Boswellic Acids and Their Role in Chronic Inflammatory Diseases.

Boswellic acids, which are pentacyclic triterpenes belong to the active pharmacological compounds of the oleogum resin of different Boswellia species. In the resin, more than 12 different boswellic acids have been identified but only KBA and AKBA received significant pharmacological interest. Biological Activity: In an extract of the resin of Boswellia species multiple factors are responsible for the final outcome of a therapeutic effect, be it synergistic or antagonistic. Moreover, the anti-inflammatory actions of BAs are caused by different mechanisms of action. They include inhibition of leukotriene synthesis and to a less extend prostaglandin synthesis. Furthermore inhibition of the complement system at the level of conversion of C3 into C3a and C3b. A major target of BAs is the immune system. Here, BEs as well as BAs including KBA and AKBA, have been shown to decrease production of proinflammatory cytokines including IL-1, IL-2, IL-6, IFN-γ and TNF-α which finally are directed to destroy tissues such as cartilage, insulin producing cells, bronchial, intestinal and other tissues. NFĸB is considered to be the target of AKBA. The complex actions of BEs and BAs in inflamed areas may be completed by some effects that are localized behind the inflammatory process as such tissue destruction. In this case, in vitro- and animal studies have shown that BAs and BEs suppress proteolytic activity of cathepsin G, human leucocyte elastase, formation of oxygen radicals and lysosomal enzymes. PHARMACOKINETICS: Whereas KBA is absorbed reaching blood levels being close to in vitro IC50, AKBA which is more active in in vitro studies than KBA, but undergoes much less absorption than KBA. However, absorption of both is increased more than twice when taken together with a high-fat meal.Clinical Studies There are a variety of chronic inflammatory diseases which respond to treatment with extracts from the resin of Boswellia species. Though, the number of cases is small in related clinical studies, their results are convincing and supported by the preclinical data. These studies include rheumatoid arthritis, osteoarthritis, chronic colitis, ulcerative colitis, collagenous colitis, Crohn's disease and bronchial asthma. It can not be expected that there is cure from these diseases but at least improvement of symptoms in about 60-70 % of the cases. Side Effects The number and severity of side effects is extremely low. The most reported complaints are gastrointestinal symptoms. Allergic reactions are rare. And most authors report, that treatment with BEs is well tolerated and the registered side effects in BE- and placebo groups are similar.

11-Keto-ß-Boswellic Acids Prevent Development of Autoimmune Reactions, Insulitis and Reduce Hyperglycemia During Induction of Multiple Low-Dose Streptozotocin (MLD-STZ) Diabetes in Mice.

The aim of the work was to study whether or not 11-keto-ß-boswellic acids prevent induction of autoimmune reactions, insulitis, and hyperglycemia in the model of multiple low-dose streptozotocin (MLD-STZ) diabetes. Using male mice (n = 6) diabetes was induced by daily i.p. injections of 40 mg/kg STZ for 5 days. In a second series together with STZ, daily i. p. injections of 11-keto-ß-boswellic acid (KBA) and O-acetyl-11-keto-ß-boswellic acid (AKBA) (7.5 and 15.0 mg/kg) were applied for 10 days. Thereafter, pro-and anti-inflammatory cytokines in the blood, histochemistry of pancreatic islets, and blood glucose levels were assayed. Five days after the last injection of STZ, a significant burst of pro-and anti-inflammatory cytokines in the blood, infiltration of lymphocytes (CD3) into pancreatic islets, and appearance of peri-insular apoptotic cells were observed. Plasma glucose increased significantly (124.4 ± 6.65 vs. 240.2 ± 27.36 mg/dl, p <0.05). Simultaneous treatment with KBA and AKBA significantly reduced pro-and anti-inflammatory cytokines (IFN-γ p < 0.01, p < 0.01; IL-1A p < 0.001, p < 0.001; IL-1B p < 0.001, p < 0.001; IL-2 p < 0.001, p < 0.001; IL-6 p < 0.01, p < 0.001; TNF-α p < 0.05, p < 0.001; IL-4 p < 0.01, p < 0.001; IL-10 p < 0.001, p < 0.001) in the blood. No infiltration of lymphocytes into pancreatic islets and appearance of peri-insular cells were detected. Moreover, KBA and AKBA reduced STZ-mediated increase of blood glucose on day 10 to 163.25 ± 16.6 (p < 0.05) and 187.6 ± 19.5 mg/dl (p < 0.05), respectively. In the model of MLD-STZ induced diabetes KBA and AKBA prevent cytokine burst, development of insulitis and reduce increase of blood glucose through "silencing" a forced-up immune reaction.

