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B Hellman

Publications and source records attributed to B Hellman.

At least 163 records · Page 9Linked to original sources
PubMed1981

Calcium and pancreatic beta-cell function. 12. Modification of 45Ca fluxes by excess of K+.

Glucose stimulation of insulin release is supposed to result from depolarization of the pancreatic beta-cells with subsequent influx of Ca2+. Isolated islets from non-inbred ob/ob-mice were employed for elucidating whether the glucose effects on the beta-cell handling of Ca2+ could be simulated by the depolarization evoked by excess of K+. Addition of 25 mM K+ was as effective as 20 mM glucose in stimulating the intracellular uptake of 45Ca. In both instances the additional amounts of incorporated 45Ca appeared in the mitochondria and the secretory granules. When analysing the washout pattern for 45Ca it was evident that the effects of raising K+ differed from those evoked by glucose. Whereas glucose inhibited 45Ca efflux during perifusion with Ca2+-deficient medium the addition of K+ resulted in a slight stimulation. Furthermore, the 45Ca incorporated in response to K+ was more readily mobilised.

Animals↗
PubMed1980

Calcium and pancreatic beta-cell function. The mechanism of insulin secretion studied with the aid of lanthanum.

La3+ was used to study the involvement of Ca2+ in insulin secretion in beta-cell-rich pancreatic islets micro-dissected from non-inbred ob/ob mice. Ultrastructural studies revealed that the localization of La3+ was entirely restricted to the exterior of the cells. Consistent with a membrane action, exposure to La3+ failed to affect glucose oxidation and either the sucrose space or the general ultrastructure of the islets. In contrast, La3+ had marked effects on insulin release and 45Ca fluxes. Exposure to La3+ resulted in pronounced inhibition of insulin release irrespective of the presence or absence of Ca2+, 3-isobutyl-1-methylxanthine or glucose. Perifusion experiments revealed that the inhibitory action was prompt, sustained and readily reversible. Removal of La3+ was associated with a subsequent prolonged stimulatory phase of insulin release even in medium deficient in Ca2+. This action could not be attributed to an increase in cyclic AMP, but was potentiated by 3-isobutyl-1-methylxanthine and abolished by L-adrenaline. La3+ displaced 45Ca from superficially located binding sites and inhibited the uptake and efflux of 45Ca. The stimulatory and inhibitory actions of glucose on 45Ca efflux were also abolished in the presence of 2 mM-La3+ Removal of La3+ was associated with the preferential mobilization of 45Ca incorporated in response to glucose. The results indicate that binding of La3+ to superficial sites in the plasma membrane leads to inhibition of insulin release by suppression of transmembrane Ca2+ fluxes. It is suggested that accumulation of Ca2+ in the cytoplasm accounts for the stimulation of insulin release seen after removal of La3+ from inhibitory binding sites in the beta-cell plasma membrane.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Calcium and pancreatic beta-cell function. 7. Evidence for cyclic AMP-induced translocation of intracellular calcium.

The effect of cyclic AMP on calcium movements in the pancreatic beta-cell was evaluated using an experimental approach based on in situ labelling of intracellular organelles of ob/ob-mouse islets with 45Ca. Whereas the glucose-stimulated 14Ca incorporation by mitochondria and secretory granules was increased under a condition known to reduce cyclic AMP (starvation), raised levels of this nucleotide (addition of 3-isobutyl-1-methylxanthine or N6,O2'-dibutyryl adenosine 3',5'-cyclic monophosphate) reduced the mitochondrial accumulation of 45Ca. Conditions with increased cyclic AMP were associated with a stimulated efflux of 45Ca from the secretory granules but not from the mitochondria. The microsomal fraction differed from both the mitochondrial and secretory granule fractions by accumulating more 45Ca after the addition of 3-isobutyl-1-methylxanthine. The results suggest that cyclic AMP potentiates glucose-stimulaated insulin release by increasing cytoplasmic Ca2+ at the expense of the calcium taken up by the organelles of the pancreatic beta-cells.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Calcium and pancreatic beta-cell function. IX. Demonstration of lanthanide-induced inhibition of insulin secretion independent of modifications in transmembrane Ca2+ fluxes.

