Spontaneous calcium oscillations in Xenopus laevis melanotrope cells are mediated by omega-conotoxin sensitive calcium channels.

The dynamics of intracellular Ca2+ signalling in single melanotrope cells of the pituitary gland of the amphibian Xenopus laevis have been studied by means of a digital imaging technique using the fluorescent dye Fura-2. When placed in vitro, the majority of the cells (77%) displayed spontaneous oscillatory changes in the free cytosolic Ca2+ concentration with a frequency of 1 +/- 0.25 (SD) min-1. The oscillations rapidly stopped when extracellular Ca2+ was reduced to nanomolar concentrations, revealing their complete dependence on Ca2+ influx. The fact that the Ca2+ oscillations were blocked by 1 microM omega-conotoxin, but not by nifedipine, at concentrations up to 50 microM, indicated that Ca2+ entered the cell via N-type rather than L-type voltage operated Ca2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca(2+)-ATPase activity, elevated the baseline Ca2+ concentration but had no effect on the occurrence of the spontaneous oscillations. This suggests that intracellular Ca2+ pools are not involved in the mechanism underlying spontaneous Ca2+ oscillations. This is the first report showing spontaneous Ca2+ oscillations mediated by N-type Ca2+ channels in melanotrope cells.