title: Distribution Profile of Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channels in α and β Cells of Pancreas: Dominant Localization in Secretory Granules and Common Error in Identification of Secretory Granule Membranes.
abstract: OBJECTIVE
  The α and β cells of pancreatic islet release important hormones in response to intracellular Ca increases that result from Ca releases through the inositol 1,4,5-trisphoshate receptor (IP3R)/Ca channels. Yet no systematic studies on distribution of IP3R/Ca channels have been done, prompting us to investigate the distribution of all 3 IP3R isoforms.
  
  
  METHODS
  Immunogold electron microscopy was performed to determine the presence and the relative concentrations of all 3 IP3R isoforms in 2 major organelles secretory granules (SGs) and the endoplasmic reticulum of α and β cells of rat pancreas.
  
  
  RESULTS
  All 3 IP3R isoforms were present in SG membranes of both cells, and the IP3R concentrations in SGs were ∼2-fold higher than those in the endoplasmic reticulum. Moreover, large halos shown in the electron microscope images of insulin-containing SGs of β cells were gap spaces that resulted from separation of granule membranes from the surrounding cytoplasm.
  
  
  CONCLUSIONS
  These results strongly suggest the important roles of SGs in IP3-induced, Ca-dependent regulatory secretory pathway in pancreas. Moreover, the accurate location of SG membranes of β cells was further confirmed by the location of another integral membrane protein synaptotagmin V and of membrane phospholipid PI(4,5)P2.
authors.affiliation: From the Department of Biochemistry, Inha University School of Medicine, Jung Gu, Incheon, Korea.
authors.forename: Yong Suk
authors.initials: YS
authors.lastname: Hur
authors.forename: Seung Hyun
authors.initials: SH
authors.lastname: Yoo
mesh_terms: D000818:Animals; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000648:ultrastructure; D050416:Glucagon-Secreting Cells / Q000737:chemistry* / Q000648:ultrastructure; D007150:Immunohistochemistry; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000032:analysis*; D050417:Insulin-Secreting Cells / Q000737:chemistry* / Q000648:ultrastructure; D008854:Microscopy, Electron; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000032:analysis; D017207:Rats, Sprague-Dawley; D022142:Secretory Vesicles / Q000737:chemistry* / Q000648:ultrastructure; D050857:Synaptotagmins / Q000032:analysis
chemical_list: D053496:Inositol 1,4,5-Trisphosphate Receptors; D019269:Phosphatidylinositol 4,5-Diphosphate; C494297:Syt5 protein, rat; D050857:Synaptotagmins