Calcium ionophores lead to apoptotic-like changes in Tilapia pituitary cells

Influx or mobilization of Ca2+ plays an important part in the signal transduction mechanisms regulating release of gonadotropin (GtH) and growth hormone (GH) in teleost fish. In mammals it may also mediate a stimulatory effect on the transcription of the genes encoding these hormones (i.e., LHbeta, FSHbeta, and GH). In the present study, exposure of tilapia pituitary cells in primary culture to two ionophores, A23187 and ionomycin, increased GtH and GH secretion over 5-24 h but led to a significant drop in mRNA levels of GtH IIbeta and GH. The mRNA levels of beta actin were also reduced by this treatment, suggesting a general, nonspecific effect in these cells. The morphology of the ionophore-exposed cells also differed markedly; they lacked cytoplasmic extensions, appeared smaller, and were less aggregated than control cells. Staining the nuclei of these cells with 4,6-diamidino-2-phenyl-dihydrochloride revealed that they had undergone condensation and fragmentation, typical of programmed cell death. Extraction of DNA from the ionophore-exposed cells and its separation on ethidium bromide-stained gels revealed that, unlike in control cells, the DNA had been broken into fragments in multiples of approximately 180-200 bp, providing further evidence of apoptotic-like effects of the ionophores on the cells. It is speculated that Ca2+, which mediates stimulation of GtH and GH release by the hypothalamic regulatory hormones, may, under certain conditions, have apoptotic-like effects, which specifically regulate the sizes of gonadotroph and somatotroph cell populations. In addition, the fact that pituitary cells exposed to ionophores may become apoptotic should be borne in mind when experiments on signal transduction are carried out with these substances.