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+)](i)), 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+)](i) in response to spontaneous action potentials. These [Ca(2+)](i) responses were not inhibited by thapsigargin or caffeine. We did find a significant correlation between the magnitude of spontaneous [Ca(2+)](i) increases and action potential duration. We conclude that intracellular calcium stores are not released during the spontaneous [Ca(2+)](i) oscillations observed in these cells, and that the magnitude of [Ca(2+)](i) oscillations is a direct result of extracellular calcium influx that is determined in part by action potential duration.
Copyright 2000 S. Karger AG, Basel