Growth hormone releasing factor depolarizes rat pituitary cells in Na+-dependent mechanism

Human growth hormone releasing factor (hGRF) is a specific stimulator of growth hormone (GH) secretion from somatotrophs both in vivo and in vitro. In this secretory process of GH adenosine 3',5'-cyclic monophosphate (cAMP) is thought to function as an intracellular mediator. The influx of Ca2+ from the extracellular space is indispensable for hGRF-induced GH secretion, which probably occurs through voltage-sensitive Ca2+ channels. We recently demonstrated that the extracellular Na+ is also essential for hGRF-induced GH secretion and proposed the possibility that hGRF depolarizes somatotrophs by increasing membrane Na+ conductance via cAMP, thereby activating voltage-sensitive Ca2+ channels, which in turn facilitate GH secretion. To directly demonstrate this possibility, we measured changes in both the membrane potentials and the intracellular Ca2+ concentration with fluorescent dyes bis-oxonol and fura 2 respectively and found that: (1) hGRF depolarized rat somatotrophs, (2) this hGRF-induced depolarization was greatly suppressed by replacing extracellular Na+ with mannitol or tris(hydroxymethyl)aminomethane (Tris+), but not by removing Ca2+ from the extracellular solution, and (3) hGRF augmented the intracellular Ca2+ concentration, which depended on both extracellular Ca2+ and Na+.