Relationship of thyrotropin-releasing hormone-induced spike and plateau phases in cytosolic free Ca2+ concentrations to hormone secretion. Selective blockade using ionomycin and nifedipine.
In clonal rat pituitary cells (GH cells), thyrotropin-releasing hormone (TRH) induced a pattern of changes in cytosolic free calcium concentrations [( Ca2+]i) composed of two phases: an acute spike phase to micromolar levels which decayed (t1/2 = 8 s) to a near-basal concentration and then rose to a prolonged plateau phase of elevated [Ca2+]i (as measured using Quin 2). Closely following these changes in [Ca2+]i, TRH stimulated a rapid "spike phase" of pronounced, but brief, enhancement of the rate of prolactin and growth-hormone secretion and then a "plateau phase" of prolonged enhancement. These two phases were dissociated using two classes of pharmacologic agents: the ionophore ionomycin, and a calcium channel antagonist nifedipine. Ionomycin (100 nM) specifically blocked (less than 90%) the spike phase of TRH action by rapidly emptying the TRH-regulated reservoir of cellular Ca2+ to generate a TRH-like spike in [Ca2+]i; nifedipine inhibited (less than 50%) the plateau phase of TRH-induced changes in [Ca2+]i and hormone secretion by preventing Ca2+ influx through voltage-dependent Ca2+ channels. These agents demonstrated that the TRH-induced spike in [Ca2+]i in GH cells is caused by release of an ionomycin-sensitive pool of cellular Ca2+ with a small component (10%) due to influx of extracellular Ca2+. The TRH-induced plateau in [Ca2+]i is due to influx of extracellular Ca2+, about half of which enters through voltage-dependent calcium channels and half of which enters via nifedipine/verapamil-insensitive influx. The TRH-induced spike in [Ca2+]i led to a burst in hormone secretion, and the plateau in [Ca2+]i produced a prolonged enhancement of secretion; the spike and plateau phases were generated independently by TRH. A spike in [Ca2+]i is necessary, but not sufficient, to induce burst release of hormone, while the prolonged rate of hormone secretion is intimately related to the steady-state [Ca2+]i.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004987:Ethers / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism; D015759:Ionomycin; D007700:Kinetics; D009543:Nifedipine / Q000494:pharmacology*; D010911:Pituitary Neoplasms; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
D004987:Ethers; D015759:Ionomycin; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D013006:Growth Hormone; D014700:Verapamil; D009543:Nifedipine; D002118:Calcium