The correlation of cyclic AMP and protein kinase activity in adipocytes with lipolysis stimulated by ACTH: the effect of adenosine deaminase and actinomycin D

ACTH at levels as low as 0.05 mU/ml stimulated lipolysis, protein kinase and cyclic AMP accumulation in isolated fat cells from fed and fasted rats. Changes in cyclic AMP levels and in the protein kinase activity ratio were well correlated temporally. The protein kinase activity ratio was potentiated by adenosine deaminase. A sudden increase or decrease in either ACTH or dibutyryl cyclic AMP concentration was associated with a rapid and corresponding change in the rate of glycerol production. With ACTH, the changes in glycerol production were accompanied by appropriate changes in cyclic AMP levels. Actinomycin-D (10 UM) did not affect lipolysis or cyclic AMP accumulation activated by ACTH in fat cells.

Transport of D-allose by isolated fat-cells: an effect of adenosine triphosphate on insulin stimulated transport

D-allose, a glucose analogue, is not metabolized by isolated fat-cells and its distribution space at equilibrium in the cells is the same as that of triated water. Uptake of allose is inhibited by glucose and 3-O-methylglucose, stimulated by insulin and virtually eliminated by cytochalasan B. Counter transport of allose out of fat-cells against a concentration gradient can be induced by exogenous glucose but not by pyruvate. It is concluded that allose is transported into fat-cells by the same carrier mediated transport system as glucose and that it is a suitable analogue with which to study the glucose transport system. Insulin stimulated allose transport, into or out of the cell, but not basal transport, is inhibited by a brief exposure of isolated fat-cells to exogenous ATP or ADP (but not AMP or AMP-PNP). The antilipolytic effect of insulin is not affected. The ATP inhibition is slowly reversible. It is suggested that ATP phosphorylates a membrane component and thereby blocks transmission of signal from the insulin receptor to the carrier system. Indirect evidence suggests that ATP does not alter the affinity of the insulin or glucose binding sites. Insulin decreases the Km of glucose metabolism of CO2 and lipid in isolated fat-cells and increases the Vmax. However,the hormone has no effect on the Ki of glucose as an inhibitor of allose transport. The glucose analogue, 3-O-methyl-glucose, also inhibits both glucose metabolism and allose transport. The Ki for both these processes is similar and is not affected by insulin. These results support the view that the effect of insulin on glucose transport is to raise the Vmax without a change in the Km. It appears further that sugar transport is not the major rate limiting step in metabolism at high glucose concentrations in the absence of insulin, or at most glucose concentrations in the presence of the hormone.

Studies on the alpha-andrenergic activation of hepatic glucose output. II. Investigation of the roles of adenosine 3':5'-monophosphate and adenosine 3':5'-monophosphate-dependent protein kinase in the actions of phenylephrine in isolated hepatocytes

The effects of the alpha-adrenergic agonist phenylephrine on the levels of adenosine 3':5'-monophosphate (cAMP) and the activity of the cAMP-dependent protein kinase in isolated rat liver parenchymal cells were studied. Cyclic AMP was very slightly (5 to 13%) increased in cells incubated with phenylephrine at a concentration (10(-5) M) which was maximally effective on glycogenolysis and gluconeogenesis. However, the increase was significant only at 5 min. Cyclic AMP levels with 10(-5) M phenylephrine measured at this time were reduced by the beta-adrenergic antagonist propranolol, but were unaffected by the alpha-blocker phenoxybenzamine, indicating that the elevation was due to weak beta activity of the agonist. When doses of glucagon, epinephrine, and phenylephrine which produced the same stimulation of glycogenolysis or gluconeogenesis were added to the same batches of cells, there were marked rises in cAMP with glucagon, minimal increases with epinephrine, and little or no changes with phenylephrine, indicating that the two catecholamine stimulated these processes largely by mechanisms not involving cAMP accumulation. DEAE-cellulose chromatography of homogenates of liver cells revealed two major peaks of cAMP-dependent protein kinase activity. These eluted at similar salt concentrations as the type I and II isozymes from rat heart. Optimal conditions for preservation of hormone effects on the activity of the enzyme in the cells were determined. High concentrations of phenylephrine (10(-5) M and 10(-4) M) produced a small increase (10 tp 16%) in the activity ratio (-cAMP/+cAMP) of the enzyme. This was abolished by propranolol, but not by phenoxybenzamine, indicating that it was due to weak beta activity of the agonist. The increase in the activity ratio of the kinase with 10(-5) M phenylephrine was much smaller than that produced by a glycogenolytically equivalent dose of glucagon. The changes in protein kinase induced by phenylephrine and the blockers and by glucagon were thus consistent with those in cAMP. Theophylline and 1-methyl-3-isobutylxanthine, which inhibit cAMP phosphodiesterase, potentiated the effects of phenylephrine on glycogenolysis and gluconeogenesis. The potentiations were blocked by phenoxybenzamine, but not by propranolol. Methylisobutylxanthine increased the levels of cAMP and enhanced the activation of protein kinase in cells incubated with phenylephrine. These effects were diminished or abolished by propanolol, but were unaffected by phenoxybenzamine. It is concluded from these data that alpha-adrenergic activation of glycogenolysis and gluconeogenesis in isolated rat liver parenchymal cells occurs by mechanisms not involving an increase in total cellular cAMP or activation of the cAMP-dependent protein kinase. The results also show that phosphodiesterase inhibitors potentiate alpha-adrenergic actions in hepatocytes mainly by a mechanism(s) not involving a rise in cAMP.

