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

The dynamics of intracellular Ca 2+ 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 Ca 2+ concentration with a frequency of 1 ± 0.25 (SD) min −1. The oscillations rapidly stopped when extracellular Ca 2+ was reduced to nanomolar concentrations, revealing their complete dependence on Ca 2+ influx. The fact that the Ca 2+ oscillations were blocked by 1 μM ω-conotoxin, but not by nifedipine, at concentrations up to 50 μM, indicated that Ca 2+ entered the cell via N-type rather than L-type voltage operated Ca 2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca 2+-ATPase activity, elevated the baseline Ca 2+ concentration but had no effect on the occurrence of the spontaneous oscillations. This suggests that intracellular Ca 2+ pools are not involved in the mechanism underlying spontaneous Ca 2+ oscillations. This is the first report showing spontaneous Ca 2+ oscillations mediated by N-type Ca 2+ channels in melanotrope cells.