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Y Giudicelli

Publications and source records attributed to Y Giudicelli.

At least 91 records · Page 5Linked to original sources
PubMed1984

Guanine nucleotide- and GTP-dependent N6-phenylisopropyladenosine stimulation of the membrane-bound cyclic AMP high affinity phosphodiesterase in rat brain.

In brain cortex, low concentrations of GTP or Gpp(NH)p activated the membrane-bound low Km cyclic AMP phosphodiesterase while higher concentrations of GTP, but not of Gpp(NH)p, reversed this activation. The adenosine analog N6-phenylisopropyladenosine (N6-PIA) elicited biphasic effect on this enzyme (activation up to 10(-8) M, complete reversion at 10(-5) M), provided that GTP was present. N6-PIA activation was reduced in the presence of Gpp(NH)p and blocked by sodium (80 mM). In contrast, the soluble low Km cyclic AMP phosphodiesterase was insensitive to GTP or N6-PIA. This study suggests that guanine nucleotides and N6-PIA exert their effects on the membrane-bound enzyme through guanine nucleotide regulatory proteins.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1984

Comparison of changes in the characteristics of beta-adrenoceptors and responsiveness of human circulating lymphocytes during and after chronic administration of pindolol and propranolol.

The characteristics of the beta-adrenoceptors (3H-dihydroalprenolol binding) and the beta-adrenergic responsiveness (cyclic AMP response to isoproterenol) were studied in circulating lymphocytes from healthy volunteers before (D 0), on the 22nd day of treatment (D 22) and on the fifth day (D 30) following discontinuation of pindolol 15 mg/day or propranolol 160 mg/day. During pindolol therapy (D 22) the beta-adrenoceptor-affinity towards dihydroalprenolol was unchanged and the beta-adrenoceptor density decreased by 50%, an effect which persisted after drug administration ceased (D 30). The receptor "down-regulation" was accompanied by a parallel decrease in the maximal lymphocyte response to isoproterenol on D 22 and 30, by a shift to the right of the concentration-response curves to isoproterenol on D 22 and 30. In contrast, no significant modification in beta-adrenoceptor density or in the maximal response to isoproterenol was observed during (D 22) or after discontinuation (D 30) or propranolol therapy; the beta-adrenoceptor affinity for dihydroalprenolol and the sensitivity of the lymphocyte response to isoproterenol were markedly reduced on D 22 but not on D 30. It is concluded that the lymphocyte beta-adrenoceptor "down regulation" and the parallel beta-adrenergic desensitization induced by chronic pindolol therapy are both due to the intrinsic sympathomimetic activity of this drug.

Adult↗
PubMed1984

In vivo desensitization of the beta, but not the alpha 2-adrenoreceptor-coupled-adenylate cyclase system in hamster white adipocytes after administration of epinephrine.

To assess the physiological relevance of the changes in adrenergic receptor observed in adipocyte membranes after in vitro exposure to catecholamines, hamster white adipocytes, which possess both beta- and alpha-adrenergic receptors, were studied after a 6-day in vivo epinephrine administration (1 mg/kg, daily). This treatment resulted in a 3-fold increase in total plasma catecholamine levels and in the following changes in the adipocytes. The lipolytic and cAMP responses to beta-agonists, ACTH, and theophylline were decreased by 55-60%, but the sensitivity of these responses to (-)isoproterenol was unchanged. Although basal adenylate cyclase activity was unaffected, (-)isoproterenol-, ACTH- or fluoride-stimulated activities were reduced by half, a defect which persisted in the presence of guanosine 5'-[beta, gamma-imido]triphosphate. Furthermore, the number of beta-adrenergic receptors was also decreased by 54%. In contrast, epinephrine treatment failed to impair the adipocyte antilipolytic response to the alpha 2-agonist clonidine, the alpha 2-component of epinephrine, and the adenosine analog N6-phenylisopropyladenosine, the adenylate cyclase inhibitory response to these compounds, and the number of alpha 2-adrenergic receptors. These results indicate that in vivo epinephrine administration does not alter the alpha 2-adrenergic system of hamster white adipocyte, but desensitizes the lipolytic response to beta-agonists and also to nonadrenergic lipolytic agents. It is therefore suggested that the likely mechanism(s) responsible for this lipolysis desensitization mainly consists in impaired adenylate cyclase coupling and possibly in altered intracellular processes, rather than in the observed beta-adrenergic receptor down-regulation.

