Phorbol esters and corticotropin releasing factor stimulate calcium influx in the anterior pituitary tumor cell line, AtT-20, through different intracellular sites of action.
The mechanisms by which an activator of cyclic AMP-dependent protein kinase, corticotropin releasing factor (CRF) and the protein kinase C stimulant, phorbol myristate acetate (PMA) regulate the level of intracellular free calcium in the mouse anterior pituitary cell line AtT-20 were examined using the fluorescence probe Quin 2. The increase in cytosolic calcium in AtT-20 cells induced by CRF and PMA was blocked by calcium channel antagonists indicating that both agents stimulate calcium influx. The ability of PMA to raise cytosolic calcium levels was prevented by the sodium channel antagonist tetrodotoxin, suggesting that phorbol esters depolarize the cell membrane or increase action potential frequency to enhance calcium influx. The K+ channel antagonists, tetraethylammonium, cesium and 4-aminopyridine, inhibited PMA-stimulated calcium influx in AtT-20 cells. Thus, one mechanism by which protein kinase C activation may lead to a depolarization of the cell membrane is through a reduction in K+ currents. In contrast, neither tetraethylammonium or cesium reduced CRF-stimulated calcium influx into AtT-20 cells. The stimulation of calcium influx by CRF, therefore, appears to not involve changes in K+ currents in AtT-20 cells. CRF activates cyclic AMP-dependent protein kinase to stimulate calcium influx either by facilitating calcium conductance directly or by modifying the membrane potential or firing activity of AtT-20 cells.
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Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia.
D000324:Adrenocorticotropic Hormone / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology*; D000242:Cyclic AMP / Q000494:pharmacology; D051379:Mice; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D010911:Pituitary Neoplasms / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D013779:Tetrodotoxin; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone; D000242:Cyclic AMP; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium