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 β-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.