Boswellic extracts and 11-keto-ß-boswellic acids prevent type 1 and type 2 diabetes mellitus by suppressing the expression of proinflammatory cytokines.

BACKGROUND: Type 1 diabetes is an autoimmune disease directed to the pancreatic islets where inflammation leads to the death of insulin-producing ß cells and insulin deficiency. Type 2 diabetes, which is closely related to overweight, is characterized by insulin resistance. In both cases, proinflammatory cytokines play an important role by causing insulitis and insulin resistance. The gum resin of Boswellia species and its pharmacologically active compounds, including 11-keto-ß-boswellic acids have been shown to suppress the expression of proinflammatory cytokines in various immune-competent cells. PURPOSE: To review the present evidence of the therapeutic effects of boswellic extracts (BE) and/or 11-keto-ß-boswellic acids in the prevention/treatment of diabetes mellitus and to provide comprehensive insights into the underlying molecular mechanisms. METHODS: This review considers all available informations from preclinical and clinical studies concerning BEs, 11-keto-ß-boswellic acids, proinflammatory cytokines and diabetes mellitus collected via electronic search (PubMed) and related publications of the author. RESULTS: Type 1 diabetes: Studies in mice with autoimmune diabetes revealed that in the model of multiple injections of low doses of streptozotocin (MLD-STZ), an extract of the gum resin of Boswellia serrata and 11-keto-ß-boswellic acid (KBA) suppressed the increase in proinflammatory cytokines in the blood, infiltration of lymphocytes into pancreatic islets and increase in blood glucose. In a second model, i.e. the nonobese diabetic (NOD) mouse, KBA prevented the infiltration of lymphocytes into pancreatic islets. Regarding the clinical effects, a case report provided evidence that BE suppressed the blood levels of tyrosine phosphatase antibody (IA2-A), a marker for insulitis, in a patient with late-onset autoimmune diabetes of the adult (LADA). Type 2 diabetes: In a preclinical study in rats where obesity was alimentary induced, the administration of BE significantly reduced food intake, overweight, proinflammatory cytokines such as interleukin-1ß (IL-1ß), and tumor necrosis factor-α (TNF-α) and ameliorated the parameters of glucose and lipid metabolism. Similar results were obtained in a second animal study, where type 2 diabetes was induced by a combination of a high-fat/high-fructose diet and a single dose of streptozotocin. Two clinical trials with patients with type 2 diabetes receiving the resin of Boswellia serrata demonstrated improvement in the blood glucose, HbA1c and lipid parameters. CONCLUSION: Preclinical and clinical data suggest that BE and/or 11-keto-ß-boswellic acids by inhibiting the expression of proinflammatory cytokines from immune-competent cells, may prevent insulitis and insulin resistance in type 1 and type 2 diabetes, respectively, and therefore may be an option in the treatment/prevention of type 1 and type 2 diabetes. It is hypothesized that molecularly, BE and 11-keto-ß-boswellic acids act via interference with the IκB kinase/Nuclear Transcription Factor-κB (IKK/NF-κB) signaling pathway through inhibition of the phosphorylation activity of IKK. However, further investigations and well-designed clinical studies are required.

Evidence for presence of insulin receptors in rat islets of Langerhans.

Binding of insulin to islets of Langerhans was studied. It was found that "specific" binding of [125I]insulin ("specific" binding equals total binding minus nonspecific binding) was saturable with respect to time and insulin concentration and depended on the number of incubated islets. Furthermore, bound insulin was displaced by native insulin in a dose-dependent manner. Bound [125I]insulin was easily dissociated and there was little [125I]insulin degradation both in the incubation medium and during the processes of binding and dissociation. Scatchard analysis of experiments with increasing [125I]insulin concentration and with displacement of insulin binding by native insulin revealed "high affinity" binding sites with a dissociation constant of 0.461 +/- 0.08 n M and 3.5 X 10(6) high affinity binding sites per islet. There also existed "low affinity" binding sites with dissociation constant (Kd) of 43.9 +/- 11.6 nM and 5.9 X 10(7) low affinity binding sites. High affinity binding sites of islets from rats pretreated with alloxan decreased by about one half, whereas Kd was unaffected. Because the Kd of specific high affinity binding and mean effective dose (ED50) of the biological effects of insulin on normal pancreatic islets are in the same range (between 0.46 and 1.19 nM), the insulin-receptor interaction may be biologically significant.

