Requirements for hormone release from permeabilized nerve endings isolated from the rat neurohypophysis

1. Isolated nerve endings from rat neurohypophyses were permeabilized with digitonin in order to gain access to the cytoplasm. Release of vasopressin (AVP), oxytocin and the neurophysins was studied under different experimental conditions. 2. Hormone release, which occurred by exocytosis, was Ca2+ dependent. Half-maximal release was observed at ca. 1.7 microM-Ca2+ in contrast to ca. 300 microM for K+-induced hormone secretion from non-permeabilized neurosecretosomes. 3. Release also occurred when the neurosecretosomes were challenged with Ca2+ 20 min after digitonin treatment. This suggests that the isolated nerve endings remain permeable after treatment with digitonin. 4. Although hormone release was potentiated in the presence of ATP, and to a lesser extent with guanosine triphosphate (GTP), secretion occurred in the absence of nucleotides. 5. Replacement of K+ as the major cation by Na+ did not modify the secretory response to a Ca2+ challenge. Release, although reduced, still occurred when KCl was replaced by sucrose. 6. Compared to glutamate, Cl-, Br- and I- did not modify the Ca2+-independent release. This release was increased in the presence of SCN-. The order of effectiveness of the anions studied in inhibiting the Ca2+-dependent release was glutamate less than Br- = Cl- = I- less than SCN-. 7. Increasing the osmolarity of the perfusate inhibited the Ca2+-dependent release of AVP and oxytocin. 8. Vincristine, which binds to microtubules, had no effect on the secretory process. 9. Ca2+ dependent AVP release was partially inhibited by the calmodulin antagonist trifluoroperazine. 10. Hormone release was potentiated by the protein kinase C activator, 4-beta-phorbol 12-myristate acetate (TPA). 11. Whereas 0.2 microM-Ca2+ induced a barely significant increase in AVP release, inositol 1,4,5-triphosphate, in the continued presence of 0.2 microM-Ca2+, produced a large secretory response. 12. 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), an inhibitor of Cl- permeability, reduced the Ca2+-dependent AVP release. 13. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), which reduces the transmembrane potential of isolated neurohypophysial granules, inhibited the Ca2+-dependent hormone secretion. 14. Maximal hormone release occurred at pH 6.6. 15. It is concluded that the permeabilized neurosecretosomes represent an excellent model for studying the minimal requirements for neurosecretion.