The endoplasmic reticulum as one continuous Ca(2+) pool: visualization of rapid Ca(2+) movements and equilibration

Area of Science:

• Cell Biology
• Physiology
• Biochemistry

Background:

• The endoplasmic reticulum (ER) serves as a critical intracellular calcium (Ca2+) store.
• Understanding the functional connectivity of the ER Ca2+ store is essential for cellular signaling.
• Previous studies have debated whether the ER Ca2+ store is a single functional unit or composed of discrete subunits.

Purpose of the Study:

• To investigate the functional connectivity of the ER Ca2+ store in pancreatic acinar cells.
• To determine if the ER Ca2+ store is a unified or fragmented system.
• To analyze Ca2+ and fluorescent probe dynamics within the ER lumen under stimulation.

Main Methods:

• Confocal microscopy was employed to monitor Ca2+ and fluorescent probe movements in the ER lumen.
• Local bleaching and uncaging techniques were utilized to assess ER lumenal dynamics.
• Pancreatic acinar cells were stimulated with acetylcholine (ACh) to induce Ca2+ release.

Main Results:

• Rapid movement and equilibration of Ca2+ and fluorescent probes within the ER lumen were observed.
• The bulk ER at the base showed diffusion into apical ER extensions, indicating partial connectivity.
• The ER Ca2+ store demonstrated robust connectivity, resisting functional fragmentation even after prolonged supramaximal ACh stimulation.
• Acetylcholine stimulation caused a uniform decrease in ER Ca2+ concentration, with repetitive cytosolic Ca2+ spikes having minimal impact on ER Ca2+ levels.

Conclusions:

• The endoplasmic reticulum (ER) operates as a functionally continuous Ca2+ store, facilitating efficient Ca2+ liberation.
• Ca2+ released from apical ER terminals is rapidly replenished from the basal rough ER.
• The ER's continuous nature is crucial for maintaining cellular calcium homeostasis and signaling efficiency.