Thyrotropin-releasing hormone-induced intracellular calcium responses in individual rat lactotrophs and thyrotrophs

Calcium responses to TRH were recorded for individual cells cultured from rat anterior pituitary tissue loaded with fura-a, and cell type was subsequently identified by immunocytochemistry. At 100 nM and 1 mu M, TRH stimulated a single transient spike of intracellular free calcium ([Ca2+](i)) in 95-100% of lactotrophs. At a concentration of 10 nM or less, the proportion of TRH-responsive cells decreased, and the [Ca2+](i) responses became more heterogeneous, consisting of a biphasic response in which an initial [Ca2+](i) spike was followed by a sustained elevation of [Ca2+](i) or [Ca2+](i) oscillations. Initiation of TRH-induced oscillations required the release of intracellular Ca2+ from thapsigargin-sensitive stores, whereas maintenance of the oscillations required influx of extracellular Ca2+ through nimodipine-sensitive Ca2+ channels. The amplitude of the initial [Ca2+](i) rise increased from 0.1-10 TRH TRH and was not significantly reduced by removal of extracellular Ca2+. The duration of the initial [Ca2+](i) transient was significantly shorter at 1 mu M than at 1 nM TRH. When TRH was added to cells that had been treated with thapsigargin to block the agonist-induced [Ca2+](i) increase, TRH often decreased [Ca2+](i), particularly in cells with high [Ca2+](i). These results suggest that TRH and elevated [Ca2+](i) act as coactivators of Ca2+ efflux, which helps terminate the agonist-evoked [Ca2+](i) transient. In addition, TRH caused increases in [Ca2+](i) in individual rat thyrotrophs, and these responses were heterogeneous. TRH stimulated a [Ca2+](i) response in a lesser proportion of thyrotrophs from euthyroid compared to hypothyroid male rats. Essentially all TRH-responsive cells stained for either PRL or TSH.