Han Xue Feng, Zhu Yun Long, Hernandez Maria, Keating Damien J, Chen Chen
Department of Physiology, Fourth Military Medical University, Shannxi, China.
Endocrine. 2005 Nov;28(2):217-24. doi: 10.1385/ENDO:28:2:217.
Ghrelin is an endogenous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca(2+) concentration ([Ca(2+)]i), which is determined by Ca(2+) influx and release from intracellular Ca(2+) storage sites. Ca(2+) influx is via voltage-gated Ca(2+) channels, which are activated by cell depolarization. Membrane potential is mainly determined by transmembrane K(+) channels. The present study investigates the in vitroeffect of ghrelin on membrane voltage-gated K(+) channels in the GH3 rat somatotrope cell line. Nystatin-perforated patch clamp recording was used to record K(+) currents under voltage-clamp conditions. In the presence of Co(2+) (1 mM, Ca(2+) channel blocker) and tetrodotoxin (1 microM, Na(+) channel blocker) in the bath solution, two types of voltage-gated K(+) currents were characterized on the basis of their biophysical kinetics and pharmacological properties. We observed that transient K(+) current (IA) represented a significant proportion of total K(+) currents in some cells, whereas delayed rectifier K(+) current (IK) existed in all cells. The application of ghrelin (10 nM) reversibly and significantly decreased the amplitude of both IA and IK currents to 48% and 64% of control, respectively. Application of apamin (1 microM, SK channel blocker) or charybdotoxin (1 microM, BK channel blocker) did not alter the K(+) current or the response to ghrelin. The ghrelin-induced reduction in K(+) currents was not affected by PKC and PKA inhibitors. KT5823, a specific PKG inhibitor, totally abolished the K+ current response to ghrelin. These results suggest that ghrelin-induced reduction of voltage-gated K(+) currents in GH3 cells is mediated through a PKG-dependent pathway. A decrease in voltage-gated K(+) currents may increase the frequency, duration, and amplitude of action potentials and contribute to GH secretion from somatotropes.
胃饥饿素是一种内源性生长激素促分泌素（GHS），可通过GHS受体促使垂体生长激素细胞释放生长激素（GH）。GH的分泌与细胞内游离钙离子浓度（[Ca(2+)]i）直接相关，而[Ca(2+)]i由钙离子内流以及细胞内钙离子储存位点的释放所决定。钙离子内流通过电压门控钙离子通道进行，该通道由细胞去极化激活。膜电位主要由跨膜钾离子通道决定。本研究调查了胃饥饿素对GH3大鼠生长激素细胞系中膜电压门控钾离子通道的体外作用。采用制霉菌素穿孔膜片钳记录法在电压钳条件下记录钾离子电流。在浴液中存在钴离子（1 mM，钙离子通道阻滞剂）和河豚毒素（1 microM，钠离子通道阻滞剂）的情况下，根据两种电压门控钾离子电流的生物物理动力学和药理学特性对其进行了表征。我们观察到，瞬时钾离子电流（IA）在某些细胞的总钾离子电流中占显著比例，而延迟整流钾离子电流（IK）存在于所有细胞中。应用胃饥饿素（10 nM）可使IA和IK电流的幅度分别可逆且显著地降低至对照的48%和64%。应用蜂毒明肽（1 microM，SK通道阻滞剂）或大蝎毒素（1 microM，BK通道阻滞剂）并未改变钾离子电流或对胃饥饿素的反应。胃饥饿素诱导的钾离子电流减少不受蛋白激酶C（PKC）和蛋白激酶A（PKA）抑制剂的影响。特异性蛋白激酶G（PKG）抑制剂KT5823完全消除了钾离子电流对胃饥饿素的反应。这些结果表明，胃饥饿素诱导的GH3细胞中电压门控钾离子电流减少是通过PKG依赖性途径介导的。电压门控钾离子电流的减少可能会增加动作电位的频率、持续时间和幅度，并有助于生长激素细胞分泌GH。
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