Abstract
The present study concerns the role of cAMP and 45Ca2+ uptake in the decreased glucose induced insulin release from 14-day cultured islets maintained in 5.5 mM glucose (A-islets), 5.5 mM glucose containing 1 mM adenosine (B-islets) and 16.7 mM glucose (C-islets), and the recovery effect of adenosine on insulin release in B-islets.
Insulin secretion, cAMP release, cAMP content and 45Ca2+ uptake were observed in freshly isolated islets (F-islets) and the cultured islets incubated for 5 to 30 min in 3.3 or 16.7 mM glucose media with or without 45Ca2+ (5 μCi/ml).
The results obtained on short incubation with 16.7 mM glucose may be summarized as follows.
(1) Insulin secretion was adequately preserved in B-and C-islets, but decreased to about onethird that in F-islets.
(2) B-islets showed a rapid, strong 45Ca2+ uptake of 1.64±0.25, 6.21±0.35 and 7.79±0.44 pmol/islet at 5, 15 and 30 min, respectively, as did C-and F-islets. On the other hand, the 45Ca2+ uptake in A-islets was remarkably low compared to the other islets throughout all the incubation periods.
(3) None of the cultured islets revealed any enhancement of cAMP content, but F-islets indicated a maximum amount of cAMP at 15 min and maintained the same level until 30 min.
(4) All the cultured islets released cAMP at about 35 fmol/islet, which was not significantly different from that on incubation with 3.3 mM glucose, for the first 5 min and the B-and C-islets continued to release cAMP gradually until 30 min. On the other hand, F-islets secreted a significant amount of cAMP over the same periods.
(5) There were no differences among the B-, C-and F-islets in the sum total of the cAMP content and the release in each incubation period, but the sum total for A-islets was significantly low compared to that in each of the other islets.
The above findings suggest that addition of adenosine may be effective in improving the decreased insulin secretion and 45Ca2+ uptake which occurred in A-islets, and that as well as failure in increase in cAMP content, certain other disturbances in the insulin release-cAMP system may account for the impaired insulin response to glucose in cultured islets.