[Involving the pharmacist in the management and care of diabetics]. / Diabetiker-Versorgung in Deutschland. Soil man die Apotheker mit einbinden, und wenn ja, wie?

Diabetes mellitus is an endemic disease in all countries of the world, with the type 2 form reaching epidemic proportions. In addition to treatment by the physician, it necessitates a considerable effort in terms of providing victims with training, care and information. Within this concept, pharmaceutical care by the pharmacist in the area of prevention must be regarded as multifaceted. In this connection his activities must be strictly differentiated from those of the physician. With regard to the qualifications needed, the pharmacist must supplement the knowledge gained during his studies by intensive courses in the field of diabetology. These are organized by the regional Chamber of Pharmacists and the regional offices of the German Diabetes Association (DDG). Thanks to the fact that he is readily accessible and well qualified, the pharmacist can thus make a major contribution to the individual management of the diabetic.

The significance of mu- and delta-receptors in rat pancreatic islets for the opioid-mediated insulin release.

The binding and the insulinotropic effects of enkephalin analogs and of morphine were investigated in rat pancreatic islets. Binding of [3H]Met-enkephalin was saturable, specific and reversible; the rank order for inhibition competition of [3H]Met-enkephalin binding by various compounds was Met-enkephalin = D-Ala2-MePhe4, Met(0)ol enkephalin) greater than Leu-enkephalin greater than morphine with half-maximal inhibitory constants (IC50) of approx. 0.3, 0.3, 100 and greater than 100 nM, respectively. Both the natural enkephalins exerted their insulinotropic effect only at stimulatory glucose concentrations. They had a dual action; whereas insulin secretion was increased at low enkephalin concentration, this effect was reversed at higher concentrations. However, the various enkephalins exerted this effect at different concentrations; only the EC50 values (half-maximal effective concentrations) of their insulinotropic effect were in the same range as the IC50 values of inhibition of [3H]met-enkephalin binding. Cysteamine pretreatment of rats (depletion of somatostatin containing D-cells and decrease in somatostatin secretion) did not change the Met-enkephalin effect on insulin secretion. In contrast to Met-enkephalin, binding of [3H]morphine to islets was not saturable, and morphine had no effect on insulin secretion unless at unphysiologically high concentrations. The data, therefore, indicate that: mu-receptors (affinity for morphine) do not play a role in rat pancreatic islets; delta-receptors (binding site for Met-enkephalin when mu-receptors are not present) mediate the insulinotropic effect of low Met-enkephalin concentrations; and the insulinotropic action of Met-enkephalin is not mediated indirectly via the paracrine effect of an inhibition of somatostatin secretion.

Properties of the insulin receptor of rat pancreatic islet.

Rat pancreatic islets have been shown to possess specific binding sites for 125I-labeled insulin. Enzymatic and chemical modification of islets are used to reveal important structures and chemical groups for insulin binding. pretreatment with trypsin, neuraminidase, 1-ethyl-3-(3-dimethylamino)carbodiimide (a carboxyl reagent), tetranitromethane (a tyrosyl and thiol reagent), and 1,3-difluoro-4,6-dinitrobenzene (modification of protein functional groups) decreased binding of insulin. This was due to the diminuation of the receptor number; in the case of trypsin-pretreatment also the receptor affinity was decreased. Inhibition of insulin binding was in each case associated with a decrease of the inhibitory effect of exogenous insulin on glucose-induced insulin secretion (not measured in the case of difluorodinitrobenzene and tetranitromethane). Phospholipase A2, (cleavage of phospholipids) did not affect these parameters. 5,5'-dithiobis(2-nitrobenzoic acid) (Ellman's reagent) and possibly p-chloromercuribenzoate (both thiol reagents) increased the number of receptors and decreased receptor affinity, but did not influence the inhibitory effect of insulin on insulin release. It is concluded that protein functional groups, sialic acid, carboxyl and tyrosyl groups, but no phospholipids and probably not sulfhydryl groups are important for the interaction of insulin with insulin receptors of rat pancreatic islets.

Evidence that cholecystokinin interacts with specific receptors and regulates insulin release in isolated rat islets of Langerhans.

