Recruitment of calcium from intracellular stores does not occur during the expression of large spontaneous calcium oscillations in GH(3) cells and lactotropic cells in primary culture.

We used simultaneous electrophysiological and intracellular calcium microfluorometry recordings to directly test for the presence of a calcium-induced calcium release mechanism in individual GH(3) cells and cells of a lactotrope-enriched primary culture. In voltage-pulse experiments, extending the duration of a depolarizing voltage-pulse increased intracellular calcium concentration (Ca(2+)), but we did not observe any evidence for recruitment of intracellular calcium stores. Furthermore, depletion of intracellular calcium stores with thapsigargin or caffeine did not change the calculated calcium buffer capacity of the cells. In current-clamp experiments, we observed elevations in Ca(2+) in response to spontaneous action potentials. These Ca(2+) responses were not inhibited by thapsigargin or caffeine. We did find a significant correlation between the magnitude of spontaneous Ca(2+) increases and action potential duration. We conclude that intracellular calcium stores are not released during the spontaneous Ca(2+) oscillations observed in these cells, and that the magnitude of Ca(2+) oscillations is a direct result of extracellular calcium influx that is determined in part by action potential duration.