Use of a trapped fluorescent indicator to demonstrate effects of thyroliberin and dopamine on cytoplasmic calcium concentrations in bovine anterior pituitary cells.

The fluorescent calcium indicator 'quin2' was used to demonstrate changes in cytoplasmic calcium concentrations in bovine anterior pituitary cells. The basal calcium concentration was 0.21 +/- 0.02 microM (mean of 4 cell preparations). Thyroliberin (TRH) (10(-10) - 10(-6) M) rapidly and at the higher concentrations transiently increased the concentration. Dopamine (10(-10) - 10(-7) M) decreased the concentration transiently and more slowly. At 10(-5) M, dopamine prevented the increase in calcium concentration caused by 10(-9) M TRH, and partially inhibited the increase caused by higher concentrations of the peptide. The data support the hypothesis that calcium is the second messenger for TRH, and suggest that dopamine inhibits TRH-induced prolactin secretion by preventing the calcium concentration from exceeding the level necessary to increase secretion.