3',5'-cyclic adenosine monophosphate augments intracellular Ca2+ concentration and gonadotropin-releasing hormone (GnRH) release in immortalized GnRH neurons in an Na+ -dependent manner.
In GT1-7 cells, cAMP increases the intracellular Ca2+ concentration ([Ca2+](i)) through activation of the voltage-gated Ca2+ channels, thereby facilitating GnRH release. To activate these channels, the membrane potential must be depolarized. In the present study we hypothesize that cAMP depolarizes the cells by increasing the membrane Na+ permeability, as in the case of somatotrophs and pancreatic beta-cells. To examine this, we analyzed [Ca2+](i) and [Na+](i) in GT1-7 cells by an intracellular ion-imaging technique along with cAMP assay by RIA. Forskolin, a direct activator of adenylyl cyclase, increased [Ca2+](i) and [Na+](i) via cAMP formation. The forskolin-induced increase in [Ca2+](i) depended on the presence of Ca2+ and Na+ in the extracellular solution. This response was blocked by the voltage-gated Ca2+ channel blocker, nifedipine; the nonselective cation channel blocker, gadolinium (Gd3+); and the cyclic nucleotide-gated channel blocker, l-cis-diltiazem. In contrast, the forskolin-induced increase in [Na+](i) depended only on extracellular Na+, not on Ca2+. Gd3+ and l-cis-diltiazem also blocked the increase in [Na+](i). Furthermore, the forskolin-induced increase in GnRH release was blunted in both low Ca2+ and low Na+ media. The results indicate that cAMP increases the membrane Na+ permeability, probably through nonselective cation channels on GT1-7 cells, thereby promoting GnRH release.
Kaneishi
Keisuke
K
Department of Physiology, Nippon Medical School, Tokyo 113-8602, Japan.
Sakuma
Yasuo
Y
Kobayashi
Hisae
H
Kato
Masakatsu
M
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002461:Cell Line, Transformed; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D007546:Isoquinolines / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D012964:Sodium / Q000494:pharmacology*; D013449:Sulfonamides; D013779:Tetrodotoxin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007546:Isoquinolines; D013449:Sulfonamides; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D007987:Gonadotropin-Releasing Hormone; D013779:Tetrodotoxin; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D012964:Sodium; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide; D002118:Calcium