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B Hellman

Publications and source records attributed to B Hellman.

At least 109 records · Page 6Linked to original sources
PubMed1987

Cadmium-induced insulin release does not involve changes in intracellular handling of calcium.

A possible interaction between Cd2+ and Ca2+ as a component in Cd2+-induced insulin release was investigated in beta cells isolated from obese hyperglycemic mice. The glucose stimulated Cd2+ uptake was dependent on the concentration of sugar. This uptake was sigmoidal with a Km for glucose of about 5 mM and was suppressed by both 50 microM of the voltage-activated Ca2+ channel blocker D-600 and 12 mM Mg2+. In the presence of 8 mM glucose 5 microM Cd2+ evoked a prompt and sustained stimulatory response, corresponding to about 3-fold of the insulin release obtained in the absence of the ion. Whereas 5 microM Cd2+ was without effect on the glucose-stimulated 45Ca efflux in the presence of extracellular Ca2+, 40 microM inhibited it. At a concentration of 5 microM, Cd2+ had no effect on the resting membrane potential or the depolarization evoked by either glucose or K+. In the absence of extracellular Ca2+ there was only a modest stimulation of 45Ca efflux by 5 microM Cd2+. Studies of the ambient free Ca2+ concentration maintained by permeabilized cells also indicate that 5 microM Cd2+ do not mobilize intracellularly bound Ca2+ to any great extent. On the contrary, at this concentration, Cd2+ even suppressed inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release. The present study suggests that Cd2+ stimulates insulin release by a direct mechanism which does not involve an increase in cytoplasmic free Ca2+ concentration.

Animals↗
PubMed1987

External ATP mimics carbachol in initiating calcium mobilization from pancreatic beta-cells conditioned by previous exposure to glucose.

1 Exposure to ATP (2-200 microM) resulted in a prominent peak of 45Ca efflux, when beta-cell-rich pancreatic islets from ob/ob-mice were perifused with a Ca2+-deficient medium. ADP and the stable alpha/beta-methylene analogues of ATP and ADP also had stimulatory effects. 2 The nucleotide initiation of 45Ca efflux mimicked that obtained with carbachol both in requiring previous exposure to glucose and in being more pronounced after replacing extracellular Na+ by K+. 3 It was possible to induce repeated peaks of stimulated 45Ca efflux, when the exposure to ATP was interrupted with intervals of perifusion with glucose-containing media. 4 The observations are consistent with the existence of P2-purinoceptors in islets, suggesting that these receptors mediate a similar mobilization of calcium as noted when activating polyphosphoinositide breakdown with carbachol. In view of the high contents of ATP and ADP in the beta-cell secretory granules, activation of P2-purinoceptors should be considered as a possible mechanism for amplification of the initial insulin secretory response.

Adenine Nucleotides↗
PubMed1987

Glucose stimulates the entry of Ca2+ into the insulin-producing beta cells but not into the glucagon-producing alpha 2 cells.

Rat pancreatic beta and alpha 2 cells were purified by autofluorescence-activated cell sorting and used for electrophysiological patch clamp studies and measurements of the initial uptake of 45Ca. Both beta and alpha 2 cells were electrically active, the action potentials of the latter cells also were detected in the absence of glucose. Furthermore, alpha 2 cells differed from beta cells in lacking a glucose-sensitive K+ channel with a single conductance of 50-60 pS (in symmetric 140 mM K+ solutions). The rate of Ca2+ entry into the alpha 2 cells was slower than that into the beta cells, being equivalent to 0.2 mmol, kg-1 dry wt min-1. Whereas raising the glucose concentration to 20 mM significantly increased the amount of Ca2+ entering the beta cells, the sugar was without effect on Ca2+ entry into the alpha 2 cells.

Animals↗
PubMed1987

Demonstration of glucose inhibition of insulin release in the presence of diazoxide.

beta-Cell-rich pancreatic islets from ob/ob mice were taken for measurements of insulin release in response to glucose after culture in RPMI 1640 medium. The stimulatory effect of 20 mmol/l glucose was converted into an inhibition when the medium was supplemented with 400 mumol/l diazoxide. Glucose inhibition of insulin release was observed when the islets had been cultured in the presence of 1 or 20 mmol/l glucose in media either containing or lacking Ca2+. The data provide further evidence for an inhibitory component in the action of glucose on insulin release, suggesting that glucose stimulation of the Ca2+ efflux is essential for the appearance of this inhibition.

Animals↗
PubMed1986

Accumulation of cadmium in pancreatic beta cells is similar to that of calcium in being stimulated by both glucose and high potassium.