Guanosine 3':5'-cyclic monophosphate binding proteins in rat tissues

Rat tissues were surveyed for proteins which bind cGMP. Binding activity was high in extracts of lung, cerebellum, and small intestine, but was low in those of liver, adipose tissue, and skeletal muscle. DEAE-cellulose chromatography resolved two peaks of cGMP-binding activity in most tissues. The binding protein in peak 1 was eluted in the flow-through volume and was most abundant in extracts of intestine. It had a sedimentation coefficient of 6S and was highly specific for cGMP at pH 7.0 (dissociation constant KD=0.05 muM). No cGMP-dependent histone kinase activity was found for this peak. The binding protein in peak 2 was eluted by 0.05-0.15 M NaCl and was the predominant binding substance in lung, cerebellum, and heart. It had a sedimentation coefficient of 8S and binding was also highly specific for cGMP, with a KD of 0.05 muM. This peak of binding activity was associated with cGMP-dependent protein kinase activity which could be purified approximately 200-fold by Sepharose 6B chromatography. Cyclic GMP dependency of kinase activity was observed only at low histone concentrations. The abundance of one or both the above binding proteins correlated with the known basal levels of cGMP in the tissues.

Carbohydrate metabolism in perfused livers of adrenalectomized and steroid-replaced rats

In livers from fasted rats perfused with bicarbonate buffer containing bovine albumin and erythrocytes, adrenalectomy decreased glycogen levels and glucose production, impaired the incorporation of 14C from [14C]lactate into glucose or glycogen, and decreased the activity of the active (I) form of glycogen synthase. Cortisol treatment restored gluconeogenesis after 1 h and glycogen synthesis after 2 h. Adrenalectomy did not alter the production of glucose or lactate or the levels of gluconeogenic intermediates in livers from fasted rats perfused with fructose, but reduced the formation of glycogen from this substrate. Adrenalectomy increased the levels of lactate and decreased the levels of P-pyruvate and subsequent intermediates in the gluconeogenic pathway. These changes were reversed by cortisol treatment. It is concluded that glucocorticoids support gluconeogenesis and glycogen synthesis in livers from fasted rats primarily by facilitating a reaction(s) located between pyruvate and P-pyruvate in the gluconeogenic pathway and by promoting the conversion of inactive to active glycogen synthase.

Stimulation by glucagon of the incorporation of U-14C-labeled substrates into glucose by isolated hepatocytes from fed rats

The effect of glucagon on the incorporation of U-14C-labeled lactate, pyruvate or alanine into glucose has been studied using isolated hepatocytes from livers of fed rats. Rates of incorporation into glucose were about the same as observed in perfused liver preparations provided precautions were taken to avoid depletion of certain metabolities by the preparative procedures. With each substrate, stimulation of the incorporation into glucose by a maximally effective concentration of glucagon (10 nM) was associated with about a 75% reduction in the substrate concentration required for a half-maximal rate and with about a 30% increase in maximum rate. Consequently, the hormone caused a substantial (2--4-fold) stimulation when any one of the above substrates was present at a near physiological concentration, but brought about only a relatively small stimulation (1.4-fold) when very high substrate concentrations were used. Provision of cytoplasmic reducing equivalents (by ethanol addition), or of precursor for acetyl-coenzyme A formation (by acetate addition)-stimulated incorporation of labeled alanine into glucose and their effects were additive with that of glucagon. This suggested that provision of either of these intermediates was not a means by which the hormone increased the incorporation of labeled substrate into glucose. NH4+ stimulated the incorporation of 20 mM [U-14C] lactate into glucose 2-fold, probably by promoting glutamate synthesis and thus enhancing the transamination of oxaloacetate to aspartate. Evidence was obtained to support the view that glucagon also increases glutamate production (presumably from endogenous protein). However, the stimulation of incorporation into glucose from 20 mM [U-14C] lactate by NH4+ plus glucagon was synergistic. This suggested that glucagon also stimulated the incorporation of labeled substrate into glucose by additional means. Stimulation of the incorporation of [U-14C] alanine into glucose by beta-hydroxybutyrate plus glucagon was also synergistic. This suggested that another action of glucagon may be to provide more intramitochondrial reducing potential.

