Voltage‐gated and agonist‐mediated rises in intracellular Ca2+ in rat clonal pituitary cells (GH3) held under voltage clamp.

1. Intracellular free calcium (Ca2+i) was estimated in single GH3 cells by dual wavelength emission spectrofluorimetry using the Ca2+ indicator dye Indo‐1, while cells were held under voltage clamp using patch clamp techniques. 2. Depolarization of cells evoked a transient rise in Ca2+i that increased with increasing duration of depolarization to a peak at about 10 s. 3. Calcium transients showed a bell‐shaped dependence on the amplitude of the depolarizing pulse. They were abolished in the absence of extracellular calcium and by application of 10 microM‐nifedipine. 4. Thyrotrophin‐releasing hormone (TRH) evoked a transient rise in Ca2+i that was followed by a more sustained period of elevated Ca2+i in some cells. The transient phase of the response but not the sustained phase was seen in the absence of extracellular calcium. 5. Ca2+i transients evoked by depolarization were not affected by pre‐release of internal Ca2+ stores with TRH. 6. The results demonstrate that voltage‐gated Ca2+ entry and Ca2+ store release can each elevate cytoplasmic free calcium in GH3 cells and may both be important for stimulus‐secretion coupling. Non‐voltage‐gated Ca2+ entry is not a major source of Ca2+ under these conditions.