The transport of Cd2+ and the effects of this ion on secretory activity and metabolism were investigated in beta cell-rich pancreatic islets isolated from obese-hyperglycemic mice. The endogenous cadmium content was 2.5 mumol/kg dry wt. After 60 min of incubation in a Ca2+-deficient medium containing 2.5 microM Cd2+ the islet cadmium content increased to 0.18 mmol/kg dry wt. This uptake was reduced by approx. 50% in the presence of 1.28 mM Ca2+. The incorporation of Cd2+ was stimulated either by raising the concentration of glucose to 20 mM or K+ to 30.9 mM. Whereas D-600 suppressed the stimulatory effect of glucose by 75%, it completely abolished that obtained with high K+. Only about 40% of the incorporated cadmium was mobilized during 60 min of incubation in a Cd2+-free medium containing 0.5 mM EGTA. It was possible to demonstrate a glucose-induced suppression of Cd2+ efflux into a Ca2+-deficient medium. Concentrations of Cd2+ up to 2.5 microM did not affect glucose oxidation, whereas, there was a progressive inhibition when the Cd2+ concentration was above 10 microM. Basal insulin release was stimulated by 5 microM Cd2+. At a concentration of 160 microM, Cd2+ did not affect basal insulin release but significantly inhibited the secretory response to glucose. It is concluded that the beta cell uptake of Cd2+ is facilitated by the activation of voltage-dependent Ca2+ channels. Apparently, the accumulation of Cd2+ mimics that of Ca2+ also involving a component of intracellular sequestration promoted by glucose.

Animals↗
PubMed1986

Unmasking of the inhibitory action of glucose on insulin release after alpha 2-adrenergic activation.

Insulin release in response to glucose was measured after culture of islets from ob/ob-mice in a Ca2+-deficient medium. The stimulatory effect of 20 mM glucose disappeared after addition of 1 microM L-epinephrine, and it was reversed into inhibition when the medium contained 0.1 to 10 microM clonidine. Glucose inhibited insulin release also after activation of the alpha 2-adrenoceptors with B-HT 933, whereas blocking of these receptors with idazoxan removed glucose inhibition in the presence of clonidine. It is concluded that alpha 2-adrenergic activation provides an efficient means of unmasking the inhibitory component in the action of glucose on insulin release.

Adrenergic alpha-Agonists↗
PubMed1986

Glucose-stimulated sequestration of Ca2+ in clonal insulin-releasing cells. Evidence for an opposing effect of muscarinic-receptor activation.

Net fluxes of Ca2+ and acid production were studied in clonal insulin-releasing cells (RINm5F) by using colour indicators and dual-wavelength spectrophotometry. After equilibration with a medium containing 10-20 microM-Ca2+, only minimal amounts of Ca2+ (0.08 mmol/kg of protein) were released from the cells by subsequent additions of the respiratory blocker antimycin A and the Ca2+ ionophore A23187. The presence of 20 mM-glucose resulted in an almost 5-fold increase of the acid production and in a stimulated net uptake of Ca2+. The latter process was independent of the extracellular Ca2+ concentration and reached saturation after 20 +/- 1 min, when it corresponded to 1.18 +/- 0.07 mmol of calcium/kg of protein. Whereas the thiol reagent iodoacetamide suppressed the acid production, interference with mitochondrial function by using antimycin A or the uncoupler carbonyl cyanide m-chlorophenylhydrazone had the opposite effect. The latter two drugs induced a selective release of Ca2+ from a pool containing 35% of that taken up during glucose exposure. Most of the remaining Ca2+ was liberated by A23187 or iodoacetamide. Carbamoylcholine was also selective in mobilizing glucose-stimulated calcium, but this calcium (17%) appeared to originate from the pool insensitive to mitochondrial poisons. The action of carbamoylcholine was blocked by atropine and did not depend on the presence of extracellular Na+. The opposite effects of glucose and muscarinic-receptor activation on a non-mitochondrial calcium pool are consistent with participation of the endoplasmic reticulum in the glucose-induced sequestration of Ca2+ in pancreatic beta-cells.

Adenoma, Islet Cell↗
PubMed1986

Glucose-induced reduction of the sodium content in beta-cell-rich pancreatic islets.

The sodium contents of beta-cell-rich pancreatic islets from ob/ob-mice were measured with an integrating flame photometer. After washing to an apparent steady state with different types of ice-cold media, islets incubated in the absence of glucose contained 79-108 mmol sodium kg-1 dry weight. Exposure to glucose resulted in 25% reduction of the islet content of sodium. This effect became manifest in the presence of 5 mM glucose, there being no additional reduction with a further increase of glucose to 20 mM. Depression of Na+ activity may partially explain why glucose, under certain conditions, can lower cytoplasmic Ca2+ and even inhibit insulin release.

Animals↗
PubMed1986

The tissue localization of m-AMSA and its effect on thymidine incorporation in various tissues in vivo.