Adenylyl Cyclases↗
PubMed1984

"Spare" beta-adrenergic receptors of rat white adipocyte membranes.

The apparent equilibrium dissociation constants for the interaction of isoproterenol with beta-receptors and adenylate cyclase were determined under the same conditions in rat adipocyte membranes and were compared with the apparent dissociation constant for the interaction of isoproterenol with cyclic AMP accumulation in the adipocyte. From these determinations, it was calculated that the occupancy of less than 4% of the receptor population is required for half-maximal stimulation of adenylate cyclase in membranes and cyclic AMP accumulation in intact cells, provided that receptor-binding and adenylate cyclase assays are performed in the presence of guanine nucleotides. Since guanine nucleotides are also required for adenylate cyclase activation in intact cells, it is concluded that the beta-receptors of rat adipocytes are "spare" receptors.

Adenylyl Cyclases↗
PubMed1983

Inhibition in vitro of acyl-CoA dehydrogenases by 2-mercaptoacetate in rat liver mitochondria.

In rat liver hypo-osmotically treated mitochondria, 2-mercaptoacetate inhibits respiration induced by palmitoyl-CoA, octanoate or butyryl-CoA only when the reaction medium is supplemented with ATP. Under this condition, NADH-stimulated respiration is not affected. In liver mitochondrial matrix, the presence of ATP is also required to observe a 2-mercaptoacetate-induced inhibition of acyl-CoA dehydrogenases tested with palmitoyl-CoA, butyryl-CoA or isovaleryl-CoA as substrate. As the oxidation of these substrates is also inhibited by the incubation medium resulting from the reaction of 2-mercaptoacetate with acetyl-CoA synthase, with conditions under which 2-mercaptoacetate has no effect, 2-mercaptoacetyl-CoA seems to be the likely inhibitory metabolite responsible for the effects of 2-mercaptoacetate. Kinetic experiments show that the main effect of the 2-mercaptoacetate-active metabolite is to decrease the affinities of fatty acyl-CoA dehydrogenases towards palmitoyl-CoA or butyryl-CoA and of isovaleryl-CoA dehydrogenase towards isovaleryl-CoA. Addition of N-ethylmaleimide to mitochondrial matrix pre-exposed to 2-mercaptoacetate results in the immediate reversion of the inhibitions of palmitoyl-CoA and isovaleryl-CoA dehydrogenations and in a delayed reversion of butyryl-CoA dehydrogenation. These results led us to conclude that (i) the ATP-dependent conversion of 2-mercaptoacetate into an inhibitory metabolite takes place in the liver mitochondrial matrix and (ii) the three fatty acyl-CoA dehydrogenases and isovaleryl-CoA dehydrogenase are mainly competitively inhibited by this compound. Finally, the present study also suggests that the inhibitory metabolite of 2-mercaptoacetate may bind non-specifically to, or induce conformational changes at, the acyl-CoA binding sites of these dehydrogenases.

Acyl Coenzyme A↗
PubMed1983

Characterization of the beta-adrenergic receptors of hamster white fat cells. Evidence against an important role for the alpha 2-receptor subtype in the adrenergic control of lipolysis.

The binding characteristics of the beta-adrenergic antagonist, [3H]dihydroalprenolol, to hamster white adipocyte membranes were studied. This binding occurred at two classes of sites, one having high affinity (Kd = 1.6 +/- 1.3 nM) but low capacity (32 +/- 17 fmol/mg membrane protein) and one having low affinity but high binding capacity. While the binding at the high-affinity sites was competitively and stereoselectively displaced by both beta-antagonists and beta-agonists, competition at the low-affinity sites occurred only with beta-antagonists and was non-stereoselective. Thus, the beta-agonist (-)-isoproterenol was further used to define nonspecific binding. Under these conditions, saturation studies showed a single class of high-affinity (Kd = 1.6 +/- 0.5 nM) binding sites with a binding capacity of 53 +/- 13 fmol/mg membrane protein (corresponding to 4000 +/- 980 sites per cell), and independent kinetic analysis provided a Kd value of 1.9 nM. Competition experiments showed that these binding sites had the characteristics of a beta 1-receptor subtype, yielding Kd values in good agreement with the Kact and the Ki values found for agonist-stimulation and for antagonist-inhibition of adenylate cyclase in membranes and of cyclic AMP accumulation and lipolysis in intact cells. Furthermore, the ability of beta-agonists to compete with this binding was severely depressed by p[NH]ppG. These results thus support the contention that the specific [3H]dihydroalprenolol binding sites defined as the binding displaceable by (-)-isoproterenol represent the physiologically relevant beta-adrenergic receptors of hamster white adipocytes. Finally, studies of the lipolytic response of these cells to (-)-norepinephrine showed that the inhibitory effect of the alpha 2-component of this catecholamine was apparent only when the effects of endogenous adenosine were suppressed, a result which argues against an important regulatory role for the alpha 2-receptors in the adrenergic control of lipolysis in hamster white adipocytes.

