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 mu M omega-conotoxin, but not by nifedipine, at concentrations up to 50 mu M, indicated that Ca2+ entered the cell via N-type rather than L-type voltage operated Ca2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca2+-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.