Membrane interactions between secretion granules and plasmalemma in three exocrine glands

Area of Science:

• Cell Biology
• Exocrine Secretion
• Membrane Dynamics

Background:

• Exocrine glands utilize exocytosis for secretion, involving complex membrane interactions.
• The role of intramembrane particles (IMP) distribution during secretory events is not fully understood.
• Understanding membrane apposition is crucial for elucidating the mechanisms of exocytosis.

Purpose of the Study:

• To investigate membrane interactions during exocytosis in rat parotid, rat lacrimal, and guinea pig pancreatic acinar cells.
• To analyze the distribution of intramembrane particles (IMP) in relation to secretory granule and plasma membrane appositions.
• To explore the potential role of mechanical forces in membrane interactions preceding exocytosis.

Main Methods:

• Utilized freeze-fracture and thin-section electron microscopy to examine exocrine cell membrane structures.
• Studied membrane interactions in vivo following pharmacological stimulation and in vitro via granule centrifugation.
• Analyzed the distribution of intramembrane particles (IMP) on plasma and granule membranes.

Main Results:

• Intramembrane particle (IMP) clearing from granule-plasma membrane contact sites was observed in stimulated parotid and lacrimal cells, forming bulges.
• Pancreatic acinar cells showed random IMP distribution post-stimulation with narrow exocytotic necks, lacking early fusion images.
• In vitro centrifugation induced IMP-free appositions in parotid granules, suggesting mechanical forces influence membrane interactions.

Conclusions:

• IMP clearing in secretory cells may result from mechanical apposition of membranes, potentially a preliminary step to fusion.
• Pancreatic cells exhibit distinct membrane interaction mechanisms compared to parotid and lacrimal cells.
• Artifactual formation of membrane blisters during fixation was identified as a potential confounding factor.