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-2, and cell type was subsequently identified by immunocytochemistry. At 100 nM and 1 microM, 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 nM TRH and was not significantly reduced by removal of extracellular Ca2+. The duration of the initial [Ca2+]i transient was significantly shorter at 1 microM 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.
Ashworth
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Department of Pharmacology and Physiology, University of Rochester School of Medicine and Dentistry, New York 14642, USA.
Hinkle
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PM
D000818:Animals; D001491:Basophils / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D005260:Female; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013972:Thyrotropin / Q000378:metabolism*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D013972:Thyrotropin; D002118:Calcium