Erik Gylfe

Comparison of the effects of leucines, non-metabolizable leucine analogues and other insulin secretagogues on the activity of glutamate dehydrogenase.

SummaryGlutamate dehydrogenase (GLDH) from bovine liver was employed in a model system for testing a possible role of GLDH in insulin release. The ability of different insulin secretagogues to stimulate the activity of the diethylstilbestrol-inhibited enzyme was tested. The two insulin-releasing amino acids, L-leucine and its non-metabolizable analogue 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid [b(−)-BCH], were the best stimulators of GLDH activity. The non-secreting stereoisomers, D-leucine and b(+)-BCH, were less effective. Glucose, L-arginine and the leucine metabolite α-ketoisocaproic acid lacked significant effects on GLDH activity. Small and diverging effects were obtained with sulfonylurea compounds: whereas carbutamide caused slight stimulation, tolbutamide and glipizide had no effect, and glibenclamide was an inhibitor. The specificity of the insulin-releasing amino acids L-leucine and b(−)-BCH in stimulating GLDH activity makes it tempting to speculate about a connection between allosteric regulation of pyridine nucleotide-dependent enzymes and insulin release.

Function of microdissected pancreatic islets cultured in a chemically defined medium. I. Insulin content and release.

SummaryMicrodissected pancreatic islets from non-inbredob/ob-mice, were cultured for 6 or 7 days in serum-free tissue culture medium 199. The insulin content of the islets decreased 60% during culture in 17 mM or 28 mM glucose and about 70% in the presence of 3.3 mM or 5.6 mM glucose. At the end of a culture period in high glucose, the sum of the insulin in the islet plus that in the culture medium was almost twice as high as the insulin content of fresh islets, indicating an active insulin biosynthesis. The maximal insulin response to glucose after culture in 17 mM or 28 mM glucose was about 40% of that in fresh islets; after culture in 3.3 mM glucose it was 10%. Half-maximal stimulation was observed at a glucose concentration of 5 mM for islets cultured with high glucose as compared to 9 mM for fresh islets. Like glucose, glibenclamide was a more effective insulin stimulator after culture with a high glucose concentration than with a low one. However, leucine-induced insulin release was not affected by the glucose concentration in the preceding culture medium. Whereas potentiation of glucose-stimulated release by arginine or dibutyryl-cAMP was independent of glucose concentration during the culture, theophylline released three times more insulin when the islets had been cultured with high glucose.

Metabolism of cold-stored pancreatic islets.

SummaryA previous study showed that the ability of glucose to stimulate insulin release was retained in islets stored at 8 °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 uptake of 86Rb+ was 20–48%. Rewarming the cold-stored islets to 37 °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.

Glucose oxidation and contents of free amino acids in pancreatic β-cells stimulated by a non-metabolizable leucine analogue

Abstract Oxidation of glucose and the contents of free amino acids were measured in β-cell-rich pancreatic islets exposed to stereoisomers of the non-metabolizable leucine analogue 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid. The insulin-releasing isomer b(−) appeared to stimulate glucose oxidation, whereas the inactive b(+) form was without effect. 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid reduced the islet contents of aspartic acid, γ-aminobutyric acid and glutamic acid and increased that of phenylalanine. Since the two isomers of 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid did not differ in their ability to alter the intracellular contents of free amino acids, these alterations are probably not significant for the regulation of insulin release. It could not be excluded that increased glucose metabolism may contribute to the stimulation of insulin release by the b(−) isomer of 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid.