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Abstract
A GTP-binding protein (G-protein), termed G-exocytosis (Ge), mediates the effects
of calcium ions in the late stages of the adrenocorticotrophin (ACTH) secretory pathway.
An activator of Ge, mastoparan, also stimulates phospholipase A(2) and so a comparison
of other phospholipase A(2)-activating peptides, melittin and phospholipase A(2)-activating
peptide was made with mastoparan to assess whether phospholipase A(2)activation was
an important component of Ge-evoked secretion. All three peptides stimulated ACTH
secretion in the effective absence of calcium ions from permeabilised cells, actions
potentiated by a phospholipase A(2)inhibitor. Ca(2+)-evoked secretion from permeabilised
cells was similarly potentiated by a phospholipase A(2) inhibitor. Furthermore, arachidonic
acid inhibited Ca(2+)- and Ge-evoked ACTH secretion, an action blocked by the cyclo-oxygenase
inhibitor ibuprofen. This study suggests that the products of phospholipase A(2)-generated
arachidonic metabolism may exert an inhibitory action on the late post-Ca(2+) stages
of the ACTH secretory pathway and that prostaglandins may be the active agents in
this capacity.