TRH raises cytosolic Ca2+ in human adenomatous lactotrophs

The effect of TRH on cytosolic free calcium concentrations, [Ca2+]i, was evaluated on cell suspensions obtained from 6 human PRL secreting pituitary adenomas. In these cells resting [Ca2+]i levels were variable (mean ±SE; 103.8 ± 6.5, n=25); the addition of 100 nM TRH caused a marked [Ca2+]i rise within 20 sec, the peak values ranging from 200 to 437 nM (285 ± 10.8 nM, n=10). The transients induced by TRH were composed by a rapid increase, due to mobilization of calcium from intracellular stores, followed within a few seconds by a lower plateau which was due to stimulated influx from the extracellular space. In fact, when EGTA and verapamil were applied after TRH they caused the Ca2+ plateau to dissipate rapidly. The addition of 1uM dopamine (DA) caused a substantial decrease of resting [Ca2+]i (about 10–30% ) as well as an inhibition of the plateau phase induced by TRH. The effect of DA completely depended on extracellular Ca2+. The TRH-induced transients observed in adenomatous cells were quite similar in size and time course to those recorded in normal rat lactotrophs. As previously observed in rat lactotrophs, in adenomatous cells treatment with pertussis toxin (PTx, 1 μg/ml for 4 h) was unable to affect the [Ca2+]i transients induced by TRH while completely abolished the effect of DA. The effects of TRH on in vivo and in vitro PRL secretion were also evaluated. Before surgery, no patient showed a positive response to the iv administration of 200 μg TRH ( serum PRL levels: 95 ± 62 ng/ml in basal conditions vs 124 ± 92 after TRH, P=NS). In in vitro secretion study, 100 nM TRH caused a slight stimulation (about 30–40%) of PRL release in 3 out of 4 adenomas and no effect in 1. By contrast, in the same experimental conditions TRH caused a marked increase of PRL release from rat lac-totrophs. In conclusion, adenomatous lactotrophs possess receptors for TRH which are coupled with the intracellular effectors operating in normal cells. Modifications of processes other than TRH signal transduction might account for the poor sensitivity to the peptide observed in prolactinomas both in vivo and in vitro.