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Under conditions of sustained hyperglycemia, glycogen accumulates in pancreatic islets, but not so in acinar pancreatic cells. We investigated whether advantage could be taken of such a situation in the perspective of the noninvasive imaging of the endocrine pancreas. Control rats or animals injected with streptozotocin (STZ) were infused with solutions of d-glucose mixed with a tracer amount of d-[U-C]glucose, and the radio-active glycogen content of both liver and pancreas was then measured. After 48 h of infusion, the radio-active glycogen content of the pancreas was 30 times lower in STZ rats than in control animals, coinciding with a 50 times lower insulin content. In the control rats, a sizable labeling of pancreatic glycogen was also recorded when d-[U-C]glucose was infused for only the last 4 h of unlabeled d-glucose infusion; such a labeling was not decreased when the animals were further infused for 1 h with only the unlabeled hexose. Moreover, a pronounced difference in the pancreatic gland and blood radioactive content of control rats was still observed when the hyperglycemic animals were killed only 40 min after the iv injection of d-[U-C]glucose. In STZ rats transplanted with islets and later infused with d-[U-C]glucose, the total radioactive content and radioactive glycogen content were both much higher in the transplanted islets than in the pancreatic gland. These results allow one to define the conditions under which the administration of either 2-deoxy-2-[F]fluoro- d-glucose or C-labeled d-glucose could conceivably be used to favor the selective labeling of the endocrine, as distinct from exocrine, pancreas. [ABSTRACT FROM AUTHOR]

The fate of 6-deoxy-6-[I[iodo- d-glucose (6-DIG), injected intravenously, was compared in control rats and animals that had received streptozotocin and were then treated with insulin or not. In the control rats, the measurement of plasma radioactivity suggested that, after an initial and rapid (up to min 10) distribution phenomenon ( K value: 12.2 × 10 min), the clearance of the iodinated hexose occurred mainly by glomerular filtration ( K value: 0.2 × 10 min). Three minutes after the injection of 6-DIG, the radioactive content of muscle, liver, and pancreas, relative to the paired value in blood, was lower in untreated diabetic rats than in control animals. In the case of muscle and liver, such a difference was no longer observed when the treatment of the diabetic rats by insulin resulted in restoration of normoglycemia. In the pancreas, however, the radioactive content, whether expressed relative to the paired blood or liver value, remained significantly lower in the insulin-treated diabetic rats than in the control animals. No significant difference between control and diabetic rats, in terms of pancreatic radioactivity, was observed 10 min after the injection of 6-DIG. These findings indicate that advantage can be taken from the vastly different time course for 6-DIG uptake by pancreatic acinar and islet cells, as recently documented in vitro, to label preferentially the endocrine moiety of the pancreatic gland shortly after 6-DIG injection. [ABSTRACT FROM AUTHOR]

Tritiated glibenclamide binds to specific receptors and is internalized in pancreatic insulin-producing B-cells. We investigated, therefore, whether tritiated glibenclamide could be used to preferentially label the endocrine, as distinct from exocrine, pancreas. In isolated rat pancreatic islets, the net uptake of H-glibenclamide reached within 30 min of incubation a near-equilibrium value, corresponding to an apparent distribution space close to three to four times the islet volume. In pieces of pancreas exposed up to 1 h to H-glibenclamide, however, its apparent distribution space progressively increased and, even at the min 60 of incubation, did not exceed a third of the wet weight of the pieces. Yet, no significant difference could be detected between the time course for H-glibenclamide uptake by pancreatic pieces from either control animals or rats injected with streptozotocin a few days before the experiments. Likewise, no significant difference in the paired ratio between the radioactive content of the pancreas and plasma could be found between the control and diabetic rats when examined 1, 5, or 24 h after the IV administration of H-glibenclamide. These findings indicate that the sulfonylurea does not represent a suitable tool for preferential labeling of the endocrine pancreas in the perspective of its imaging by a noninvasive procedure. [ABSTRACT FROM AUTHOR]

Tritiated glibenclamide binds to specific receptors and is internalized in pancreatic insulin-producing B-cells. We investigated, therefore, whether tritiated glibenclamide could be used to preferentially label the endocrine, as distinct from exocrine, pancreas. In isolated rat pancreatic islets, the net uptake of H-glibenclamide reached within 30 min of incubation a near-equilibrium value, corresponding to an apparent distribution space close to three to four times the islet volume. In pieces of pancreas exposed up to 1 h to H-glibenclamide, however, its apparent distribution space progressively increased and, even at the min 60 of incubation, did not exceed a third of the wet weight of the pieces. Yet, no significant difference could be detected between the time course for H-glibenclamide uptake by pancreatic pieces from either control animals or rats injected with streptozotocin a few days before the experiments. Likewise, no significant difference in the paired ratio between the radioactive content of the pancreas and plasma could be found between the control and diabetic rats when examined 1, 5, or 24 h after the IV administration of H-glibenclamide. These findings indicate that the sulfonylurea does not represent a suitable tool for preferential labeling of the endocrine pancreas in the perspective of its imaging by a noninvasive procedure. [ABSTRACT FROM AUTHOR]

The exchange of protons and deuterons by phosphoglucoisomerase during the single passage conversion of D-[2-C,1-H]fructose 6-phosphate in HO or D-[2-C]fructose 6-phosphate in DO to D-[2-C]glucose 6-phosphate, as coupled with the further generation of 6-phospho-D-[2-C]gluconate in the presence of excess glucose-6-phosphate dehydrogenase was investigated by C NMR spectroscopy of the latter metabolite. In HO, the intramolecular deuteron transfer from the C of D-fructose 6-phosphate to the C of D-glucose 6-phosphate amounted to 65%, a value only slightly lower than the 72% intramolecular proton transfer in DO. Both percentages, especially the latter one, were lower than those previously recorded during the single passage conversion of D-[1-C,2-H]glucose 6-phosphate in HO or D-[1-C]glucose 6-phosphate in DO to D-fructose 6-phosphate and then to D-fructose 1,6-bisphosphate. These differences indicate that the sequence of interactions between the hexose esters and the binding sites of phosphoglucoisomerase is not strictly in mirror image during, respectively, the conversion of the aldose phosphate to ketose phosphate and the opposite process. [ABSTRACT FROM AUTHOR]

The discrimination between the isotopes of hydrogen in the reaction catalyzed by yeast phosphoglucoisomerase is examined by NMR, as well as by spectrofluorometric or radioisotopic methods. The monodirectional conversion of D-glucose 6-phosphate to D-fructose 6-phosphate displays a lower maximal velocity with D-[2-H]glucose 6-phosphate than unlabelled D-glucose 6-phosphate, with little difference in the affinity of the enzyme for these two substrates. About 72% of the deuterium located on the C of D-[1-C,2-H]glucose 6-phosphate is transferred intramolecularly to the C of D-[1-C,1-H]fructose 6-phosphate. The velocity of the monodirectional conversion of D-[U-C]glucose 6-phosphate (or D-[2-H]glucose 6-phosphate) to D-fructose 6-phosphate is virtually identical in HO and DO, respectively, but is four times lower with the tritiated than C-labelled ester. In the monodirectional reaction, the intramolecular transfer from the C of D-[2-H]glucose 6-phosphate is higher in the presence of DO than HO. Whereas prolonged exposure of D-[1-C]glucose 6-phosphate to DO, in the presence of phosphoglucoisomerase, leads to the formation of both D-[1-C,2-H]glucose 6-phosphate and D-[1-C,1-H]fructose 6-phosphate, no sizeable incorporation of deuterium from DO on the C of D-[1-C]fructose 1,6-bisphosphate is observed when the monodirectional conversion of D-[1-C]glucose 6-phosphate occurs in the concomitant presence of phosphoglucoisomerase and phosphofructokinase. The latter finding contrasts with the incorporation of hydrogen from HO or tritium from HO in the monodirectional conversion of D-[2-H]glucose 6-phosphate and unlabelled D-glucose 6-phosphate, respectively, to their corresponding ketohexose esters. [ABSTRACT FROM AUTHOR]

The rate of conversion of D-glucose 6-phosphate to D-fructose 6-phosphate as catalyzed by yeast phosphoglucoisomerase is about fourfold lower when H, rather than H, is present on the C2 of D-glucose 6-phosphate. This difference appears to be due mainly to a change in maximal velocity, rather than affinity. Phosphoglucoisomerase also distinguishes between H and H in terms of either their intramolecular transfer from C2 to C1 or their incorporation from water on the C1 of D-fructose 6-phosphate. [ABSTRACT FROM AUTHOR]

In rat pancreatic islets, the apparent space of distribution of galactose is not different from that of other hexoses. In homogenates of islets or tumoral insulin-producing cells, galactose is phosphorylated at a very low rate relative to either glucose phosphorylation in the same tissues or galactose phosphorylation by liver homogenates. In intact islets, galactose increases modestly the glucose 6-phosphate content and is oxidized at a much lower rate than glucose. Galactose slightly increases insulin output in the presence of a stimulatory concentration of glucose but fails to provoke insulin release in the absence of glucose, whether in islets removed from rats fed a normal or galactose-rich diet. The low rate of galactose oxidation and its poor insulinotropic capacity appear attributable to the weak activity of galactokinase in pancreatic islets. [ABSTRACT FROM AUTHOR]

