Hyperosmolality inhibits exocytosis in sea urchin eggs by formation of a granule-free zone and arrest of pore widening

Carrie J. Merkle, Douglas E. Chandler

Research output: Contribution to journalArticle

11 Scopus citations

Abstract

Hyperosmolality is known to inhibit membrane fusion during exocytosis. In this study cortical granule exocytosis in sea urchin eggs is used as a model system to determine at what step this inhibition occurs. Strongylocentrotus purpuratus eggs were incubated in hyperosmotic seawater (Na2SO4, sucrose or sodium HEPES used as osmoticants), the eggs activated with 20 μm A23187 to trigger exocytosis, and then quick frozen or chemically fixed for electron microscopy. Thin sections and freeze-fracture replicas show that at high osmolality (2.31 osmol/kg), there is a decrease in cortical granule size, a 90% reduction in granule-plasma membrane fusion, and formation of a granulefree zone between the plasma membrane and cortical granules. This zone averages 0.64 μm in thickness and prevents the majority of granules from docking at the plasma membrane. The remaining granules (∼10%) exhibit early stages of fusion which appear to have been stabilized; the matrix of these granules remains intact. We conclude that exocytosis is blocked by two separate mechanisms. First, the granule-free zone prevents granule-plasma membrane contact required for fusion. Second, in cases where fusion does occur, opening of the pocket and dispersal of the granule contents are slowed in hyperosmotic media.

Original languageEnglish (US)
Pages (from-to)223-232
Number of pages10
JournalThe Journal of Membrane Biology
Volume112
Issue number3
DOIs
StatePublished - Dec 1989

Keywords

  • cortex
  • exocytosis
  • freeze fracture
  • hyperosmolality
  • membrane fusion
  • sea urchin egg

ASJC Scopus subject areas

  • Biophysics
  • Physiology
  • Cell Biology

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