To determine the nature of the pancreatic islet cell cholecystokinin (CCK) receptor, we studied CCK receptor binding and biologic activity in isolated rat pancreatic islets. Binding of 70 pM 125I-CCK to collagenase-prepared isolated rat pancreatic islets at 24 degrees C was one-half maximal after 5 min and maximal at 60 min. At 60 min, specific binding was 12% of total radioactivity per 100 micrograms islet protein; nonspecific binding (in the presence of 1 microM CCK 8) was less than 2% of total radioactivity. Unlabeled CCK 33 inhibited labeled hormone binding one-half maximally at 2 nM; Scatchard analysis showed one binding site (Kd, 2.3 +/- 0.4 nM; Bmax, 8.1 pmol/mg protein). The agonist selectivity of this binding site was: CCK 8 = CCK 33 greater than desulfated-CCK 8 greater than CCK 4. Two CCK antagonists were studied; N-carbobenzoxy-L-tryptophan was more potent than dibutyryl-cGMP. When the effect of CCK on insulin release from the islets was studied, the order of potency of CCK agonists and antagonists on insulin secretion was the same as the order of their ability to inhibit 125I-CCK binding. The effect of CCK on insulin secretion was dependent on the glucose concentration in the media. CCK had no effect at 5.6 mM glucose and was fully effective at 11.0 mM glucose. These data, therefore, indicate that: specific binding sites for CCK are present in rat pancreatic beta cells; and CCK acts in concert with glucose to stimulate insulin secretion.

6-Phosphogluconate/glucose-6-phosphate ratio in rat pancreatic islets during inhibition of insulin release by exogenous insulin.

Inhibition of glucose-stimulated insulin release by exogenous insulin has been demonstrated in pancreatic islets to be associated with a decrease of the NADPH/NADP ratio and the pentose-phosphate cycle activity. Batches of five islets were incubated for 15 and 90 minutes in 1 ml. of KRB buffer with 2 per cent albumin containing 3 mg./ml. glucose and 0, 200, 400, or 800 microU./ml. of rat insulin, and the glucose-6-phosphate (G6P) and 6-phosphogluconate (6PG) contents were determined by enzymatic cycling. In response to a rise in the concentration of insulin, the 6PG/G6P ratio decreased. A close relationship was observed between this decrease of 6PG/G6P ratio and the net insulin release, the absolute rate of glucose oxidation via the pentose phosphate cycle, and the NADPH/NADP ratios measured under similar conditions. The results suggest that exogenous insulin, directly or indirectly, regulates the pentose cycle activity in the pancreatic islets at the G6P dehydrogenase step.

Effect of leucine on the pyridine nucleotide contents of islets and on the insulin released--interactions in vitro with methylene blue, thiol oxidants, and p-chloromercuribenzoate.

In the presence of glucose (2 mg/ml), leucine (10 mM) noticeably increased islets' NADPH contents as well as the NADPH:NADP ratio; the changes occurred as soon as 1 min after its addition. NADH concentrations were also increased by leucine. The NADPH:NADP ratio as well as insulin release stimulated by glucose plus leucine were markedly decreased by methylene blue. The thiol oxidants diamide and tert-butyl hydroperoxide also inhibited insulin secretion in response to glucose plus leucine. Employing the perfused pancreas technique, the insulin-releasing action of p-chloromercuribenzoate was further enhanced by leucine. The combined effects were inhibited by tert-butyl hydroperoxide, however. Our data suggest that the insulin-releasing action of leucine depends on the islets' NADPH and reduced glutathione (GSH); in addition, leucine may contribute to insulin secretion by increasing the islet NADPH:NADP ratio and the NADH:NAD ratio. From the data, we assume that the observed increase of NADPH may lead via GSH to an increase in the number of such thiol groups in the beta-cell membrane, which are believed to be related to stimulation of insulin release and, thus, to increase the sensitivity of the beta-cell to stimulation by glucose and/or leucine.

Thiol-dependent and non-thiol-dependent stimulations of insulin release.

