INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR SUBTYPE-3 IN PANCREATIC-ISLET CELL SECRETORY GRANULES REVISITED

It has been reported that the inositol 1,4,5-trisphosphate receptor su btype 3 is expressed in islet cells and is localized to both insulin a nd somatostatin granules [Blondel, O., Moody, M. M., Depaoli, A. M., S harp, A. H., Boss, C. A., Swift, H. & Bell, G. I. (1994) Proc. Natl. A cad. Sci. USA 91, 7777-7781], This subcellular localization was based on electron microscope immunocytochemistry using antibodies (affinity- purified polyclonal antiserum AB3) directed to a 15-residue peptide of rat inositol trisphosphate receptor subtype 3, We now show that these antibodies cross-react with rat, but not human, insulin, Accordingly, the anti-inositol trisphosphate receptor subtype 3 (AB3) antibodies l abel electron dense cores of mature (insulin-rich) granules of rat pan creatic beta cells, and rat granule labeling was blocked by preabsorpt ion of the AB3 antibodies with rat insulin, The immunostaining of imma ture, Golgi-associated proinsulin-rich granules with AB3 antibodies wa s very weak, indicating that cross-reactivity is limited to the hormon e and not its precursor, Also, the AB3 antibodies labeled pure rat ins ulin crystals grown in vitro but failed to stain crystals grown from p ure human insulin, By immunoprecipitation, the antibodies similarly di splayed a higher affinity for rat than for human insulin, We could not confirm the labeling of somatostatin granules using AB3 antibodies.