Abstract
The cytosolic free calcium concentration, [Ca 2+] i, was monitored in single isolated goldfish gonadotrophs with the fluorescent probe Indo-1. It was found that goldfish gonadotrophs exhibit both spontaneous and secretagogue-induced [Ca 2+] i rises. Spontaneous [Ca 2+] i transients showed striking kinetic features and a sensitivity to external Ca 2+ suggesting that they were the consequence of transient Ca 2+ entries. Two kinetically distinct patterns of [Ca 2+] i rises were generated in response to the two native forms of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In a part of the gonadotrophs, GnRHs triggered a plateau [Ca 2+] i rise whereas in other responsive cells they induced a series of [Ca 2+] i bursts, each consisting of grouped [Ca 2+] i transients. Both plateau and burst [Ca 2+] i response patterns were due to Ca 2+ entry through plasma membrane Ca 2+ channels, inasmuch as they were suppressed with external Ca 2+ removal. No contribution of Ca 2+ release from thapsigargin-sensitive stores was observed in either response pattern. While in mammalian gonadotrophs GnRH raises [Ca 2+], by mostly acting on internal Ca2+ sequestering stores, our results show that GnRH-stimulated goldfish gonadotrophs rapidly increase Ca 2+ entry to enhance their [Ca 2+] i levels.
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