Aminooxyacetate, an inhibitor of cytosolic transamination reactions, inhibited insulin release evoked by either 2-ketoisocaproate or L-leucine in rat pancreatic islets incubated in the presence of L-glutamine or L-asparagine. As a rule, aminooxyacetate also inhibited the oxidation of these nutrient secretagogues and impaired the respiratory response of the islets to the combinations of nutrients. However, the oxidative and secretory response to the combination of L-leucine and L-glutamine was less severely affected by aminooxyacetate than that evoked by the three other combinations of exogenous nutrients. These findings reinforce the view that the stimulus-secretion coupling of insulin release in response to L-leucine and 2-ketoisocaproate in association with either L-glutamine or L-asparagine tightly depends on the oxidation of these nutrient secretagogues, on their effect upon O uptake and, within limits, on the intracellular site of generation of reducing equivalents in the pancreatic islet cells. [ABSTRACT FROM AUTHOR]

Nutrients which stimulate insulin secretion are currently thought to initiate the series of cellular events eventually leading to insulin release either by interacting with a stereospecific receptor system (the regulatory site hypothesis) or by acting as a fuel (the substrate site hypothesis) in the pancreatic B-cell. The latter hypothesis is supported by a number of observations indicating that the capacity of nutrients to stimulate insulin release is indeed highly dependent on their capacity to increase catabolic fluxes in isolated pancreatic islets. However, these observations do not rule out the existence of nutrient receptors in islet cells. For instance, a nonmetabolized analog of L-leucine stimulates insulin release by causing allosteric activation of glutamate dehydrogenase, which should be considered, therefore, as a receptor for certain amino acids. Likewise, the increase in glycolytic flux, which is associated with the process of glucose-stimulated insulin release, is attributable not solely to a mass action phenomenon but also to the activation of phosphofructokinase by fructose 2.6-bisphosphate. The biosynthesis of this activator may involve a glucose receptor system. The fact that certain nutrient secretagogues (e.g D-glucose and L-leucine) act in the B-cell both as substrates and enzyme activators permits reconciliation of the substrate site and regulatory site hypotheses for insulin release. [ABSTRACT FROM AUTHOR]

In order to investigate the phenomenon of B-cell desensitization to D-glucose, rat pancreatic islets were cultured for 20-44h in the presence of increasing concentrations of D-glucose in the 5.6 to 27.8 mM range, and then incubated for 30 to 120 min for measurement of secretory, metabolic and ionic variables. After culture in the presence of 5.6 mM D-glucose, the release of insulin evoked by D-glucose (16.7 mM) was less marked than that seen in islets cultured in the presence of 11.1 mM D-glucose. In the latter islets, the secretory response to D-glucose (8.3 mM or more) was still modest, especially over short periods of incubation, but was markedly enhanced by either theophylline or forskolin. The release of insulin evoked by D-glucose in the presence of theophylline was little affected by either Ca concentration of the culture medium or length of culture period (20h vs 44h). The culture-induced alteration in the responsiveness to D-glucose coincided with a smaller relative increase of D-[5H]glucose utilization, D-[U-C]glucose oxidation or netCa uptake at increasing concentrations of the hexose. It contrasted with a well-preserved secretory response to nonnutrient secretagogues. Although these findings could be interpreted as evidence of B-cell desensitization to D-glucose, the fact that the secretory behavior of the islets was not vastly different whether they were first cultured at physiological (8.3 mM) or higher (11.1 to 27.8 mM) concentrations of D-glucose suggests that this experimental design may not be an optimal model for the functional alteration of the B-cell in hyperglycemic non-insulin-dependent diabetic subjects. [ABSTRACT FROM AUTHOR]

Insulin release induced by 3-phenylpyruvate in rat pancreatic islets was unaffected by L-norleucine, L-valine, L-alanine and L-asparagine, but potentiated by L-glutamine, glycine and L-serine. The latter two amino acids also enhanced insulin secretion evoked by the combination of 3-phenylpyruvate and L-glutamine. Since 3-phenylpyruvate is known to stimulate insulin release by acting as a transamination partner and facilitating the catabolism of endogenous amino acids, the present findings suggest that the secretory efficiency of such a biochemical sequence depends upon the subcellular location of the transamination reaction and upon the interference of 3-phenylpyruvate with the transport of 2-keto acids into mitochondria. [ABSTRACT FROM AUTHOR]

Pancreatic polypeptide (PP) is a recently identified hormone produced by pancreatic endocrine cells. The islets of genetically obese mice (ob/ob, C57 BL/6J), which are suspected to lack a circulating satiety factor, contain relatively few of the PP-producing cells. Administration of bovine pancreatic polypeptide (bPP) reduces food intake and suppresses body weight gain in the hyperphagic obese mice. It is postulated that PP participates in the regulation of food intake in a manner as yet undefined. [ABSTRACT FROM AUTHOR]