Abstract
Secretory output of insulin from the pancreatic beta cell is regulated in a sophisticated manner by nutrients, hormones and neurotransmitters,having impact on the different stages of the sequential events leading to release into the blood to maintain glucose homeostasis. Although the final step of the secretory process, exocytosis, has relatively been well investigated, the significance of pre-exocytotic processes such as granule movement and interaction with the plasma membrane has received less attention. Recently, we and a few other groups directly analysed the nature of intracellular traffic of insulin granules as well as their distribution. Glucose, the most important insulin secretagogue, was found to activate pre-exocytotic events as well as actual exocytosis, whereas some hormones, neurotransmitters and pharmacological substances were found to act solely on the earlier stages to potentiate insulin release. Mechanisms regulating granule traffic, access, docking and priming were found to be distinct from those for granule exocytosis, which dominantly depends on Ca2+ influx from the extracellular space. Recent studies have suggested that overloading of Ca2+ may cause apoptotic death of the beta cell. We, therefore, suggest here that elucidation of the pre-exocytotic steps may allow an increase in insulin output with a minimal rise in the intracellular Ca2+ concentration, which would be helpful to prevent beta-cell loss and maintain beta cell functions under diabetic conditions.
Keywords: Pre-exocytotic, Insulin, Myosin, Glucagon, Phosphatidilinositol, N-ethylmaleimide, Microtubule
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