beta-Cell-rich pancreatic islets were microdissected from noninbred ob/ob-mice and used to examine the mode of action of trivalent lanthanide ions on insulin secretion. La3+, Sm3+, and Tm3+ were equally effective inhibitors of basal and glucose-stimulated insulin release. As indicated by perifusion experiments with Tm3+, the inhibitory action was prompt, sustained, and readily reversible. Despite the similarities among the lanthanides in inhibiting insulin secretion, these cations differed considerably in their ability to impair transmembrane 45Ca fluxes. Using 10 different members of the lanthanide series, it was possible to demonstrate that their effectiveness to inhibit 45Ca uptake increased with ionic radius. La3+ markedly inhibited intracellular uptake and superficial binding of 45Ca at both 3 and 20 mM glucose. However, Tm3+ failed to affect intracellular 45Ca uptake and only reduced superficial binding of 45Ca at 3 mM glucose. In efflux experiments, Tm3+ did not affect basal or glucose-stimulated 45Ca washout from islets perifused with a medium containing 1.28 mM Ca2+. In a Ca2+-deficient medium, Tm3+ caused a slight transient increase, followed by reduction of 45Ca washout. However, when glucose was omitted, there was a prompt increase in the washout of radioactivity in the presence of Tm3+. Accordingly, the potent inhibitory action of Tm3+ on insulin secretion is not matched by changes in transmembrane Ca2+ fluxes. Since the lanthanides do not penetrate intracellularly, we propose the existence of cationic binding sites in the beta-cell plasma membrane with direct inhibitory effects on insulin secretion.

Animals↗
PubMed1980

Ca2+ transport in pancreatic beta-cells during glucose stimulation of insulin secretion.

The role of Ca2+ in the regulation of insulin secretion was evaluated using beta-cell-rich pancreatic islets isolated from ob/ob-mice. The glucose stimulation of the secretory activity is supposed to result from accumulation of Ca2+ in the submembrane cytoplasmic space. It is likely that this process reflects the balance between increased entry of Ca2+ into the beta-cells and an enhanced sequestration of Ca2+ in the organelle sinks. The proposed model can explain the cAMP potentiation of glucose-stimulated insulin release with suppression of the mitochondrial Ca2+ uptake. Furthermore, differences in the Ca2+ buffering capacity of the secretory granules may account for other characteristic features of glucose-stimulated insulin release, in particular its biphasic nature and sensitivity to suppression on withdrawal of nutrients.

Adenosine Triphosphate↗
PubMed1980

Calcium movements in relation to glucose-stimulated insulin secretion.

The influence of glucose on the beta-cell handling of Ca2+ was studied in pancreatic islets isolated from ob/ob mice. Glucose had both stimulatory and inhibitory effects on the washout of radioactivity from islets preloaded with 45Ca. Although the phenomenon of stimulation may be essentially associated with an increased turnover of 45Ca incorporated in response to glucose, the inhibitory effect might merely reflect a lowering of Ca2+ in the cytoplasm following from its accumulation into secretory granules and mitochondria. It is suggested that the Ca2+ uptake by the granules is mediated by an ATP-dependent proton gradient and that these organelles serve as a regulator of the cytoplasmic Ca2+ involved in stimulus-secretion coupling. Alterations in the beta-cell handling of Ca2+ may not only explain the role of glucose as an initiator of insulin release, but also the potentiation by cAMP of the action of glucose. The latter effect can tentatively be ascribed to an increase of cytoplasmic Ca2+ following cAMP-induced inhibition of the net Ca2+ uptake by mitochondria.

1-Methyl-3-isobutylxanthine↗
PubMed1979

Calcium and pancreatic beta-cell function. 6. Glucose and intracellular 45Ca distribution.

