Hormonal regulation of cytosolic free calcium and its functional consequences: the GH-cell model

Use of clonal strains of prolactin (PRL)- and growth hormone-producing rat pituitary cells has proven informative in elucidating a number of the early biochemical, ionic, and secretory events regulated by the hypothalamic tripeptide, thyrotropin-releasing hormone (TRH). TRH causes biphasic changes in the concentration of cytosolic free calcium [( Ca2+]i) in GH4C1 cells and biphasic changes in hormone secretion. Early changes occur on a msecond to second scale and late changes, on a time scale of minutes. Although increases in [Ca2+]i are essential for enhanced secretion, at least in the case of the rapid initial phase, the TRH-induced increase in [Ca2+]i is necessary, but not sufficient to enhance secretion. A co-mediator with calcium appears to be diacylglycerol. The majority of the calcium involved in the early phase of rise in [Ca2+]i induced by TRH is derived from intracellular sources, while essentially all of the calcium rise observed in the late phase is derived from extracellular calcium entering the cell through both voltage-dependent and voltage-independent conductances. Because TRH causes an elevation of inositol(1,4,5) trisphosphate [Ins(1,4,5)P3] within seconds, but not mseconds, further studies are required before it can be concluded unequivocally that Ins(1,4,5)P3 is the sole mediator of the rapid phase of rise in [Ca2+]i induced by TRH in GH-cells.(ABSTRACT TRUNCATED AT 250 WORDS)