Lussier B T, French M B, Moor B C, Kraicer J
Department of Physiology, University of Western Ontario, London, Canada.
Endocrinology. 1991 Jan;128(1):592-603. doi: 10.1210/endo-128-1-592.
GH-releasing factor (GRF)-stimulated GH release is dependent on a biphasic increase in free intracellular Ca2+ concentration [( Ca2+]i), resulting from an influx of Ca2+ into somatotrophs, while the inhibitory action of somatostatin (SRIF) on basal and GRF-induced GH release results from its ability to lower [Ca2+]i by inhibiting Ca2+ influx. This study was carried out to investigate the mechanism by which GRF and SRIF regulate [Ca2+]i to control GH release. The roles of ion channels, cAMP-dependent processes, and protein kinase-C (PKC) were investigated by measuring changes in [Ca2+]i, 45Ca influx, and GH release when purified rat somatotrophs were exposed to high K+, cAMP analogs, prostaglandin E2, as well as the PKC activators 1,2-dioctanoyl-glycerol and phorbol 12-myristate 13-acetate. High K+ depolarization produced a rapid and transient increase in [Ca2+]i, while cAMP and prostaglandin E2 led to a sustained elevated [Ca2+]i. PKC activators produced a transient increase in [Ca2+]i, followed by a decrease to below baseline. All secretagogues tested raised [Ca2+]i by stimulating Ca2+ influx through L-type voltage-sensitive Ca2+ channels (VSCC), since the increases in [Ca2+]i were blocked by incubation in Ca2(+)-free medium and by the dihydropyridine Ca2+ antagonist nifedipine. SRIF lowered [Ca2+]i by blocking the Ca2+ influx stimulated by all of these GH secretagogues except high K+. These results are consistent with the model in which GRF initiates its action by increasing Na+ conductance to depolarize the somatotroph via cAMP. This depolarization would stimulate Ca2+ influx through VSCC, which would result in the first phase of the GRF-dependent increase in [Ca2+]i. This increase in [Ca2+]i would stimulate Ca2+ removal from the cytosol by activating Ca-ATPase via Ca-calmodulin and/or PKC. This would result in the lowering of [Ca2+]i to the plateau level of the second phase of the GRF response. SRIF prevents the GRF-induced increase in [Ca2+]i by increasing K+ conductance and, thus, hyperpolarizing the cell. Hyperpolarization would close VSCC, leading to a decrease in Ca2+ influx, with a subsequent drop in [Ca2+]i.
生长激素释放因子（GRF）刺激的生长激素释放依赖于细胞内游离钙离子浓度[Ca2+]i的双相增加，这是由于钙离子流入生长激素分泌细胞所致，而生长抑素（SRIF）对基础和GRF诱导的生长激素释放的抑制作用源于其通过抑制钙离子内流来降低[Ca2+]i的能力。本研究旨在探讨GRF和SRIF调节[Ca2+]i以控制生长激素释放的机制。当纯化的大鼠生长激素分泌细胞暴露于高钾、环磷酸腺苷类似物、前列腺素E2以及蛋白激酶-C（PKC）激活剂1,2 - 二辛酰甘油和佛波醇12 - 肉豆蔻酸酯13 - 乙酸酯时，通过测量[Ca2+]i、45Ca内流和生长激素释放的变化，研究了离子通道、环磷酸腺苷依赖性过程和PKC的作用。高钾去极化使[Ca2+]i迅速短暂增加，而环磷酸腺苷和前列腺素E2导致[Ca2+]i持续升高。PKC激活剂使[Ca2+]i短暂增加，随后降至基线以下。所有测试的促分泌剂均通过刺激钙离子通过L型电压敏感性钙离子通道（VSCC）内流来提高[Ca2+]i，因为在无钙培养基中孵育以及使用二氢吡啶类钙离子拮抗剂硝苯地平可阻断[Ca2+]i的增加。SRIF通过阻断除高钾外所有这些生长激素促分泌剂刺激的钙离子内流来降低[Ca2+]i。这些结果与以下模型一致，即GRF通过增加钠离子电导使生长激素分泌细胞去极化，从而启动其作用，这一去极化会刺激钙离子通过VSCC内流，导致GRF依赖性[Ca2+]i增加的第一阶段。[Ca2+]i的这种增加会通过钙调蛋白和/或PKC激活钙-ATP酶，从而刺激钙离子从细胞质中清除。这将导致[Ca2+]i降至GRF反应第二阶段的平台水平。SRIF通过增加钾离子电导使细胞超极化，从而阻止GRF诱导的[Ca2+]i增加。超极化会关闭VSCC，导致钙离子内流减少，随后[Ca2+]i下降。
