Effect of bromocriptine on thyroid hormone-induced growth hormone production in cultured GH1 cells

L-triiodothyronine induces a three-fold increase in growth hormone production in cultured GH1 cells. The dopaminergic agonist, bromocriptine, inhibits the thyroid hormone-induced growth hormone production to the level of that produced by the control cells. This effect can be observed within eight hours of incubation and persists as long as forty-eight hours in culture and is not due to a change in cell population. In contrast, bromocriptine appears to have no effect on growth hormone production in the control cultures. The estimated concentration of bromocriptine which gives a half-maximal inhibitory effect is 0.3 nM, and concentrations above 5 nM give a complete inhibitory effect. The intracellular growth hormone concentration represents only a very small fraction of total hormone production and does not appear to be influenced by bromocriptine. Therefore, the inhibitory effect of bromocriptine on growth hormone production in thyroid hormone treated cultured GH1 cells may result from its action on the synthesis of growth hormone.