Unit of Endocrinology and Metabolism, University of Louvain Faculty of Medicine, B-1200 Brussels, Belgium.
Endocrinology. 2012 Oct;153(10):4644-54. doi: 10.1210/en.2012-1450. Epub 2012 Sep 4.
Insulin secretion is triggered by an increase in the cytosolic calcium concentration (Ca(2+)) in β-cells. Ca(2+)-induced exocytosis of insulin granules can be augmented by metabolic amplification (unknown signals generated through glucose metabolism) or neurohormonal amplification (in particular cAMP mediated). Functional actin microfilaments are not required for metabolic amplification, but their possible role in cAMP-mediated amplification is unknown. It is also uncertain whether cAMP (generated in response to glucose) is implicated in metabolic amplification. These questions were addressed using isolated mouse islets. cAMP levels were increased by phosphodiesterase inhibition (with isobutylmethylxanthine) and adenylate-cyclase stimulation (with forskolin or glucagon-like peptide-1, 7-36 amide). Raising cAMP levels had no steady-state impact on actin polymerization in control islets. Neither disruption (depolymerization by latrunculin) nor stabilization (polymerization by jasplakinolide) of actin microfilaments was counteracted by cAMP. Both changes increased both phases of glucose- or tolbutamide-induced insulin secretion but did not prevent further amplification by cAMP. These large changes in secretion were not caused by changes in Ca(2+), which was only slightly increased by cAMP. Both phases of insulin secretion were larger in response to glucose than tolbutamide, although Ca(2+) was lower. This difference in secretion, which reflects metabolic amplification, was independent of microfilaments, was not attributable to differences in cAMP, and persisted in presence of dibutyryl-cAMP or when cAMP levels were variably raised by isobutylmethylxanthine + forskolin or glucagon-like peptide-1, 7-36 amide. We conclude that metabolic and cAMP-mediated amplification of insulin secretion are distinct pathways that accelerate acquisition of release competence by insulin granules that can access exocytotic sites without intervention of microfilaments.
胰岛素的分泌是由β细胞胞质中钙离子浓度(Ca(2+))的增加引发的。胰岛素颗粒的 Ca(2+)-诱导胞吐作用可以通过代谢放大（通过葡萄糖代谢产生的未知信号）或神经激素放大（特别是 cAMP 介导的）来增强。代谢放大不需要功能性肌动蛋白微丝，但它们在 cAMP 介导的放大中的可能作用尚不清楚。也不确定 cAMP（响应葡萄糖产生）是否参与代谢放大。这些问题是使用分离的小鼠胰岛来解决的。通过磷酸二酯酶抑制（用异丁基甲基黄嘌呤）和腺苷酸环化酶刺激（用 forskolin 或胰高血糖素样肽-1，7-36 酰胺）来增加 cAMP 水平。在对照胰岛中，cAMP 水平的升高对肌动蛋白聚合没有稳态影响。肌动蛋白微丝的破坏（用 latrunculin 解聚）或稳定（用 jasplakinolide 聚合）都不会被 cAMP 抵消。这两种变化都增加了葡萄糖或甲苯磺丁脲诱导的胰岛素分泌的两个阶段，但不能阻止 cAMP 的进一步放大。这些分泌的巨大变化不是由 cAMP 引起的，cAMP 仅略微增加 Ca(2+)。与甲苯磺丁脲相比，葡萄糖刺激的胰岛素分泌的两个阶段都更大，尽管 Ca(2+)较低。这种分泌差异反映了代谢放大，独立于微丝，不是由于 cAMP 的差异引起的，并且在存在二丁酰基-cAMP 或当 cAMP 水平通过异丁基甲基黄嘌呤+ forskolin 或胰高血糖素样肽-1，7-36 酰胺可变升高时仍然存在。我们得出结论，胰岛素分泌的代谢和 cAMP 介导的放大是不同的途径，它们通过加速可以访问胞吐部位的胰岛素颗粒获得释放能力来加速胰岛素颗粒获得释放能力，而无需微丝的干预。
