 3-O-methylglucose and 2-deoxyglucose into thigh muscles of the perfused hindlimbs was stimulated fivefold by insulin, but was unaffected by epinephrine. Epinephrine also did not inhibit the stimulation of uptake by insulin. Epinephrine decreased the phosphorylation of 2-deoxyglucose, however, and caused the intracellular accumulation of free glucose. These last two effects were more prominent in the presence of insulin. Whereas epinephrine caused large rises in glucose-6-P and fructose-6-P, insulin did not alter the concentration of these metabolites either in the absence or presence of epinephrine.THESE DATA INDICATE THAT: (a) epinephrine has a stimulatory effect on glucose uptake by perfused rat hindlimbs that does not appear to be exerted on skeletal muscle; (b) epinephrine does not affect hexose transport in skeletal muscle; (c) epinephrine inhibits insulin-stimulated glucose uptake in skeletal muscle by inhibiting glucose phosphorylation. It is hypothesized that the inhibition of glucose phosphorylation is due to the stimulation of glycogenolysis, which leads to the accumulation of hexose phosphates, which inhibit hexokinase.

Adrenergic alpha-Antagonists↗
PubMed1981

Mechanisms of hormonal regulation of liver metabolism.

The mechanism of actions of glucagon, alpha- and beta-adrenergic agonists, vasopressin and angiotensin II in the liver proposed in this article are summarized in Fig. 8. The actions of glucagon and beta-adrenergic agonists in liver can be entirely ascribed to their interaction with specific plasma membrane receptors which activate adenylate cyclase leading to the intracellular accumulation of cAMP and activation of cAMP-dependent protein kinase. This enzyme phosphorylates phosphorylase b kinase, glycogen synthase, L-type pyruvate kinase, and other liver proteins resulting in alterations in their activities which can account for several of the known hepatic responses to glucagon. There is no clear evidence that Ca2+ ions are involved in the hepatic actions of this hormone. Glucocorticoids, but not thyroid hormones, are required for normal responsiveness of the liver to glucagon. The steroids do not modify cAMP accumulation or cAMP-dependent protein kinase activation, but may act by modulating the action of the kinase on its substrates. Glucocorticoids and thyroid hormones decrease beta-adrenergic responses in the liver apparently by decreasing the number of beta-receptors. Insulin inhibits the actions of physiological concentrations of glucagon by decreasing cAMP accumulation: its mechanism of action is unknown. The actions of alpha-adrenergic agonists, vasopressin and angiotensin II on the liver resemble those of glucagon, but do not involve accumulation of cAMP or activation of cAMP-dependent protein kinase. These agents appear to act by increasing cytosolic Ca2+ thus altering the activities of Ca2+-sensitive enzymes such as phosphorylase b kinase and calmodulin-dependent glycogen synthase kinase. Their receptors appear to be located exclusively on the plasma membrane and a major mechanism by which they raise cytosolic Ca2+ is by inducing the release of this cation from mitochondria. These considerations imply the existence of an intracellular messenger(s) for these agents which is generated at the plasma membrane in response to receptor activation and exerts effects on mitochondria or perhaps other intracellular structures. Glucocorticoids and thyroid hormones increase alpha-adrenergic responses in the liver apparently by increasing the number of alpha-receptors. Insulin inhibits the responses of the liver to alpha-agonists, but not to vasopressin or angiotensin II.

Adrenergic alpha-Agonists↗
PubMed1980

The role of phosphorylation in the alpha-adrenergic-mediated inhibition of rat hepatic pyruvate kinase.

Phenylephrine in the presence of 1-methyl-3-isobutylxanthine and propanolol caused a 40-50% inhibition of pyruvate kinase (type L) activity in isolated hepatocytes, which was accompanied by a 2-3-fold increase in the phosphate content of the enzyme. These changes were blocked by the alpha-adrenergic antagonist dihydroergocryptine and could not be accounted for by the slight increase in cyclic AMP-dependent protein kinase activity generated by the alpha-adrenergic agonist. It is concluded that a significant component of the inhibition of hepatic pyruvate kinase mediated by alpha-adrenergic agonists can be attributed to a cyclic AMP-independent alteration in the phosphorylation state of the enzyme.

Animals↗
PubMed1980

Calcium regulation of guanine nucleotide activation of hepatic adenylate cyclase.

Pretreatment of isolated rat liver plasma membranes by washing with NaHCO3 buffer or by exposure to the chelator ethyleneglycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) with or without the ionophore A23187, produced a decrease in the sensitivity of adenylate cyclase (ATP pyrophosphate-lyase (cyclizing) EC 4.6.1.1) to subsequent stimulation by NaF or guanosine 5'-(beta-gamma-imino)triphosphate (GPP(NH)P). Sensitivity to activation by the nucleotide could be restored by addition of the lyophilized and ashed wash or by addition of Ca2+, Mg2+ or Mn2+. The factor extracted from the membranes by these various treatments which was responsible for loss of stimulation was identified as Ca2+. Determination of the metal ion content of isolated membranes by atomic absorption spectrometry indicated that Ca2+ was the only divalent cation present in sufficient concentration to support persistent activation by either NaF or GPP(NH)P. Pretreatment of liver plasma membranes with trifluoperazine, which inhibits the action of Ca2+-dependent regulator protein in other enzyme systems, reduced GPP(NH)P activation of adenylate cyclase and caused marked depletion of membrane Ca2+. The effects of low concentrations (less than 100 microM) of the phenothiazine could be reversed totally by Ca2+ and partly by regulator protein. At higher concentrations of trifluoperazine, slight restoration of enzyme activation was seen with either agent. The hypothesis is presented that Ca2+ interacts with the nucleotide (GTP or GDP) regulatory site(s) of the adenylate cyclase. This interaction may be regulator-protein-dependent and may be important in determining the sensitivity of the enzyme to nucleotide activation in vivo.

Adenylyl Cyclases↗
PubMed1980

Effects of trypsin on binding of [3H]epinephrine and [3H]-dihydroergocryptine to rat liver plasma membranes. Evidence for interconversion of binding sites.

Treatment of liver plasma membranes with trypsin at low concentrations (1 to 2 microgram/mg of protein) caused at 3- to 4-fold increase in alpha-specific [3H]epinephrine binding. The change was due to an increase in the number of high affinity binding sites, with no change in the dissociation constant. With increasing trypsin concentrations, the dissociation constant was decreased and there was a progressive loss of binding. Elastase, papain, and thermolysin caused similar effects, whereas the thrombin, leucine aminopeptidase, phospholipase A2, phospholipase C, phospholipase D, and detergents did not cause an increase in [EH]epinephrine binding. The increase in epinephrine high affinity binding sites was correlated with a loss of high affinity [3H]-dihydroergocryptine binding sites which also bind [3H]epinephrine with low affinity (El-Refai, M. F., Blackmore, P. F., and Exton, J. H. (1979) J. Biol. Chem. 254, 4375-4386). Incubation of membranes with the alpha blockers dihydroergocryptine (50 nM) and phenoxybenzamine (20 nM) prior to protease treatment diminished the increase in [3H]epinephrine binding induced by trypsin (1.5 microgram/mg). The concentration dependence and time course of trypsin actions on 70 nM [3H]epinephrine binding and 10 nM [3H]dihydroergocryptine binding are consistent with a trypsin-mediated conversion of low affinity epinephrine binding sites to high affinity epinephrine binding sites.

Animals↗
PubMed1980

Stable changes to calcium fluxes in mitochondria isolated from rat livers perfused with alpha-adrenergic agonists and with glucagon.

