Effect of methylxanthines on alloxan inhibition of insulin release

The toxicology of cocoa and methylxanthines: a review of the literature

The critical review of the literature cited on pharmacology, toxicology, metabolism, and safety assessment clearly demonstrates that cocoa per se has not attracted a great deal of scientific interest because of its long-term usage with no reported adverse effects that would be injurious to man. On the other hand, a great deal of research has been directed towards understanding the pharmacological properties of the methylxanthines--caffeine, theobromine, and theophylline. Much of the emphasis on metabolism, toxicology, teratogenic potential, and safety assessment has been on the evaluation of caffeine. In light of the serious health concerns ascribed to the effects of caffeine and the lack of basic information on theobromine and theophylline, it is imperative that a major research program be undertaken to evaluate these methylxanthines and, of course, cocoa, coffee, and tea. It only will be through elucidating their mechanism of action that we will be in a position to assess their safety. Before committing research efforts to evaluating the long-term effects of these methylxanthines and their respective foodstuffs, which serve as our primary source of exposure, it is critical to initiate more basic research on the metabolism of caffeine, theophylline, and theobromine in several animal species and man. While published reports do appear in this area, it is essential to understand fully the similarities and differences between various animals and man. The influence of dietary factors and drug interactions must also be determined. Before establishing dosage levels for a chronic toxicity study, the pharmacokinetics of the dose must be determined in the species that will be used in long-term studies. This is necessary if there is a dose-dependency in the animal above which saturation may occur and the plasma half-life kinetics change, or shifts occur either in the metabolic pathways of degradation and/or in the route of excretion from the body. The area of teratology must also be thoroughly evaluated. Studies undertaken should include identification and quantitation of the metabolites of caffeine, theophylline, and theobromine in the pregnant animal, the respective pharmacokinetics of each compound, dose-dependency (if this is the case), and their potential teratogenicity. In addition, the influence of other drugs or dietary variables must be included. In addition to teratology, a great deal of research is needed to assess and quantitate fetal and neonatal metabolism of these compounds.(ABSTRACT TRUNCATED AT 400 WORDS)

Alloxan-induced luminol luminescence as a tool for investigating mechanisms of radical-mediated diabetogenicity

Chemiluminescence of luminol in a cell-free system was used to investigate the mechanism of alloxan-dependent free-radical generation. In the presence of alloxan and reduced glutathione (GSH), luminescence was greatly stimulated by FeSO4. Replacing GSH by oxidized glutathione or NAD(P)(H), or replacing FeSO4 by CuSO4, ZNSO4 or FeCl3, did not yield chemiluminescence. The chemiluminescence of a mixture of alloxan. GSH, FeSO4 and luminol was inhibited by catalase, superoxide dismutase, scavengers of hydroxyl radicals (sodium benzoate, n-butanol, D-mannitol, dimethyl sulphoxide) or metal-ion chelators (EDTA, diethylenetriaminepenta-acetic acid, diethyldithiocarbamate. desferroxamine), D-glucose, L-glucose, D-mannose, D-fructose, 3-O-methyl-D-glucose, NAD+, NADH, NADP+ or NADPH, but not by urea or enzymically inactive superoxide dismutase. The results support the hypothesis that the diabetogenic action of alloxan is mediated by hydroxyl radicals generated in an iron-catalysed reaction. Protection against alloxan in vivo depends both on the chemical reactivity of protector with radicals or radical-generating systems and on the stereospecific requirement of some strategic site in the B-cell.

[Effects of uric acid on the B cell in the isolated perfused rat pancreas (author's transl)]

Methylxanthines, such as caffeine and theophylline, show effects increasing the secretion of insulin. Perfusion experiments were intended to find out whether insulin secretion is influenced by uric acid, which is chemically closely related. Besides, it was to be demonstrated that uric acid causes no damages to islet cell structures for it is well established that alloxan, which is chemically related to uric acid, produces necroses in B cells. Isolated rat pancreata were stimulated by glucose at concentrations of 100 mg/100 ml and 300 mg/100 ml. In part of the experiments uric acid was added to the perfusion medium at a concentration of 12 mg/100 ml. We found that uric acid has no influence on insulin secretion if it is stimulated by glucose at a concentration of 100 mg/100 ml. However, if B cells are stimulated by glucose at a concentration of 300 mg/100 ml insulin secretion is enhanced by the addition of uric acid by more than 100%. This indicates that this substance exhibits a glucose-potentiating effect. The characteristic dynamics of insulin secretion demonstrate together with light- and electron-microscopic investigations that uric acid causes a real additional secretion and no leaking of intracellular insulin as a result of destruction of islet cell structures by an effect of uric acid similar to that of alloxan.

Trypan Blue as a marker of plasma membrane permeability in alloxan-treated mouse islet cells

Suspensions of pancreatic islet cells from noninbred ob/ob-mice were incubated with Trypan Blue. Microscope photometry showed that apparently viable cells excluded the dye completely, whereas the nuclei of nonviable cells accumulated Trypan Blue by a saturable process. The nucleus-to-medium dye gradient was more then 30:1 in media containing 0.1% or less Trypan Blue. The apparent affinity constant for nuclear binding of the dye was 3.1 X 10(4)l/mol. Albumin partially inhibited the nuclear staining. More than 0.5% Trypan Blue in the medium was toxic per se. In the absence of albumin, 0.5 or 20 mmol/l alloxan, 1 mmol/l N-ethylmaleimide, or 0.1 mmol/l chloromercuribenzene-p-sulphonic acid, but not 20 mmol/l streptozotocin, increased the frequency of islet cells stained with 0.1% Trypan Blue. The absorbance of nuceli was also increased in cells treated with alloxan or N-ethylmaleimide, but not in those treated with chloromercuribenze-p-sulphonic acid. It is concluded that alloxan rapidly increases the permeability of the plasma membrane in mouse beta-cells. This action of alloxan appears to be more acute than any such effect of streptozotocin.

Possible toxic effects of normal and diabetic patient serum on pancreatic B-cells

Serum from normal blood-donors and juvenile diabetic patients inhibited Rb+ accumulation and stimulated release of 51Cr and insulin in suspensions of dispersed pancreatic islet cells prepared from ob/ob mouse islets, which are rich in B-cells. The effects indicate the presence of a B-cytotoxic factor in human serum. Serum from mouse and fetal calf also inhibited the islet cell accumulation of Rb+. Toxicity was not suppressed by treating serum with protein A-Sepharose and did not correlate with islet cell binding of fluorescent antibodies to human immunoglobulin. Whereas all sera inhibited Rb+ accumulation, 3 of 6 diabetic patient sera, but no blood-donor serum, made the cells fluoresce on exposure to the fluorescent antibodies. Supporting a dependence on complement, toxicity remained after dialysis, but was destroyed by treating serum with zymosan-A or heating at 56 degrees for 30 min.