Osmotic pressure regulation of prolactin and growth hormone release from bovine secretory granules.
PRL and GH are stored in bovine pituitary secretory granules in part as intermolecular disulfide-linked multimers (as many as 50 monomers/oligomer). In vitro incubation of granules with reduced glutathione (GSH) results in the production of monomeric hormones and increased hormone release. We tested the hypothesis that increases in intragranular osmolality due to conversion of oligomers to lower mol wt forms are key to granule rupture. Hormone release was measured after granule incubations in medium made hyperosmotic by the addition of raffinose or sucrose. PRL release at pH 7.8 with 0.7 M raffinose was reduced to 43.5% of the control value, whereas GH release ranged from 70-100% of the control value. Augmented release stimulated by 2 mM GSH was also diminished by raffinose, to 17.9% (PRL) and 75.6% (GH) of control values. The IC50 for raffinose inhibition of PRL release was 0.62 M; this shifted to 0.38 M in the presence of 2 mM GSH. Raffinose also blunted the granule response to GSH at all other GSH concentrations tested and decreased stimulated release at pH 9 as well as at 7.8. Interference with the ability to maintain an osmotic gradient, a maneuver accomplished by partial disruption of granule membranes with 0.5% deoxycholate treatment, reduced or abolished the raffinose inhibition. Molecular sizing of granule fractions on Superose-6 (Pharmacia) indicated that PRL monomers remaining in the pelletable granule fraction increased with raffinose about 2-fold under both basal and GSH-stimulated conditions. These data support the concept that thiol:disulfide equilibria may be important in PRL and GH storage and secretion by their influence on intragranular osmolality. In exocytosis in vivo, osmotically mediated granule rupture may be triggered by thiols during granule:plasma membrane fusion.
D000818:Animals; D002417:Cattle; D002474:Cell-Free System; D003594:Cytoplasmic Granules / Q000378:metabolism; D003840:Deoxycholic Acid / Q000494:pharmacology; D005089:Exocytosis; D005978:Glutathione / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism*; D066298:In Vitro Techniques; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000378:metabolism* / Q000648:ultrastructure; D011388:Prolactin / Q000378:metabolism*; D011887:Raffinose; D013395:Sucrose
D003840:Deoxycholic Acid; D013395:Sucrose; D011388:Prolactin; D013006:Growth Hormone; D005978:Glutathione; D011887:Raffinose