A potential code of luteinizing hormone-releasing hormone-induced calcium ion responses in the regulation of luteinizing hormone secretion among individual gonadotropes

Luteinizing hormone-releasing hormone (LHRH) induces two Ca2+ responses in single gonadotropes: a Ca2+ spike/plateau or oscillation. Similar receptor-mediated Ca2+ signals have been reported in many cell types but their functional significance is obscure. Accordingly, we have determined the concentration-response properties of LHRH-induced luteinizing hormone (LH) release at the single cell level. We demonstrate a critical single cell LHRH threshold for LH release. Each gonadotrope had a particular LHRH threshold value and a range of different single cell thresholds was distributed in the gonadotrope population. The physiological significance of the threshold was demonstrated by a striking reduction (delta ED50 = 153 nM) of the LHRH threshold immediately before the preovulatory surge of LH release. The metestrous phenotype of secretion resembled a quantal process in contrast with the graded process of the proestrous phenotype. That is, the quantity of hormone secreted per metestrous gonadotrope was independent of LHRH concentration and more all-or-none than graded. The LHRH threshold and the quantal secretion process of metestrous gonadotropes was further studied by measuring cytosolic Ca2+ using fura-2 and digital imaging microscopy. We provide evidence suggesting that the Ca2+ spike/plateau and oscillation are the respective responses to subthreshold and suprathreshold concentrations of LHRH. It is proposed therefore that the Ca2+ oscillation and spike/plateau response form a binary intracellular signaling code that functions as an on-off switch. It is further proposed that this potential code unraveled here for the regulation of hormone secretion may also regulate other gonadotrope functions. Thus, while the Ca2+ spike/plateau response is strongly associated with LH release, it may be associated with reduced levels of LH-beta mRNA, and reduced numbers of LHRH receptors. Conversely, while the Ca2+ oscillation appears to be unrelated to LH release, it may be associated with increased levels of LH-beta mRNA, and increased numbers of LHRH receptors. This model may explain in molecular terms the long-standing observation that an invariant, albeit pulsatile, pattern of LHRH release is sufficient to support the preovulatory surge of LH release.