The distribution of 14C-labelled m-AMSA was studied in rats and pigmented mice using whole body autoradiography. The agent rapidly disappeared from the blood, accumulating in significant amounts in large parenchymal organs, certain endocrine tissues, and the retina of the pigmented mouse eye. The hemopoietic and lymphoid tissues showed a moderate uptake of radioactivity with the highest concentration observed in the thymus. The autoradiograms indicated a rapid excretion of radioactivity via the liver, kidney and the glandular part of the gastric mucosa. The distribution pattern of label from 14C-m-AMSA remained unaffected by pretreatment of animals with high dose (500 mg kg-1 b.w.) of cytosine arabinoside. Injection of unlabelled m-AMSA (7 mg kg-1 b.w.) to growing rats 24 h before sacrifice resulted in a highly significant (P less than 0.001) inhibition of 3H-thymidine incorporation into the DNA of thymus and spleen. A less pronounced reduction was observed in the kidney, adrenal, lung and testes. The thymidine incorporation into the DNA of bone marrow was markedly suppressed when calculated per dry weight, but increased when related to the DNA content, suggesting early regeneration of the remaining cells. In contrast, no significant effects were observed on the DNA synthesis in small intestine and liver.

Aminoacridines↗
PubMed1986

Effects of carcinogenic halogenated aliphatic hydrocarbons on [3H]thymidine incorporation into various organs of the mouse. A comparison between 1,2-dibromoethane and 1,2-dichloroethane.

The effects of 1,2-dibromoethane (DBE) and 1,2-dichloroethane (DCE) on the incorporation of [3H]thymidine into DNA were evaluated in various tissues of mice. The compounds were given intraperitoneally 24 h before sacrifice in an equimolar dose (293 mumoles/kg body weight). 2 h before the animals were killed, 0.5 mu Ci [3H]thymidine/g body weight was injected intraperitoneally. Both agents inhibited the [3H]thymidine incorporation in the forestomach, a site for their carcinogenic action. Whereas DBE also suppressed the [3H]thymidine incorporation in the nasal mucosa, the thymus, and the "glandular stomach", DCE was inhibitory only in the kidney. The observed difference in the effect of DBE and DCE on the thymus had its counterpart in a DBE-induced decrease of acid-insoluble radioactivity, demonstrated with whole-body autoradiography. The results indicate that in vivo screening of [3H]thymidine incorporation into various organs of an intact experimental animal is a sensitive technique for comparing cyto- and/or genotoxic effects of chemicals with a similar chemical structure.

Animals↗
PubMed1986

Evidence for stimulatory and inhibitory effects of cadmium on the [3H]thymidine incorporation into various organs of the mouse.

The effects of cadmium chloride on the DNA turnover in various organs of the mouse were evaluated by measuring the incorporation of intraperitoneally injected [6-3H]thymidine. This approach is considered to be a useful complement to other short-term in vivo tests in the screening for genotoxic properties of chemicals. A moderate amount of CdCl2 (1 mg/kg body wt) lacked inhibitory effects on the incorporation of [3H] thymidine, but produced a significantly increased uptake of the DNA precursor into the liver. Whereas the genotoxic polycyclic aromatic hydrocarbon 3-methylcholanthrene suppressed the [3H]thymidine incorporation into several organs when given in a dose of 30 mg/kg body wt, cadmium chloride was inhibitory only when injected in a sublethal dose (4 mg/kg body wt). When the injected amount of CdCl2 was 4 mg/kg, an initial and transient inhibition of the [3H]thymidine incorporation was observed in several organs. After extending the time between the injection of cadmium and sacrifice to 72 h, such a high dose of cadmium produced potent stimulatory effects on the [3H]thymidine incorporation not only into the liver but also into the pancreas, kidney, small intestine, and testis. The mechanism behind the cadmium-induced stimulation of the DNA synthesis remains obscure but may be due to an increased biosynthesis of the cytoplasmatic protein metallothionein. The stimulatory effects of cadmium on the incorporation of [3H]thymidine correlate well both with reported sites of extensive accumulation of the heavy metal and the presence of high concentrations of cadmium-induced metallothionein.

Animals↗
PubMed1986

Rate of incorporation of [3H]thymidine in various tissues of the mouse. A basis for the evaluation of genotoxic effects of chemicals.

[3H]Thymidine has been extensively used as a selective precursor to DNA in studies on the kinetics of cell proliferation. We have become interested in measuring early inhibition of the DNA synthesis in various organs of intact animals for detecting genotoxic properties of chemicals. Such experiments should, for convenience and to achieve a large capacity, be performed in the simplest way possible. The present paper deals with some practical aspects on the use of [3H]thymidine in vivo. [6-3H]Thymidine was injected intraperitoneally in mice and the uptake of radioactivity was evaluated by using whole-body autoradiography and liquid scintillation spectrometry. Autoradiograms of sections washed with trichloroacetic acid and methanol were compared with those subjected only to freeze-drying. Liquid scintillation counting was performed of total, non-volatile, acid-insoluble and DNA-associated radioactivities. A rapid increase of the [3H]thymidine incorporation was seen during the first hour after the injection. Further prolongation of the survival time did not result in any significant increase of the incorporated radioactivity. Moreover, there were only slight differences between the autoradiograms from extracted and non-extracted sections. Radioactivities associated with DNA closely correlated to those representing acid-insoluble material, indicating that acid-insoluble radioactivity provides a good estimate of the [3H]thymidine incorporation into DNA.

