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

Ca2+ signals are involved in the signal transduction of neuroendocrine regu lators. In goldfish, two endogenous gonadotropin-releasing hormones, salmon (s)GnRH and chicken (c)GnRH-II, control maturational gonadotropin secretio n. Although considerable evidence suggests that sGnRH and cGnRH-II exert th eir activity on goldfish gonadotropes through a single population of recept ors, differences in signal transduction mechanisms between these peptides h ave been demonstrated. We used ratio-metric Fura-2 Ca2+ imaging of single m orphologically identified gonadotropes to quantitatively compare the Ca2+ s ignals 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 ma ximal concentrations, Ca2+ signals generated by cGnRH-II rose significantly faster than those elicited by sGnRH, while other parameters such as the ma ximum amplitude, average Ca2+ increase, and latency did not differ between the two peptides. Ca2+ signals evoked by sGnRH or cGnRH-II were often spati ally restricted to one part of the cell over the duration of the response. We provide a comprehensive account of the spatial and temporal aspects, inc luding calculated kinetics, of GnRH-evoked Ca2+ signals in single identifie d gonadotropes. This is the first report of quantified differences in Ca2signals generated by two endogenous GnRH neuropeptides, which may act throu gh the same receptor population in this cell type. (C) 1999 Academic Press.