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R Lupi

Publications and source records attributed to R Lupi.

At least 37 records · Page 2Linked to original sources
PubMed1998

Characterization of sulfonylurea receptors in isolated human pancreatic islets.

Current information on pancreatic islet sulfonylurea receptors has been obtained with laboratory animal pancreatic beta cells or stable beta-cell lines. In the present study, we evaluated the properties of sulfonylurea receptors of human islets of Langherans, prepared by collagenase digestion and density-gradient purification. The binding characteristics of labeled glibenclamide to pancreatic islet membrane preparations were analyzed, displacement studies with several oral hypoglycemic agents were performed, and these latter compounds were tested as for their insulinotropic action on intact human islets. [3H]glibenclamide saturable binding was shown to be linear at < or =0.25 mg/ml protein; it was both temperature and time dependent. Scatchard analysis of the equilibrium binding data at 25 degrees C indicated the presence of a single class of saturable, high-affinity binding sites with a Kd value of 1.0+/-0.07 nM and a Bmax value of 657+/-48 fmol/mg of proteins. The displacement experiments showed the following rank order of potency of the oral hypoglycemic agents we tested: glibenclamide = glimepiride > tolbutamide > chlorpropamide >> metformin. This binding potency order was parallel with the insulinotropic potency of the evaluated compounds.

ATP-Binding Cassette Transporters↗
PubMed1997

Peripheral benzodiazepine receptors in isolated human pancreatic islets.

Peripheral benzodiazepine receptors have been shown in some endocrine tissues, namely the testis, the adrenal gland, and the pituitary gland. In this work we evaluated whether peripheral benzodiazepine receptors can be found in the purified human pancreatic islets and whether they may have a role in insulin release. Binding of the isoquinoline compound [3H]1-(2-chlorophenyl-N-methyl-1-methyl-propyl)-3- isoquinolinecarboxamide (13H]PK-11195) a specific ligand of peripheral benzodiazepine receptors, to cellular membranes was saturable and Scatchard's analysis of the saturation curve demonstrated the presence of a single population of binding sites, with an affinity constant value of 9.20 +/- 0.80 nM and a maximum number of binding sites value of 8913 +/- 750 fmol/mg of proteins. PK-11195 and 7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4- benzodiazepine-2-on (Ro 5-4864) significantly potentiated insulin secretion from freshly isolated human islets at 3.3 mM glucose. These results show the presence of peripheral benzodiazepine receptors in purified human pancreatic islets and suggest their role in the mechanisms of insulin release.

Benzodiazepinones↗
PubMed1997

Modulatory role of GTP-binding proteins in the endogenous ADP-ribosylation of cytosolic proteins.

The endogenous ADP-ribosylation of cytosolic proteins and the pattern of NAD degradation were analyzed in subcellular fractions of rat liver in order to investigate the modulation of these reactions by GTP-binding (G) proteins. We could show that intracellular membranes from rat liver have a guanine nucleotide- and divalent cation-dependent pyrophosphatase activity able to rapidly degrade NAD to AMP. This enzymatic activity was investigated by two different approaches: the degradation of [32P]-NAD in the presence of intracellular membranes and the mono-ADP-ribosylation of cytosolic proteins. Divalent cations, preferentially Zn2+ and Mn2+, were required for the pyrophosphatase activity, since in the presence of the Zn2+ chelator TPEN (N,N,N',N'-tetrakis(2-pyridyl-methyl)ethylenediamine) or EDTA, the NAD degradation was inhibited by about 50%. Accordingly, in the presence of TPEN the endogenous ADP-ribosylation of cytosolic proteins was enhanced, whereas Zn2+ caused a significant inhibition of this reaction. GDP beta S was able to strongly activate the mono-ADP-ribosylation of cytosolic proteins. This effect was abolished by GTP gamma S, suggesting that a G protein, or rather one of the subunits of a heterotrimeric G protein, is involved in the modulation of the pyrophosphatase and consequently, of endogenous ADP-ribosylation. We propose that a regulatory pathway involving a heterotrimeric G protein modulates enzymes affecting the NAD turnover and availability of NAD for endogenous mADPRTs.

Adenosine Diphosphate Ribose↗
PubMed1997

Effects of glibenclamide and metformin (alone or in combination) on insulin release from isolated human pancreatic islets.

Isolated human pancreatic islets were prepared by collagenase digestion and density gradient purification, and the effects of glibenclamide (0.5 and 5.0 mumol/l) and metformin (20 and 200 mumol/l), alone or in combination, on insulin release were evaluated at varying glucose concentrations. At 3.3 mmol/l glucose level, the addition of 5.0 mumol/l glibenclamide or 5.0 mumol/l glibenclamide plus 200 mumol/l metformin caused a significant increase of insulin release, compared with glucose alone. At 16.7 mmol/l glucose concentration, a significant increase of insulin secretion, compared with glucose alone, was produced by the addition of either 5.0 mumol/l glibenclamide, 200 mumol/l metformin, or both 5.0 mumol/l glibenclamide and 200 mumol/l metformin. The effect of the combination of the two drugs was significantly higher than that with either drug used alone. Thus, glibenclamide was shown to have an insulinotropic effect on human islets at both low and high glucose concentrations, and metformin at high glucose concentrations. A possible synergistic effect of glibenclamide and metformin at high glucose concentrations is also suggested.

Cell Separation↗