EVIDENCE FOR TRH-INDUCED INFLUX OF EXTRACELLULAR CA2+ IN PITUITARY GH4C1 CELLS

The aim of the study was to investigate the relationship between thyrotropin-releasing hormone (TRH)-induced changes in intracellular free Ca2+ ([Ca2+]i), and influx of extracellular Ca2+ in Fura 2 loaded pituitary GH4C1 cells. Stimulating the cells with TRH in a Ca2+-containing buffer induced a biphasic change in [Ca2+]i. First, a transient increase in [Ca2+]i, followed by a sustained phase. In cells stimulated with TRH in a Ca2+-free buffer, the transient increase in [Ca2+]i was decreased (p < 0.05), and the sustained phase was totally abolished. Addition of Ni2+ prior to TRH blunted the component of the TRH-induced transient increase in [Ca2+]i dependent on influx of Ca2+. In the presence of extracellular Mn2+, TRH stimulated quenching of Fura 2 fluorescence. This quenching was blocked by Ni2+. The results indicate that both the TRH-induced transient increase in [Ca2+]i as well as the sustained phase in [Ca2+]i in GH4C1 cells is dependent on influx of extracellular Ca2+. © 1991 Academic Press, Inc.