Adipose Tissue↗
PubMed1982

Effects of 2-mercaptoacetate in isolated liver mitochondria in vitro. Competitive inhibition of 3-hydroxybutyrate dehydrogenase and depression of the beta-oxidation pathway.

The effects of 2-mercaptoacetate on the respiration rates induced by different substrates were studied in vitro in isolated liver mitochondria. With palmitoyl-L-carnitine or 2-oxoglutarate as the substrate, the ADP-stimulated respiration (State 3) was dose-dependently inhibited by 2-mercaptoacetate. with glutamate or succinate as the substrate. State-3 respiration was only slightly inhibited by 2-mercaptoacetate. In contrast, the oxidation rate of 3-hydroxybutyrate was competitively inhibited by 2-mercaptoacetate in both isolated mitochondria and submitochondrial particles. In uncoupled mitochondria and in mitochondria in which ATP- and GTP-dependent acyl-CoA biosynthesis was inhibited, the inhibitory effect of 2-mercaptoacetate on palmitoyl-L-carnitine oxidation was abolished; under the same conditions, however, inhibition of 3-hydroxybutyrate oxidation by 2-mercaptoacetate still persisted. These results led to the following conclusions: 2-mercaptoacetate itself enters the mitochondrial matrix, inhibits fatty acid oxidation through a mechanism requiring an energy-dependent activation of 2-mercaptoacetate and itself inhibits 3-hydroxybutyrate oxidation through a competitive inhibition of the membrane-bound 3-hydroxybutyrate dehydrogenase. This study also strongly suggests that the compound responsible for the inhibition of fatty acid oxidation is 2-mercaptoacetyl-CoA.

3-Hydroxybutyric Acid↗
PubMed1982

Characteristics of beta-adrenergic-agonist binding to rat adipocyte membranes. Evidence that (+/-)-[3H]hydroxybenzylisoproterenol interacts selectively with the adipocyte beta-adrenergic receptors.

The binding characteristics of the beta-adrenergic agonist (+/)-[3H]hydroxybenzylisoproterenol to rat adipocyte membranes were studied. Binding was rapid, reaching equilibrium within 10 min at 37 degrees C (second order rate constant K1 = 1.37 . 10(7) . M-1 . min-1). Dissociation of specific binding by 0.5 mM (-)-isoproterenol suggested dissociation from two different sites with respective dissociation rate constants k2 of 0.106 . min-1 and 0.011 . min-1 . [3H]Hydroxybenzylisoproterenol binding was saturable (Bmax = 690 +/- 107 fmol/mg protein), yielding curvilinear Scatchard plots. Computer modeling of these data were consistent with the existence of two classes of [3H]hydroxybenzylisoproterenol binding sites, one having high affinity (KD = 3.5 +/- 0.7 nM) but low binding capacity (10% of the total sites) and one having low affinity (KD = 101 +/- 20 nM) but high binding capacity (90% of the sites). Adrenergic ligands competed with [3H]hydroxybenzylisoproterenol binding with the following order of potency = (-)-propranolol greater than (-)-isoproterenol greater than (-)-norepinephrine approximately (-)-epinephrine greater than greater than (+)-isoproterenol = (+)-propranolol, which is consistent with binding to beta 1-adrenergic receptors. Competition curves of [3H]hydroxybenzylisoproterenol binding by the beta-agonist (-)-isoproterenol were shallow and modeled to two affinity states of binding, whereas, competition curves by beta-antagonist (-)-propranolol were steeper with Hill number near to one. Gpp[NH]p severely reduced [3H]hydroxybenzylisoproterenol binding, an effect which apparently resulted from the reduction of the number of both the high and low affinity sites. In membranes which had been previously exposed to (-)-isoproterenol, the number of [3H]hydroxybenzylisoproterenol binding sites was reduced by 50%, an effect which apparently resulted from the loss of part of both the high and low affinity state binding sites. Finally, the ability of (-)-isoproterenol to stimulate adenylate cyclase correlated closely with the ability of (-)-isoproterenol to displace [3H]hydroxybenzylisoproterenol binding. Comparison of these findings with the binding characteristics of the beta-antagonist [3H]dihydroalprenolol to rat adipocyte membranes, led to conclude that [3H]hydroxybenzylisoproterenol can be successfully used to label the beta-adrenergic receptors of rat fat cells and suggests that it might be a better ligand than [3H]dihydroalprenolol in these cells.

