Thyrotropin-releasing hormone-induced spike and plateau in cytosolic free Ca2+ concentrations in pituitary cells. Relation to prolactin release

Abstract

Using the acetoxymethyl ester of 'Quin 2', a fluorescent Ca2+-indicator, we have loaded prolactin (PRL)-producing rat pituitary cells with non-toxic concentrations of Quin 2 and quantitated changes in cytosolic free calcium concentration ([Ca2+](i)) during stimulation of PRL release by thyrotropin-releasing hormone (TRH) and 40 mM K+. TRH induced a biphasic response, with an immediate (< 1 s) spike in [Ca2+](i) from basal levels (350 ± 80 nM) to a peak of 1-3 μM, which decayed rapidly (t( 1/2 ) = 8 s) to a near basal nadir, then rising to a plateau in [Ca2+](i) of 500-800 nM. The TRH-induced spike phase was attenuated but not abolished by prior addition of EGTA, while the plateau phase was eliminated by EGTA. Addition of 40 mM K+ caused an immediate spike in [Ca2+](i) to 1-3 μM which equilibrated slowly (t( 1/2 ) = 1 min) directly to a plateau of 600-800 nM. The K+-induced spike and plateau phases were both abolished by prior addition of EGTA. The biphasic nature of TRH action on [Ca2+](i) parallels the biphasic actions of TRH on 45Ca2+ fluxes and the biphasic release of PRL by GH cells in suspension. These findings provide evidence that Ca2+-dependent agonist-mediated increases in [Ca2+](i) and hormone release are linked, and may generally have two modes; an acute 'spike' mode, dependent primarily on redistribution of intracellular Ca2+ stores; and a sustained 'plateau' mode, dependent on influx of extracellular Ca2+.