Glucose stimulates the uptake of 45Ca into beta-cell-rich pancreatic islets isolated from ob/ob-mice. The distribution of the incorporated radioactivity was analysed by labelling the organelles with 45Ca in their cellular environment. The radioactive content of the organelles was measured after homogenization and fractionation of the islets under conditions preventing 45Ca redistribution. The 45Ca taken up in response to glucose appeared essentially in the secretory granule fraction and in that enriched in mitochondria. Modification of the 45Ca loading procedure, involving reduction of the oxygen tension and incubation volume, resulted in the disappearance of the glucose effect on the mitochondrial fraction whereas part of the stimulatory effect on the secretory granules persisted. Buffering of calcium by the secretory granules and mitochondria may be important for regulating the cytoplasmic Ca2+ involved in stimulus-secretion coupling.

Animals↗
PubMed1979

Effects of ouabain on insulin release, adenosine 3',5'-monophosphate and guanine 3',5'-monophosphate in pancreatic islets.

Isolated pancreatic islets of noninbred ob/ob mice were used to test the hypothesis that adenylate cyclase responds to changes of the transmembrane milieu or electric field in intact beta-cells. In the presence of a phosphodiesterase inhibitor, ouabainstimulated both the release of insulin and the islet content of cAMP. Ouabain had no noticeable effect on the islet content of cGMP. These results support the hypothesis at test. However, because ouabain also had some stimulatory effect on cAMP in islet homogenates, a direct action of ouabain on adenylate cyclase cannot be ruled out.

1-Methyl-3-isobutylxanthine↗
PubMed1979

Evidence for divergent glucose effects on calcium metabolism in pancreatic beta- and alpha 2-cells.

Elucidation of the role of Ca2+ in the secretion of insulin and glucagon is complicated by the presence of different types of cells in the pancreatic islets. Visualization of calcium in sections of guinea pig pancreas with the histochemical reagent glyoxal bis-2-hydroxyanil revealed the most intense staining in the endocrine part but no differences between various islet cell types. A procedure for eliminating the majority of the beta-cells by streptozotocin injection in the guinea pig enabled a comparison of collagenase-isolated islets rich in alpha 2-cells with islets from untreated animals rich in beta-cells. The latter islets contained 24.6 +/- 2.4 mmol calcium/kg dry wt, as estimated by flameless atomic absorption spectrophotometry. This is twice as much as noted for the exocrine pancreas or the islets rich in alpha 2-cells. After storage for 3 days in culture medium, the two types of islets contained similar amounts of calcium. The cultured islets displayed differences related to cellular composition when measuring the incorporation of 45Ca into a lanthanum-nondisplaceable (intracellular) pool. In the presence of 3 mM glucose, more 45Ca was incorporated into the islets rich in alpha 2-cells. Increasing the glucose concentration to 20 mM with or without further addition of 30 U/liter bovine insulin was without effect on the 45Ca uptake into the islets rich in alpha 2-cells but stimulated that into islets rich in beta-cells. The different calcium dependence on glucose in the two types of islets may indicate that increased uptake of Ca2+ is a component of the mechanism for the secretion of both insulin and glucagon.

Animals↗
PubMed1979

Calcium and pancreatic beta-cell function. 5. Mobilisation of a glucose-stimulated pool of intracellular 45Ca by metabolic inhibitors and the ionophore A-23187.

Glucose is believed to stimulate incorporation of calcium into the secretory granules of the pancreatic beta-cells. The mechanism of the glucose-stimulated accumulation of calcium in the granule pool was evaluated by measuring fluxes of 45Ca in beta-cell-rich pancreatic islets microdissected from ob/ob-mice. The incorporation of lanthanum-nondisplaceable 45Ca in response to phosphate in being suppressed by 10 micrometer antimycin A, 0.3 mM 2,4-dinitrophenol or 1 mM N-ethylmaleimide. Exposure to each of these metabolic inhibitors also resulted in a protracted efflux of the glucose-sensitive 45Ca under conditions when neither the 45Ca incorporated in the presence of 3 mM glucose nor in response to phosphate was significantly affected. The glucose-stimulated intracellular 45Ca existed in a state allowing it to be washed out with the ionophore A-23187. The results suggest that the glucose-stimulated incorporation of calcium into the secretory granules is mediated by transport against a concentration gradient into the granule sac.

