Stimulation by melittin of adrenocorticotropin and beta-endorphin release from rat adenohypophysis in vitro |
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Authors: | W. Knepel Ch. Gerhards |
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Affiliation: | 1. Department of Logistics and Operations Management, HEC Montréal, H3T 2A7, Canada;2. IMT Atlantique, LS2N, UMR CNRS 6004, Nantes, France |
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Abstract: | The effect of melittin on the release of adrenocorticotropin (ACTH) and β-endorphin from the corticotropic cells of the rat adenohypophysis was examined in vitro. Anterior pituitary quarters were perifused or incubated in vitro and ACTH- (ACTH-IR) or β-endorphin-like immunoreactivity (β-End-IR) in the medium was measured by radioimmunoassays. Melittin stimulated ACTH-IR and β-End-IR release. This effect was rapid in onset, reversible, and concentration-related (50–5000 ng/ml) and dependend on the presence of calcium ions in the incubation medium. Melittin also elevated the tissue content of unesterified 3H-arachidonic acid that had previously been incorporated into lipids. Purported phospholipase A2 inhibitors, mepacrine (up to 1 mM), dexamethasone (0.5 mg/kg in vivo, 50 nM in vitro), or p-bromophenacylbromide (100 μM), did not decrease the Melittin (500 ng/ml) — induced β-End-IR release, although mepacrine and dexamethasone may have inhibited phospholipase A2 activity as indicated by an inhibition of melittin-evoked prostaglandin E2 formation. After stimulation by melittin (500 ng/ml), β-End-IR release was not affected by the cyclooxygenase inhibitor inddomethacin (up to 140 μM), whereas nordihydroguaiaretic acid (100 μM), a lipoxygenase inhibitor, or BW755C (250 μM), an inhibitor of both cyclooxygenase and lipoxygenase, abolished melittin-induced hormone secretion. We conclude that melittin generates a signal in the corticotropic cells of the rat adenohypophysis which induces hormone secretion by exocytosis. This signal may be unrelated to the activation by melittin of phospholipase A2. |
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