The effects of reduced glutathione (GSH) and diamide (an oxidant of GSH) on insulin release induced by glucose, glyceraldehyde, leucine, tolbutamide, glibenclamide, Ca-ionophore A-23187, isoprenaline, and dbcAMP were studied using isolated rat pancreatic islets. In the absence or presence of low glucose (5.6 mM) neither GSH (0.1 mM) nor diamide (0.1 mM) affected insulin release. Insulinotropic action of glucose (11.1 mM) and glyceraldehyde (11.1 mM), and that of tolbutamide (0.1 mg/ml) and leucine (10 mM) both in the presence of 11.1 mM glucose was further augmented by GSH and inhibited by diamide. GSH (0.05-1 mM) and diamide (0.1 mM) failed to affect the insulin secretion evoked by glibenclamide (5 micrograms/ml) + glucose (11.1 mM), Ca-ionophore A-23187 (50 micrograms/ml) + glucose (5.6 mM), isoprenaline (1 microM) + glucose (5.6 mM), and db-cAMP (1 mM) + glucose (5.6 mM). The data suggest that the insulin-releasing capacity of glucose, glyceraldehyde, tolbutamide, and leucine depends on the redox state of islet thiols, whereas the insulin-releasing effect of glibenclamide, Ca-ionophore, isoprenaline, and db-cAMP does not. The possibility that thiol dependency is associated with those compounds increasing Ca uptake but not with compounds acting as Ca-ionophores or only by increasing intracellular cAMP is discussed.

Islet glutathione and insulin release.

In isolated rat pancreatic islets, glucose (5.6, 11.1, and 16.7 mM) significantly increased reduced glutathione (GSH) and decreased oxidized glutathione (GSSG) levels in a dose-related manner. This was paralleled by a concomitant increase of NADPH and a decrease of NADP. The change of the GSH level occurred as quickly as one minute after addition of glucose. Exogenous insulin (200, 400, and 800 microU/ml) significantly decreased islet GSH levels in the presence of 5.6 and 16.7 mM glucose and significantly inhibited the insulin-releasing effect of the thiol reagent parachloromercuribenzoate (p-CMB) and tolbutamide. These data, together with earlier observations, suggest that GSH levels in pancreatic islets are increased by glucose and decreased by exogenous insulin via their effects on the pentose phosphate shunt and NADPH. Our results are compatible with the hypothesis that glucose and exogenous insulin, by modifying the redox state of the NADPH/NADP and GSH/GSSG systems, modulate the sensitivity of the beta-cell to the insulin-triggering actions of glucose, p-CMB, and tolbutamide.

Cysteine analogues potentiate glucose-induced insulin release in vitro.

In rat pancreatic islets, cysteine analogues, including glutathione, acetylcysteine, cysteamine, D-penicillamine, L-cysteine ethyl ester, and cysteine-potentiated glucose (11.1 mM) induced insulin secretion in a concentration-dependent manner. Their maximal effects were similar and occurred at approximately 0.05, 0.05, 0.1, 0.5, 1.0, 1.0 mM, respectively. At substimulatory glucose levels (2.8 mM), insulin release was not affected by these compounds. In contrast, thiol compounds, structurally different from cysteine and its analogues, such as mesna, tiopronin, meso-2,3-dimercaptosuccinic acid (DMSA), dimercaprol (BAL), beta-thio-D-glucose, as well as those cysteine analogues that lack a free-thiol group, including L-cystine, cystamine, D-penicillamine disulfide, S-carbocysteine, and S-carbamoyl-L-cysteine, did not enhance insulin release at stimulatory glucose levels (11.1 mM); cystine (5 mM) was inhibitory. These in vitro data indicate that among the thiols tested here, only cysteine and its analogues potentiate glucose-induced insulin secretion, whereas thiols that are structurally not related to cysteine do not. This suggests that a cysteine moiety in the molecule is necessary for the insulinotropic effect. For their synergistic action to glucose, the availability of a sulfhydryl group is also a prerequisite. The maximal synergistic action is similar for all cysteine analogues tested, whereas the potency of action is different, suggesting similarity in the mechanism of action but differences in the affinity to the secretory system.

Encapsulation of islets in rough surface, hydroxymethylated polysulfone capillaries stimulates VEGF release and promotes vascularization after transplantation.

