Gonadotropin secretion from perifused tilapia pituitary in relation to gonadotropin-releasing hormone, extracellular calcium, and activation of protein kinase C

Gonadotropin (taGTH) secretion from perifused fragments of tilapia pituitaries was stimulated in a dose-dependent manner by an analog of gonadotropin-releasing hormone ([d-Ala⁶] des Gly¹⁰ ethylamide LHRH; GnRHa) in a dose range of 1.28 to 128 pM. The baseline secretion rate and taGTH secretion in response to GnRHa were both reduced when the perifusion medium lacked Ca²⁺. Calcium ionophore (A23187; 0.1 mM mimicked the effect of GnRHa but only in the presence of Ca²⁺. The addition of cobalt chloride to the medium at 0.6 mM initially caused an increase in taGTH secretion which was followed by its decrease. At a CoCl₂ concentration of 1.3 mM, the baseline secretion rate remained low and the effect of GnRHa on taGTH secretion was attenuated. Withdrawal of CoCl₂ from the medium was followed by an elevated basal secretion rate. Five-minute pulses of the protein kinase C activator, 1 oleyl-2-acetyl-rac-glycerol (OAG; 0.25 to 10.4 mM) stimulated taGTH secretion in the presence of Ca²⁺. With the reservation that the experiments were performed on fragments containing more than one pituitary cell type, the results indicate that the stimulation of GTH secretion in this fish is dependent, as in mammals, on extracellular Ca²⁺ and probably involves the activation of protein kinase C. However, the fact that taGTH may be stimulated to some extent in the absence of extracellular calcium or in the presence of 1.3 mM Co²⁺ may point to the possibility that Ca²⁺ is mobilized from intracellular stores as a result of GnRH stimulation or to the involvement of an additional mechanism of GnRH action in fish independent of calcium.