Adipose Tissue↗
PubMed1982

Catecholamine binding to the alpha-adrenergic receptors of hamster adipocytes. Evidence that guanine nucleotides regulate this binding to the alpha 2-receptor subtype.

The binding characteristics of the alpha-component of (-)-[3H]norepinephrine to hamster adipocyte membranes were studied. Binding was rapid, equilibrium in 20 min at 25 degrees C. Dissociation of specific binding by 10 microM phentolamine suggested dissociation from two different sites. The time course of dissociation induced by a 50-fold dilution was unchanged by the addition of norepinephrine, suggesting the absence of cooperative binding sites. [3H]norepinephrine binding was saturable, yielding curvilinear Scatchard plots. Computer modeling of these data further supported the existence of two classes of binding sites, one with high affinity (KD = 23 nM) but low binding capacity (96 fmol/mg protein) and one with low affinity (KD = 400 nM) but high binding capacity (1000 fmol/mg protein). Adrenergic ligands competed with [3H]norepinephrine binding in the following order of potency: (-)-norepinephrine greater than (-)-epinephrine much greater than (+)-norepinephrine greater than (-)-isoproterenol. Displacement by the selective alpha-adrenergic drugs prazosin, clonidine, and yohimbine yielded biphasic curves consistent with binding of [3H]norepinephrine to both alpha 1-(14-22%) and alpha 2-(78-86%) receptor subtypes. Although Gpp(NH)p failed to alter the binding of [3H] dihydroergocryptine, it severely reduced the binding affinity of (-)-epinephrine, (-)-norepinephrine and the selective alpha 2-agonist, clonidine. The inhibitory effects of clonidine and of the alpha-component of (-)-epinephrine on the adrenocorticotropin-stimulated cyclic AMP production in the intact adipocyte were closely correlated with their effects on the binding of both [3H]norepinephrine and [3H]dihydroergocryptine. Conversely, yohimbine but not prazosin markedly antagonised the alpha-inhibitory effect of norepinephrine on cyclic AMP production. These data led to conclude that [3H]norepinephrine can be successfully used to study the entire alpha-adrenergic receptor population of hamster fat cells and that the predominant alpha 2-receptor subtype exists in two different affinity states for agonists, the proportions of which are modulated by guanine nucleotides.

Adipose Tissue↗
PubMed1981

2-Mercaptoacetate administration depresses the beta-oxidation pathway through an inhibition of long-chain acyl-CoA dehydrogenase activity.

To elucidate the mechanisms through which 2-mercaptoacetate administration inhibits fatty acid oxidation in the liver, the respiration rates induced by different substrates were studied polarographically in rat hepatic mitochondria isolated 3 h after 2-mercaptoacetate administration. Palmitoyl-L-carnitine oxidation was almost completely inhibited in either the absence or presence of malonate. Octanoate oxidation was also inhibited, and the intramitochondrial acyl-CoA content was markedly increased. The oxidation rate of pyruvate and 2-oxoglutarate on the one hand and of 3-hydroxybutyrate, succinate and glutamate on the other was either normal or only slightly decreased. In the presence of 2,4-dinitrophenol, the extent of the inhibition of palmitoyl-L-carnitine oxidation was unchanged. All these results are consistent with the hypothesis that the 2-mercaptoacetate inhibition of fatty acid oxidation is due to an inhibition of the beta-oxidation pathway itself. Finally, the mitochondrial defect responsible for this inhibition was shown to be an inhibition of palmitoyl-CoA dehydrogenase activity (EC 1.3.99.3).

Acyl-CoA Dehydrogenase, Long-Chain↗