Animals↗
PubMed1986

Effects of N-nitrosopyrrolidine and N-nitrosoproline on the incorporation of 3H-thymidine into the DNA of various organs of the mouse: tissue specificity and effects of ethanol consumption.

The effects of the carcinogenic N-nitrosamine N-nitrosopyrrolidine (NPYR) and the non-carcinogenic N-nitrosamino acid N-nitrosoproline (NPRO) on 3H-thymidine incorporation into DNA were evaluated in various organs of male C57BL mice. The N-nitroso compounds were given intraperitoneally 24 hrs before sacrifice in equimolar amounts (148 mumol/kg b.wt.). Two hours before the mice were killed, they were given an intraperitoneal injection of 3H-thymidine. NPYR, but not NPRO, induced a tissue-specific inhibition of 3H-thymidine incorporation into DNA. The inhibition occurred only in organs reported to be involved in the biotransformation of NPYR, i.e., the liver, lung, and nasal mucosa. Daily oral consumption of ethanol (1.8 ml 30% ethanol for 30 days) had no effects in itself on 3H-thymidine incorporation, but resulted in an enhancement of the inhibitory action of NPYR in the lung and nasal mucosa.

Animals↗
PubMed1986

Modifications of the pancreatic beta-cell function after lowering their potassium content.

beta-cell-rich pancreatic islets from ob/ob-mice were kept for 3 days in a culture medium and analysed for their content of potassium. In a normal ionic milieu intracellular potassium was calculated as 190-260 mM. Whereas this concentration remained essentially unaffected after lowering extracellular K+ to 1.5 mM, further reduction to 0.15 mM depressed islet potassium to 5% or less of its original value. Irrespective of its medium concentration, potassium in the islets increased when glucose was raised from 1 to 20 mM. Depression of the islet potassium was associated with a rise of intracellular calcium. Despite profound depletion of potassium, the beta-cells maintained their insulin content and could still oxidize glucose at a substantial rate. When potassium was suppressed to 25% or less of the original content, the beta-cells responded to glucose with a paradoxical inhibition of insulin release. After 3 days of potassium depletion, exposure to a normal ionic milieu neither restored the intracellular content of potassium nor a stimulated insulin secretory response to glucose.

Animals↗
PubMed1986

Mobilization of different intracellular calcium pools after activation of muscarinic receptors in pancreatic beta-cells.

Exposure to carbachol resulted in a biphasic stimulation of 45Ca efflux when beta-cell-rich pancreatic islets from ob/ob mice were perifused with a Ca2+-deficient medium. The pattern of stimulated 45Ca efflux was markedly modified by glucose. Whereas the initial carbachol-stimulated phase was conditional on previous exposure to glucose, the subsequent phase was completely suppressed by 20 mmol/l of the sugar. The stimulatory could be clearly separated also on the basis of a Na+ dependence. Removal of extracellular Na+ resulted in a disappearance of the second phase, but it was still possible to induce a prominent initial peak if depletion of intracellular K+ was prevented when Na+ was omitted. It is concluded that activation of muscarinic receptors in the pancreatic beta-cells results in mobilization of calcium from more than one intracellular pool. Whereas the second phase of stimulated efflux can be explained in terms of an increased entry of Na+ into the beta-cells, the initial stimulation may be due to receptor-mediated breakdown of polyphosphoinositides.

Animals↗
PubMed1986

Inositol 1,4,5-trisphosphate mobilizes glucose-incorporated calcium from pancreatic islets.

Mobilization of intracellular calcium from beta-cell-rich pancreatic islets of ob/ob-mice was studied by measuring unidirectional 45Ca efflux at 37 degrees and 18 degrees C during perifusion with a K+-rich medium deficient in Ca2+ and Na+. Addition of 100 microM carbachol induced a prominent peak of Ca2+ efflux from islets preexposed to glucose. After cell permeabilization with digitonin D-myo-inositol 1,4,5-trisphosphate (IP3) caused glucose-dependent mobilization of calcium. In demonstrating that not only carbachol but also IP3 can mobilize calcium incorporated in response to glucose, the present data suggests that the endoplasmic reticulum participates in glucose-induced lowering of cytoplasmic Ca2+ activity in the pancreatic beta-cells.

Animals↗