Hormonal control of cyclic 3':5'-AMP levels and gluconeogenesis in isolated hepatocytes from fed rats

Glucagon can stimulate gluconeogenesis from 2 mM lactate nearly 4-fold in isolated liver cells from fed rats; exogenous cyclic adenosine 3':5'-monophosphate (cyclic AMP) is equally effective, but epinephrine can stimulate only 1.5-fold. Half-maximal effects are obtained with glucagon at 0.3 nM, cyclic AMP at 30 muM and epinephrine at 0.2 muM. Insulin reduces by 50% the stimulation by suboptimal concentrations of glucagon (0.5 nM). A half-maximal effect is obtained with 0.3 nM insulin (45 microunits/ml). Glucagon in the presence of theophylline (1 mM) causes a rapid rise and subsequent fall in intracellular cyclic AMP with a peak between 3 and 6 min. Some of the fall can be accounted for by loss of nucleotide into the medium. This efflux is suppressed by probenecid, suggesting the presence of a membrane transport mechanism for the cyclic nucleotide. Glucagon can raise intracellular cyclic AMP about 30-fold; a half-maximal effect is obtained with 1.5 nM hormone. Epinephrine (plus theophylline, 1 mM) can raise intracellular cyclic AMP about 2-fold; the peak elevation is reached in less than 1 min and declines during the next 15 min to near the basal level. Insulin (10 nM) does not lower the basal level of cyclic AMP within the hepatocyte, but suppresses by about 50% the rise in intracellular and total cyclic AMP caused by exposure to an intermediate concentration of glucagon. No inhibition of adenylate cyclase by insulin can be shown. Basal gluconeogenesis is not significantly depressed by calcium deficiency but stimulation by glucagon is reduced by 50%. Calcium deficiency does not reduce accumulation of cyclic AMP in response to glucagon but diminishes stimulation of gluconeogenesis by exogenous cyclic AMP. Glucagon has a rapid stimulatory effect on the flux of 45Ca2+ from medium to tissue.

Regulation of adenosine 3:5-monophosphate-dependent protein kinase

The effects of epinephrine, glucagon, insulin and 1-methyl-3-isobutylxanthine on adenosine 3:5-monophosphate (cAMP)-dependent protein kinase activity were investigated in the perfused rat heart. The conditions for homogenization of heart tissue and assay of protein kinase are described. The activation state of the enzyme is expressed as the ratio of the rate of phosphorylation of histone in the absence to that in the presence of 2 mu-M cAMP. This activity ratio is stable in crude homogenates over 15 min of incubation; it is not affected by up to 30-fold dilution of the tissue volume. The ratio is elevated to a variable degree in hearts taken immediately from the animal but falls to a stable, basal level of 0.15 to 0.20 after 15 min of perfusion in vitro. An optimal concentration of epinephrine (10 mu-M) in the perfusate elevates cAMP from 0.5 to 1.3 nmol per g of tissue and increases the protein kinase activity ratio from 0.20 to 0.65. When hearts are perfused with a steady, submaximal concentration of epinephrine (0.4 mu-M), the level of cAMP and the protein kinase activity ratio rise in parallel within 15 s and remain elevated for at least 10 min. When epinephrine is removed from the perfusion medium, the level of cAMP and enzyme activity ratio decline rapidly to basal levels. Both glucagon and the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine also increase the cardiac cAMP levels and protein kinase activity ratio in a dose-dependent manner. Glucagon acts as rapidly as does epinephrine whereas 1-methyl-3-isobutylxanthine requires at least 30 s before any effect can be observed. Insulin by itself does not significantly affect the cyclic nucleotide level or enzyme activity. The hormone has not been observed to lower the cAMP level or protein kinase activity in the heart under any conditions tested. In concentrations of 10 microunits per ml or greater, it does, however, cause a slight rise in the tissue level of cAMP and the protein kinase activity when these have been elevated to intermediate levels by exposure to epinephrine. This effect could only be observed when hearts were treated with catecholamine and could not be detected with glucagon or 1-methyl-3-isobutylxanthine. In all cases tested, slight increases in the protein kinase activity ratio (from 0.2 to 0.3) were accompanied by much greater increases in the amount of phosphorylase in the a form (20% to 70%). It was observed that at perfusion times greater than 3 min, there was a significant reduction in phosphorylase activity even though both the cAMP level and protein kinase activity remained elevated. In these studies, changes in the protein kinase activity correlate well with the tissue cAMP levels under all conditions tested.

