Green Anne K, Stratton Rebecca C, Squires Paul E, Simpson Alec W M
Department of Biological Sciences, The University of Warwick, Gibbet Hill Road, Coventry, UK.
J Biol Chem. 2007 Nov 23;282(47):34542-54. doi: 10.1074/jbc.M707115200. Epub 2007 Sep 24.
Elevations in intracellular Ca(2+) concentration and calpain activity are common early events in cellular injury, including that of hepatocytes. Atrial natriuretic peptide is a circulating hormone that has been shown to be hepatoprotective. The aim of this study was to examine the effects of atrial natriuretic peptide on potentially harmful elevations in cytosolic free Ca(2+) and calpain activity induced by extracellular ATP in rat hepatocytes. We show that atrial natriuretic peptide, through protein kinase G, attenuated both the amplitude and duration of ATP-induced cytosolic Ca(2+) rises in single hepatocytes. Atrial natriuretic peptide also prevented stimulation of calpain activity by ATP, taurolithocholate, or Ca(2+) mobilization by thapsigargin and ionomycin. We therefore investigated the cellular Ca(2+) handling mechanisms through which ANP attenuates this sustained elevation in cytosolic Ca(2+). We show that atrial natriuretic peptide does not modulate the release from or re-uptake of Ca(2+) into intracellular stores but, through protein kinase G, both stimulates plasma membrane Ca(2+) efflux from and inhibits ATP-stimulated Ca(2+) influx into hepatocytes. These findings suggest that stimulation of net plasma membrane Ca(2+) efflux (to which both Ca(2+) efflux stimulation and Ca(2+) influx inhibition contribute) is the key process through which atrial natriuretic peptide attenuates elevations in cytosolic Ca(2+) and calpain activity. Moreover we propose that plasma membrane Ca(2+) efflux is a valuable, previously undiscovered, mechanism through which atrial natriuretic peptide protects rat hepatocytes, and perhaps other cell types, against Ca(2+)-dependent injury.
细胞内钙离子浓度升高和钙蛋白酶活性增强是细胞损伤（包括肝细胞损伤）早期常见的事件。心房利钠肽是一种循环激素，已被证明具有肝脏保护作用。本研究的目的是检测心房利钠肽对细胞外ATP诱导的大鼠肝细胞胞质游离钙离子浓度潜在有害升高及钙蛋白酶活性的影响。我们发现，心房利钠肽通过蛋白激酶G，减弱了单个肝细胞中ATP诱导的胞质钙离子升高的幅度和持续时间。心房利钠肽还可防止ATP、牛磺石胆酸刺激钙蛋白酶活性，以及毒胡萝卜素和离子霉素引起的钙离子动员。因此，我们研究了心房利钠肽减弱胞质钙离子持续升高的细胞钙离子处理机制。我们发现，心房利钠肽不调节钙离子从细胞内储存库的释放或再摄取，但通过蛋白激酶G，既刺激质膜钙离子外流，又抑制ATP刺激的钙离子流入肝细胞。这些发现表明，刺激质膜净钙离子外流（钙离子外流刺激和钙离子内流抑制均有贡献）是心房利钠肽减弱胞质钙离子升高和钙蛋白酶活性的关键过程。此外，我们提出质膜钙离子外流是心房利钠肽保护大鼠肝细胞以及可能其他细胞类型免受钙离子依赖性损伤的一种有价值的、此前未被发现的机制。