Animals↗
PubMed1978

Effect of Na+, K+ and Mg2+ on 45Ca+ uptake by pancreatic islets.

Microdissected pancreatic islets of noninbred ob/ob-mice were used to study ionic effects on the lanthanum-nondisplaceable 45Ca2+ uptake by islet cells. Omission of Mg2+ from the incubation medium had no effect, but the 45Ca2+ uptake was increased by omission of Na+ and decreased by omission of K+. Excess Mg2+ (1.2--15 mM) inhibited and excess K+ (4.7--25 mM) stimulated the 45Ca2+ uptake in a concentration-dependent manner. Stimulation of 45Ca2+ uptake in Na+-deficient islets was associated with an enhancement of the basal insulin release. Total abolishment of glucose-stimulated 45Ca2+ uptake in K+-deficient islets did not preclude a significant secretory reponse to glucose. It is concluded that the lanthanum-nondisplaceable 45Ca2+ uptake shows a partial correlation to insulin release.

Animals↗
PubMed1978

Calcium and pancreatic beta-cell function. 4. Evidence that glucose and phosphate stimulate calcium-45 incorporation into different intracellular pools.

beta-Cell-rich pancreatic islets were microdissected from ob/ob-mice and used for studies of 45Ca uptake and washout. Irrespective of whether the experiments were performed at 21 or 37 degrees C both glucose and phosphate stimulated the net uptake of lanthanum-nondisplaceable 45Ca. The stimulatory effect of phosphate was additive to that produced by glucose. 45Ca incorporated in response to phosphate differed from that taken up in the presence of 20 mM glucose in being easily washed out although it was not affected by the glucose concentration of the washing medium. The efflux of 45Ca was reduced after introducing phosphate into a medium used to perifuse islets which had accumulated 45Ca in response to 20 mM glucose. This suggests that the outward calcium transport can be influenced also by intracellular trapping of the cation. The glucose-stimulated insulin release was inhibited by phosphate; an effect reversed by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. It is concluded that a common effect of glucose and phosphate is to trap calcium in the pancreatic beta-cells but that there are fundamental differences between their effects on intracellular distribution of calcium and on insulin release.

Animals↗
PubMed1978

Calcium and pancreatic beta-cell function. 2. Mobilisation of glucose-sensitive 45Ca from perifused islets rich in beta-cells.

beta-Cell-rich pancreatic islets were microdissected from ob/ob-mice and loaded with 45Ca in the presence of 3 or 20 mM glucose. Subsequent measurements of the effluxes of radioactivity in a perifusion apparatus revealed that the slowly exchangeable 45Ca taken up in response to glucose was also preferentially mobilised by this compound. Glucose stimulation of 45Ca efflux was abolished after omission of calcium from the perifusion medium but persisted when insulin release was inhibited by prolonged starvation, addition of L-epinephrine or lowering of temperature. The presence of a stimulated efflux of radioactivity even under conditions of inhibited insulin release indicates that sources other than beta-granules ejected by exocytosis contribute to the additional 45Ca released after raising the glucose concentration of the perifusion medium. It is suggested that the beta-cell depolarisation as such may account for part of the 45Ca mobilised by glucose.

Animals↗
PubMed1978

Metabolism of cold-stored pancreatic islets.

A previous study showed that the ability of glucose to stimulate insulin release was retained in islets stored at 8 degrees C for one week provided that glucose was present in a high concentration in the storage medium. The metabolic properties of islets stored in the cold have now been further explored in an attempt to clarify the protective effect of glucose. During storage in the cold the islet formation of 3H2O from (5--3H) glucose and oxygen consumption were only a few per cent of that of fresh islets whereas the putake of 86Rb+ was 20--48%. Rewarming the cold-stored islets to 37 degrees C after one week of cold-storage restored the 86Rb+ uptake, the formation of 3H2O and 14CO2 from labelled glucose and oxygen consumption to 75, 80, 60 and 40% respectively of fresh islet levels. The results emphasize the usefulness of cold-storage for preservation of functionally intact isolated islets.

Animals↗