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

Influx or mobilization of Ca2+ plays an important part in the signal transd uction mechanisms regulating release of gonadotropin (GtH) and growth hormo ne (GH) in teleost fish. In mammals it may also mediate a stimulatory effec t on the transcription of the genes encoding these hormones (i.e., LH beta, FSH beta, and GH). In the present study, exposure of tilapia pituitary cel ls in primary culture to two ionophores, A23187 and ionomycin, increased Gt H and GH secretion over 5-24 h but led to a significant drop in mRNA levels of GtH II beta and GH. The mRNA levels of beta actin were also reduced by this treatment, suggesting a general, nonspecific effect in these cells. Th e morphology of the ionophore-exposed cells also differed markedly; they la cked cytoplasmic extensions, appeared smaller, and were less aggregated tha n control cells. Staining the nuclei of these cells with 4,6-diamidino-2-ph enyl-dihydrochloride revealed that they had undergone condensation and frag mentation, typical of programmed cell death. Extraction of DNA from the ion ophore-exposed cells and its separation on ethidium bromide-stained gels re vealed that, unlike in control cells, the DNA had been broken into fragment s in multiples of approximately 180-200 bp, providing further evidence of a poptotic-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 ef fects, which specifically regulate the sizes of gonadotroph and somatotroph cell populations. In addition, the fact that pituitary cells exposed to io nophores may become apoptotic should be borne in mind when experiments on s ignal transduction are carried outwith these substances. (C) 1999 Academic Press.