Multivesicular exocytosis in rat pancreatic beta cells

Area of Science:

• Cell Biology
• Endocrinology
• Physiology

Background:

• Insulin secretion from pancreatic beta cells is crucial for glucose homeostasis.
• Exocytosis, the process of vesicle fusion and release, is the primary mechanism for insulin secretion.
• Understanding the nuances of exocytosis, such as compound exocytosis, is key to understanding beta cell function.

Purpose of the Study:

• To investigate the occurrence and functional significance of compound exocytosis in insulin-secreting beta cells.
• To determine how compound exocytosis is modulated by secretagogues like glucose and carbachol.
• To elucidate the role of compound exocytosis in augmented insulin secretion.

Main Methods:

• Electrophysiological, biochemical, and optical methods were used to monitor exocytosis in rat beta cells.
• Confocal imaging, transmission electron microscopy, and block face scanning electron microscopy provided ultrastructural evidence.
• Two-photon sulforhodamine imaging visualized exocytotic events and membrane structures.

Main Results:

• Compound exocytosis contributed minimally (<5%) to glucose- or depolarization-induced exocytosis alone.
• Stimulation with glucose and carbachol increased compound exocytosis to 15-20% of release events.
• Carbachol enhanced glucose-induced insulin secretion by ~40%, mimicking elevated intracellular calcium levels.
• Ultrastructural analysis revealed fused multigranular aggregates, increasing fourfold with carbachol stimulation.

Conclusions:

• Compound exocytosis is quantitatively significant during elevated cytoplasmic calcium, as induced by muscarinic receptor activation.
• This process is a major contributor to the augmented insulin secretion observed with muscarinic receptor stimulation.
• Compound exocytosis represents an important mechanism for fine-tuning insulin release under specific physiological conditions.