Mitochondria isolated from rat liver after a short-term perfusion with the alpha-adrenergic agonist phenylephrine or with glucagon exhibited enhanced rates of uptake of Ca2+ and prolonged retention of Ca2+ in the presence of 4mm-P(i). The effect of Ca2+ retention was apparent after perfusion with phenylephrine for only 1min and was maximal after 7min of treatment. The changes induced by glucagon, although similar, were less rapid. Adrenaline caused similar changes to phenylephrine and its effects were blocked by the alpha-adrenergic antagonist phenoxybenzamine, but not by the beta-antagonist propranolol. The Ca2+ content of the isolated mitochondria decreased by 30% 1min after the onset of perfusion with phenylephrine; by 6min it had begun to return to the original value which was reached at 10min. A similar loss in calcium content was induced by glucagon but the changes were not as great and occurred more slowly. Mitochondria from phenylephrine-treated livers exhibited decreased rates of Ca2+ efflux induced by addition of 2mm-EGTA, a 50% increase in the contents of ADP and total adenine nucleotides, a small increase in the transmembrane pH gradient, and a reduced rate of oxaloacetate-induced NADPH oxidation. This study thus shows that stimulation of liver by alpha-adrenergic agonists, like that by glucagon, induces within minutes a stable modification of mitochondria leading to alterations in the Ca2+-translocation cycle (increased Ca2+ uptake and retention) and alterations in mitochondrial energy-linked reactions.

Adenine Nucleotides↗
PubMed1980

Activation of endogenous phosphorylase kinase in liver glycogen pellet by cAMP-dependent protein kinase.

Liver glycogen phosphorylase associated with the glycogen pellet was activated by a MgATP-dependent process. This activation was reduced by 90% by ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid, not affected by the inhibitor of the cAMP-dependent protein kinase, and increased 2.5-fold by the catalytic subunit of cAMP-dependent protein kinase. Low levels of free Ca2+ (8 x 10(-8) M) completely prevented the effects of the chelator. The activation of phosphorylase by MgATP was shown not to be due to formation of AMP. DEAE-cellulose chromatography of the glycogen pellet separated phosphorylase from phosphorylase kinase. The isolated phosphorylase was no longer activated by MgATP in the presence or absence of the catalytic subunit of cAMP-dependent protein kinase. The isolated phosphorylase kinase phosphorylated and activated skeletal muscle phosphorylase b and the activation was increased 2- to 3-fold by the catalytic subunit of cAMP-dependent protein kinase. Mixing the isolated phosphorylase and phosphorylase kinase together restored the effects of MgATP and the catalytic subunit of cAMP-dependent protein kinase on phosphorylase activity. These findings demonstrate that the phosphorylase kinase associated with liver glycogen has regulatory features similar to those of muscle phosphorylase kinase.

Animals↗
PubMed1980

Epinephrine regulation of skeletal muscle glycogen metabolism. Studies utilizing the perfused rat hindlimb preparation.

Studies of rat skeletal glycogen metabolism carried out in a perfused hindlimb system indicated that epinephrine activates phosphorylase via the cascade of phosphorylation reactions classically linked to the beta-adrenergic receptor/adenylate cyclase system. The beta blocker propranolol completely blocked the effects of epinephrine on cAMP, cAMP-dependent protein kinase, phosphorylase, and glucose-6-P, whereas the alpha blocker phentolamine was totally ineffective. Omission of glucose from the perfusion medium did not modify the effects of epinephrine. Glycogen synthase activity in control perfused and nonperfused muscle was largely glucose-6-P-dependent (-glucose-6-P/+glucose-6-P activity ratios of 0.1 and 0.2, respectively). Epinephrine perfusion caused a small decrease in the enzyme's activity ratio (0.1 to 0.05) and a large increase in its Ka for glucose-6-P (0.3 to 1.5 mM). This increase in glucose-6-P dependency correlated in time with protein kinase activation and was totally blocked by propranolol and unaffected by phentolamine. Comparison of the kinetics of glycogen synthase in extracts of control and epinephrine-perfused muscle with the kinetics of purified rat skeletal muscle glycogen synthase a phosphorylated to various degrees by cAMP-dependent protein kinase indicated that the enzyme was already substantially phosphorylated in control muscle and that epinephrine treatment caused further phosphorylation of synthase, presumably via cAMP-dependent protein kinase. These data provide a basis for speculation about in vivo regulation of the enzyme.

Animals↗
PubMed1980

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues.

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Animals↗
PubMed1980

Insulin regulation of skeletal muscle glycogen metabolism.

Using the perfused rat hindlimb preparation, the role of insulin in the regulation of glycogen metabolism in voluntary skeletal muscle has been characterized. A maximally effective concentration of insulin (1 mU/ml) caused a threefold increase in glucose clearance by 5 min. However, the -glucose-6-P/+glucose-6-P activity ratio of glycogen synthase was not significantly increased before 20 min. Insulin concentrations as low as 0.1 mU/ml significantly modified the glycogen synthase activity ratio and the half-maximal activation constant (A0.5) for glucose-6-P at 30 min, but had no effect on tissue cAMP. These changes were not dependent on the presence of glucose and were not modified by fasting. These results indicate that high physiological concentrations of insulin activate glycogen synthase in voluntary skeletal muscle and that this effect is independent of changes in glucose uptake or tissue cyclic AMP.

Animals↗
PubMed1980

The role of calcium in alpha-adrenergic inactivation of glycogen synthase in rat hepatocytes and its inhibition by insulin.

The effects of epinephrine, vasopressin, and A23187 on glycogen synthase and phosphorylase were examined in isolated rat liver parenchymal cells from fed animals. In normal calcium-containing hepatocytes, epinephrine, vasopressin, and A23187 were more potent at inactivating glycogen synthase, previously activated with 30 mM glucose, than at activating phosphorylase. In calcium-depleted hepatocytes (cells washed and incubated with 1 mM EGTA), the effect of epinephrine on both enzyme activities was impaired, while the effects of vasopressin and A23187 were completely abolished. Insulin was more effective at inhibiting the effects of epinephrine in calcium-depleted cells, but it was without effect on vasopressin and A23187 actions. The ability of epinephrine, vasopressin, and A23187 to elicit calcium efflux from cells was not altered by the presence of 30 mM glucose. These findings are consistent with the idea that the alpha-adrenergic inactivation of liver glycogen synthase may be a result of the increased stimulation of a calcium-dependent protein kinase, possibly phosphorylase b kinase.

Animals↗
PubMed1979

Effects of adrenalectomy on hormone action on hepatic glucose metabolism. Impaired glucagon activation of glycogen phosphorylase in hepatocytes from adrenalectomized rats.

The effects of adrenalectomy on glucagon activation of liver glycogen phosphorylase and glycogenolysis were studied in isolated hepatocytes. Adrenalectomy resulted in reduced responsiveness of glycogenolysis and phosphorylase to glucagon activation. Stimulation of cAMP accumulation and cAMP-dependent protein kinase activity by glucagon was unaltered in cells from adrenalectomized rats. Adrenalectomy did not alter the proportion of type I and type II protein kinase isozymes in liver, whereas this was changed by fasting. Activation of phosphorylase kinase by glucagon was reduced in hepatocytes from adrenalectomized rats, although the half-maximal effective concentration of glucagon was unchanged. No difference in phosphorylase phosphatase activity between liver cells from control and adrenalectomized rats was detected. Glucagon-activated phosphorylase declined rapidly in hepatocytes from adrenalectomized rats, whereas the time course of cAMP increase in response to glucagon was normal. Addition of glucose (15 mM) rapidly inactivated glucagon-stimulated phosphorylase in both adrenalectomized and control rat hepatocytes. The inactivation by glucose was reversed by increasing glucagon concentration in cells from control rats, but was accelerated in cells from adrenalectomized rats. It is concluded that impaired activation of phosphorylase kinase contributes to the reduced glucagon stimulation of hepatic glycogenolysis in adrenalectomized rats. The possible role of changes in phosphorylase phosphatase is discussed.

Adrenalectomy↗