Two endogenous gonadotropin-releasing hormones generate dissimilar Ca2+ signals in identified goldfish gonadotropes

Ca2+ signals are involved in the signal transduction of neuroendocrine regulators. In goldfish, two endogenous gonadotropin-releasing hormones, salmon (s)GnRH and chicken (c)GnRH-II, control maturational gonadotropin secretion. Although considerable evidence suggests that sGnRH and cGnRH-II exert their activity on goldfish gonadotropes through a single population of receptors, differences in signal transduction mechanisms between these peptides have been demonstrated. We used ratio-metric Fura-2 Ca2+ imaging of single morphologically identified gonadotropes to quantitatively compare the Ca2+ signals evoked by sGnRH and cGnRH-II. The amplitude and the rate of rise of sGnRH- and cGnRH-II-evoked Ca2+ signals increased with concentration. At maximal concentrations, Ca2+ signals generated by cGnRH-II rose significantly faster than those elicited by sGnRH, while other parameters such as the maximum amplitude, average Ca2+ increase, and latency did not differ between the two peptides. Ca2+ signals evoked by sGnRH or cGnRH-II were often spatially restricted to one part of the cell over the duration of the response. We provide a comprehensive account of the spatial and temporal aspects, including calculated kinetics, of GnRH-evoked Ca2+ signals in single identified gonadotropes. This is the first report of quantified differences in Ca2+ signals generated by two endogenous GnRH neuropeptides, which may act through the same receptor population in this cell type. (C) 1999 Academic Press.