Mitochondria in Calcium Signaling in the Exocrine Pancreas

Research Article

Mitochondria in Calcium Signaling in the Exocrine Pancreas

Ren Sato^1*, Yuki Takahashi², Kenta Yamamoto³ and Mei Kobayashi²

1Department of Virology, Nagoya University, Nagoya, Japan
2Department of Microbiology, Hokkaido University, Sapporo, Japan
3Department of Immunology, Tohoku University, Sendai, Japan

Published: 16 May 2016

Abstract

Background: Pancreatic acinar cells secrete digestive enzymes in response to hormonal and neural stimuli that trigger complex intracellular calcium (Ca2+) signals. These signals, often originating in the apical pole and exhibiting oscillatory patterns at physiological stimulus strengths, are crucial for driving exocytosis. Mitochondria, dynamic organelles essential for cellular energy metabolism, are also known to accumulate Ca2+ and are strategically positioned near Ca2⁺ release sites in many cell types, including pancreatic acinar cells. However, the precise contribution of mitochondria to shaping physiological Ca2⁺ signals and maintaining cellular homeostasis in the exocrine pancreas requires detailed investigation.

Objective: This study aimed to elucidate the role of mitochondria in modulating agonist-induced cytosolic Ca2⁺ signals and regulating cellular bioenergetics in isolated rat pancreatic acinar cells.

Methods: Pancreatic acinar cells or small acinar clusters were isolated from rat pancreas. Cytosolic Ca2⁺ ([Ca2⁺] cyt) was measured using ratiometric imaging of Fura-2 or confocal imaging of Fluo-4. Mitochondrial Ca2⁺ ([Ca2⁺] mito) and Membrane Potential were monitored using Rhod-2 and TMRM, respectively, with confocal microscopy. Cells were stimulated with physiological Concentrations of Cholecystokinin (CCK) or acetylcholine analogue carbachol (CCh). Mitochondrial function was manipulated using the mitochondrial Ca2⁺ uniporter (MCU) inhibitor Ruthenium 360 (Ru360) and mitochondrial uncouplers/inhibitors (FCCP, oligomycin). Cellular ATP levels were measured using a luciferase-based assay.

Results: Stimulation with CCK or CCh induced repetitive cytosolic Ca2⁺ oscillations localized primarily to the apical, granule-rich pole of the acinar cell. These cytosolic Ca2⁺ rises were closely followed by transient increases in mitochondrial Ca2⁺, indicating rapid Ca2⁺ uptake by perigranular mitochondria. Inhibition of mitochondrial Ca2⁺ uptake using Ru360, or dissipation of using FCCP (which prevents uptake), significantly altered the pattern of cytosolic Ca2⁺ signals. Specifically, the decay phase of individual Ca2⁺ transients was prolonged, peak amplitudes were sometimes increased, and the oscillatory pattern often transitioned towards a more sustained elevation, particularly at moderate agonist concentrations. Physiological mitochondrial Ca2⁺ uptake was associated with a transient hyperpolarization or stabilization and maintained cellular ATP levels during stimulation. Conversely, conditions mimicking pathological Ca2⁺ overload led to sustained mitochondrial Ca2⁺ elevation, depolarization, and a decline in ATP levels.   

Conclusion: Mitochondria play a critical dual role in shaping Ca2⁺ signals and maintaining bioenergetic homeostasis in pancreatic acinar cells. By rapidly taking up Ca2⁺ released from apical ER stores, they act as dynamic buffers that influence the amplitude, duration, and oscillatory pattern of physiological cytosolic Ca2⁺ signals. This Ca2⁺ uptake simultaneously stimulates mitochondrial ATP production, providing the energy required for Ca2⁺ extrusion and secretory processes. However, this system is vulnerable, as excessive Ca2⁺ influx can overwhelm mitochondrial capacity, leading to dysfunction potentially relevant to pancreatic pathology.

 Keywords: Mitochondria; Calcium Signaling; Exocrine Pancreas; Acinar Cells; Calcium Buffering; ATP Production; Mitochondrial Membrane