Studies on the Mechanism of Insulin Release from Rat Pancreatic Islets: Correlation of Glucose Metabolism, Especially TCA Cycle, and Insulin Secretion in B Cells

Abstract

The effects of glucose metabolism on insulin secretion have been investigated with rat pancreatic islets which were prepared by Lacy and Costianovsky's method.
The results were obtained as follows:
1. Effect of inhibitors of glycolysis and TCA cycle on glucose induced insulin release was as follows:
a) Glucose-induced insulin release significantly decreased when glucose and 2-deoxyglucose were present in equal concentration.
b) 1mM iodoacetate inhibited markedly insulin release induced by 16.7 mM glucose and the inhibition was significantly removed by addition of 10mM pyruvic acid though the insulin secretion was not completely recovered to its previous level.
c) 10mM malonic acid reduced remarkably insulin secretion even in high glucose concentration (16.7mM).
2. Effect of inhibition of oxidative phosphorylation on glucose-induced insulin release was as follows:
a) The stimulating effect of glucose in a high concentration (16.7mM) was lost under 95% N2-5% CO₂ gassing and its inhibitory effect was not repaired by addition of 200μg/mli tolbutamide, 250μg/ml theophylline, and 0.1, 0.5 or 1.0mM db-cAMP.
b) 2, 4 dinitrophenol did not diminish insulin release in a low glucose concentration (3.3mM), but inhibited it remarkably in a high glucose concentration (16.7mM).
3. 1 or 10mM ATP did not effect insulin release induced by 3.3 or 16.7mM glucose in vitro.
4. Relationship between insulin secretion and ATP concent in B cell was as follows:
a) 1 or 10mM adenosine significantly stimulated insulin release, and increased ATP content in rat pancreatic islets.
b) Insulin secretion and islet ATP contcent increased in parallel when the glucose concentration was elevated from 0 to 27.8mM.
c) Inhibition of insulin release by 10mM malonic acid was significantly reduced by addition of 1 mM adenosine, but at that time, dissociation between insulin secretion and islet ATP content was recognized.
These observations suggest that glycolysis and the TCA cycle, namely respiratory chain phosphorylation and perhaps ATP are involved in insulin release induced by glucose stimulation: that ATP contents in the islet of Langerhans are composed of several compartments in special reference to the physiological function, at least one of them having a relation to glucose-induced insulin release, and that the increment of ATP content in the islet is involved in the second phase rather than in the first of insulin release by glucose stimulation.