On the question of translocation of heart cAMP-dependent protein kinase

Rat hearts were perfused with epinephrine and/or 1-methyl-3-isobutylxanthine for 2 min. These agents raised the concentration of cAMP and increased the fraction of cAMP-dependent protein kinase (EC 2.7.1.70) in the active form. However, the content of cAMP-dependent protein kinase in the soluble fraction of homogenates of these hearts was reduced and the amount in the particulate fraction was increased. A similar redistribution was obtained by adding cAMP to homogenates of control hearts. The reduction in soluble protein kinase content was due to apparent binding of the free catalytic subunit of the enzyme to particulate material (12,000 times g pellet) in media of low ionic strength (smaller than 100 mM KCl). The amount bound was, therefore, proportional to the dissociation of the holoenzyme. The binding was not altered by prior boiling or trypsin treatment of the particulate material, but it was prevented or reversed by the addition of 150 mM KCl. The catalytic subunit of the protein kinase from heart also bound to particulate fractions from liver or Escherichia coli and to various denatured proteins. These findings suggest that the protein kinase activity of membranes and particulate fractions has frequently been overestimated, since isolation of particulate materials has usually been carried out at low ionic strength. The data also imply that intracellular translocation of the protein kinase catalytic subunit, at least in heart tissue, is of questionable physiological significance.

The distribution and dissociation of cyclic adenosine 3':5'-monophosphate-dependent protein kinases in adipose, cardiac, and other tissues

In crude extracts of adipose tissue the protein kinase dissociates slowly at 30 degrees into regulatory and catalytic subunits in the presence of 700 mug per ml of histone or 0.5 M NaCl. If the kinase is first dissociated by adding 10 muM adenosine 3':5'-monophosphate (cAMP), reassociation occurs instantaneously after removal of the cAMP by Sephadex G-25 chromatography. In contrast, in crude xtracts of heart, the protein kinase dissociates rapidly in the presence of 700 mug per ml of histone or 0.5 M NaCl and reassociates slowly after removal of cAMP. These differences are accounted for by the existence of two types of protein kinases in these tissues, referred to as types I and II. DEAE-cellulose chromatography of extracts of adipose tissue produces only one peak of cAMP-dependent protein kinase activity (type II) which elutes between 0.15 and 0.25 M NaCl. Similar chromatography of heart extracts resolves enzyme activity into two peaks; a type I enzyme which elutes between 0.05 and 0.1 M and predominates (greater than 75% of total activity), and a type II enzyme which elutes between 0.15 and 0.25 M NaCl. The dissociation properties of the types I and II enzymes from heart and adipose tissue are retained after partial purification by DEAE-cellulose and Sepharose 6B chromatography. Rechromatography of the separated peaks of the cardiac enzymes does not change the elution pattern. Sucrose density gradient centrifugation and gel filtration studies indicate that the molecular weights of these enzymes are very similar. The type II enzyme isolated by DEAE-cellulose chromatography of heart extracts resembles the adipose tissue enzyme, i.e. it undergoes slow dissociation at 30 degrees in the presence of histone or 0.5 M NaCl. The adipose tissue kinase and the heart type II kinase are not identical, however, since they do not elute at exactly the same point on DEAE-cellulose columns. A survey of several tissues indicates the presence of type I and II protein kinases similar to the enzymes in adipose tissue and heart as determined by DEAE-cellulose chromatography of crude extracts and by dissociation of the enzymes with histone. The presence of MgATP prevents dissociation of type I enzyme from heart by 0.5 M NaCl or histone. The profile of the enzyme on DEAE-cellulose, however, is not changed...

Relationship of some hepatic actions of insulin to the intracellular level of cyclic adenylate

Glucagon and/or the catecholamines elevate tissue cyclic adenylate (cAMP) and thereby change the activity of a number of metabolic processes, as studied in the isolated perfused rat liver. Insulin can lower cAMP in liver and thereby opposes these changes. In liver, in the basal state, there appears to be a substantial quantity of cAMP which is compartmentalized, unaffected by insulin and physiologically inactive. The hormone effects are exerted on a pool of “free cAMP,” and changes in the level of this pool control the minute-to-minute output of glucose and presumably effect rapid regulation of potassium fluxes and other processes. The level of cAMP in the active pool in the perfused liver is reflected by the efflux of the nucleotide into the medium. Not all insulin effects in liver or other tissues are mediated by a fall in cAMP.