The L beta T2 clonal gonadotrope: A model for single cell studies of endocrine cell secretion

We have used single gonadotropes from the newly derived line, L beta T2, to investigate the modulation of Ca2+ signaling and exocytosis by the steroid hormone environment. This cell line, derived by targeted oncogenesis in transgenic mice, has recently been shown to secrete LH in response to GnRH. We have characterized the effects of both GnRH and membrane depolarization on exocytosis and intracellular [Ca2+] ([Ca2+](i)) in individual L beta T2 cells. GnRH (1-100 nM) evoked concentration-dependent increases in [Ca2+](i) and secretion, as monitored by measurement of plasma membrane capacitance (C-m) using the whole-cell perforated-patch technique, and the extent of these changes were dependent upon steroid hormone background. GnRH treatment of cells cultured in medium containing charcoal-treated FBS (ct-FBS) showed smaller changes in [Ca2+](i) than cells cultured in untreated FBS. However, when estradiol (E(2)) and dexamethasone (Dex) were added to the ct-FBS medium (E(2)Dex-ct-FBS), the elevations in [Ca2+](i) stimulated by GnRH increased almost 2-fold. Additionally, the rates of secretion in the E(2)/Dex-ct-FBS-cultured cells were greater than in either ct-FBS- or FBS-cultured cells. The increase in secretory response observed with E(2)/Dex-ct-FBS appeared to be due to both an increase in the peak [Ca2+](i) stimulated by GnRH and a shift toward increased sensitivity of the Ca2+ dependency of exocytosis. In contrast to GnRH-evoked responses, the increases in [Ca2+](i) elicited by depolarization were greater in cells cultured in ct-FBS than in E(2)/Dex-ct-FBS; however, the secretory rates were no different in the two groups. Likewise, there was no apparent effect of steroid treatment on the Ca2+ dependency of depolarization-evoked exocytosis. In summary, these results 1) clearly demonstrate the utility of this cell line for single-cell studies of both agonist- and depolarization-evoked secretion; 2) reveal that steroid hormone background has profound effects on L beta T2 cells, both on stimulus-induced calcium mobilization and on the apparent Ca2+ sensitivity of exocytosis; and 3) show that expression of the steroid hormone effect on Ca2+-sensitivity is dependent upon receptor occupation by GnRH.