The transplantation of encapsulated islets of Langerhans is one approach to treat type 1 diabetes without the need of lifelong immunosuppression. Capillaries have been used for macroencapsulation because they have a favorable surface-to-volume ratio and because they can be refilled. It is unclear at present whether the outer surface of such capillaries should be smooth to prevent, or rough to promote, cell adhesions. In this study we tested a new capillary made of modified polysulfone (MWCO: 50 kDa) with a rough, open-porous outer surface for islet transplantation. Compared with free-floating islets, encapsulation of freshly isolated rat islets affected neither the kinetics nor the efficiency of glucose-induced insulin release in perifusion experiments. Free-floating islets maintained insulin secretion during cell culture but encapsulated islets gradually lost their glucose responsiveness and released VEGF. This indicated hypoxia in the capillary lumen. Transplantation of encapsulated rat islets into diabetic rats significantly reduced blood glucose concentrations from the first week of implantation. This hypoglycaemic effect persisted until explantation 4 weeks later. Transplantation of encapsulated porcine islets into diabetic rats reduced blood glucose concentrations depending on the islet purity. With semipurified islets a transient reduction of blood glucose concentrations was observed (2, 8, 18, 18 days) whereas with highly purified islets a sustained normoglycaemia was achieved (more than 28 days). Explanted capillaries containing rat islets were covered with blood vessels. Vascularization was also observed on capillaries containing porcine islets that were explanted from normoglycaemic rats. In contrast, on capillaries containing porcine islets that were explanted from hyperglycemic rats a fibrous capsule and lymphocyte accumulations were observed. No vascularization on the surface of transplanted capillaries was observed in the absence of islets. In conclusion, encapsulated islets can release VEGF, which appears to be an important signal for the vascularization of the capillary material. The rough, open-porous outer surface of the polysulfone capillary provides a site well suited for vascular tissue formation and may allow a prolonged islet function after transplantation.

L-type calcium channels in insulin-secreting cells: biochemical characterization and phosphorylation in RINm5F cells.

Opening of dihydropyridine-sensitive voltage-dependent L-type Ca2+-channels (LTCCs) represents the final common pathway for insulin secretion in pancreatic beta-cells and related cell lines. In insulin-secreting cells their exact subunit composition is unknown. We therefore investigated the subunit structure of (+)-[3H]isradipine-labeled LTCCs in insulin-secreting RINm5F cells. Using subunit-specific antibodies we demonstrate that alpha1C subunits (199 kDa, short form) contribute only a minor portion of the total alpha1 immunoreactivity in membranes and partially purified Ca2+-channel preparations. However, alpha1C forms a major constituent of (+)-[3H]isradipine-labeled LTCCs as 54% of solubilized (+)-[3H]isradipine-binding activity was specifically immunoprecipitated by alpha1C antibodies. Phosphorylation of immunopurified alpha1C with cAMP-dependent protein kinase revealed the existence of an additional 240-kDa species (long form), that remained undetected in Western blots. Fifty seven percent of labeled LTCCs were immunoprecipitated by an anti-beta-antibody directed against all known beta-subunits. Isoform-specific antibodies revealed that these mainly corresponded to beta1b- and beta3-subunits. We found beta2- and beta4-subunits to be major constituents of cardiac and brain L-type channels, respectively, but not part of L-type channels in RINm5F cells. We conclude that alpha1C is a major constituent of dihydropyridine-labeled LTCCs in RINm5F cells, its long form serving as a substrate for cAMP-dependent protein kinase. beta1b- and beta3-Subunits were also found to associate with L-type channels in these cells. These isoforms may therefore represent biochemical targets for the modulation of LTCC activity in RINm5F cells.

Binding of insulin to rat pancreatic islets: comparison between pancreatic human insulin and biosynthetic human insulin.

Human pancreatic insulin, biosynthetic human insulin (BHI), and pork insulin were compared in terms of their binding characteristics to insulin receptors on rat pancreatic islets. There was no difference in binding or on biologic effect, i.e., ability to inhibit insulin secretion.

Effect of thioloxidant diazene dicarboxylic acid bis-(N'-methylpiperazide) (DIP) on 45Ca2+ net uptake into rat pancreatic islets.

The effect of DIP (an oxidant of glutathione) on 45Ca2+ net uptake induced by a variety of stimulators of insulin secretion was studied in rat pancreatic islets. In addition the effect of exogenous glutathione (GSH) on 45Ca2+ net uptake in response to glucose was tested. DIP (0.1 mM) inhibited the increase of 45Ca2+ net uptake in the presence of glucose (16.7 mM) and glyceraldehyde (10 mM). A similar inhibitory effect could be demonstrated, when 45Ca2+ net uptake was enhanced by tolbutamide (100 micrograms/ml), glibenclamide (0.5 micrograms/ml), b-BCH (20 mM), 2-ketoisocaproate (20 mM), arginine (20 mM) in the presence of 3 mM glucose or by high extracellular potassium (20 mM). The increase of 45Ca2+ net uptake stimulated by leucine (20 mM) plus glucose (3 mM) was further augmented by DIP. Exogenous GSH did not affect 45Ca2+ net uptake in the presence of (5.6-16.7 mM) glucose. It is suggested that 45Ca2+ net uptake of pancreatic islets depends on the redox state of islet thiols regardless of whether uptake is promoted via inhibition of potassium efflux (nutrients, sulfonylureas) or by high potassium and arginine. The voltage sensitive calcium-channel is the site of action of critical thiols. It is possible that these thiols are localized at the inner side of the plasma membrane.

Insulin release from pancreatic islets of fetal rats mediated by leucine b-BCH, tolbutamide, glibenclamide, arginine, potassium chloride, and theophylline does not require stimulation of Ca2+ net uptake.

In pancreatic islets of fetal rats the effect of glucose (3 and 16.7 mM), glyceraldehyde (10 mM), leucine (20 mM), b-BCH (20 mM), tolbutamide (100 micrograms/ml), glibenclamide (0.5 and 5.0 micrograms/ml) arginine (20 mM), KCl (20 mM) and theophylline (2.5 mM) on 45Ca2+ net uptake and secretion of insulin was studied. All compounds tested failed to stimulate 45Ca2+ net uptake. However, in contrast to glucose and glyceraldehyde, leucine, b-BCH, tolbutamide, glibenclamide, arginine, KCl and theophylline significantly stimulated release of insulin. This effect could not be inhibited by the calcium antagonist verapamil (20 microM). Elevation of the glucose concentration from 3 to 5.6 mM did not alter 86Rb+ efflux of fetal rat islets but inhibited 86Rb+ efflux of adult rat islets. Stimulation of 86Rb+ efflux with tolbutamide (100 micrograms/ml), leucine (20 mM) or b-BCH (20 mM) in the presence of 3 mM glucose was also ineffective in fetal rat islets. Our data suggest that stimulation of calcium uptake via the voltage dependent calcium channel is not possible in the fetal state. They also provide evidence that stimulators of insulin release which are thought not to act through their metabolism, initiate insulin secretion from fetal islets by a mechanism which is different from stimulation of calcium influx.

MEK-1/2 inhibition prevents 5-lipoxygenase translocation in N-formylpeptide-challenged human neutrophils.

In order to elucidate the role of mitogen-activated protein kinase kinase (MEK-1/2) in 5-lipoxygenase (5-LO) activation we studied the N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced 5-LO translocation in human blood neutrophils (PMNs). In non-primed, Ca(2+)-repleted PMNs, fMLP consistently stimulated MEK-1/2 phosphorylation, but induced 5-LO translocation and product formation (430+/-128 pmol; SEM, n=13) only in 13 of 18 PMN preparations from different healthy donors. In fMLP-responsive cells, the MEK-1/2 inhibitor PD098059 (50 microM) attenuated MEK phosphorylation and abolished 5-LO activation at the translocation step. The fMLP-mediated 5-LO product formation was also sensitive to MEK inhibition by U0126 and to p38 inhibition by SB203580. But in contrast to PD098059, U0126 at 10 microM and SB203580 at 20-50 microM impaired 5-LO activity in the cell-free assay setting, suggesting direct actions of higher concentrations of U0126 and SB203580 on 5-LO apart from MEK and p38 inhibition, respectively. These data show that fMLP initiates 5-LO product formation in non-primed, Ca(2+)-repleted human blood PMNs from healthy donors, and that MEK signaling is pivotal, but not sufficient for 5-LO activation in response to the receptor agonist fMLP.

Pyridine nucleotides in pancreatic islets during inhibition of insulin release by exogenous insulin.

In vitro addition of rat insulin (200, 400 or 800 muU/ml) to collagenase-isolated pancreatic islets of adult rats diminished glucose (3 mg/ml)-induced insulin release which was correlated with a decrease of the ratio of total NADPH/NADP and inhibition of glucose oxidation via the pentose phosphate shunt (PPS). NADH and NAD levels were not affected. It is suggested that exogenous insulin diminishes the islet total NADPH/NADP ratio by a direct or indirect decrease in PPS activity. However, it is also conceivable that insulin decreases this ratio through another mechanism than PPS. It is possible that inhibition of insulin secretion by exogenous insulin is at least in part due to the decrease of the NADPH/NADP ratio.