Fröhlich Otto, Aggarwal Deepak, Klein Janet D, Kent Kimilia J, Yang Yuan, Gunn Robert B, Sands Jeff M
Department of Physiology, Emory University School of Medicine, Atlanta, GA 30322, USA.
Am J Physiol Renal Physiol. 2008 Mar;294(3):F518-24. doi: 10.1152/ajprenal.00349.2007. Epub 2008 Jan 2.
Trans-epithelial tracer urea flux across Madin-Darby canine kidney (MDCK) cells permanently expressing the urea transporter UT-A1 is stimulated by agents that activate the cAMP signaling pathway, such as vasopressin or forskolin, thus mimicking the activation of urea permeability in the inner medullary collecting duct in the presence of vasopressin. Here, we report that UT-A1-mediated urea flux is also activated two-to-threefold over background by exposing the cells to media containing LiCl. This is in contrast to reports on cortical and medullary collecting duct tubules where acute and chronic exposure to lithium (Li) suppresses the osmotic water permeability, which is also regulated by cAMP levels. The Li concentration dependence of urea flux activation was linear up to 150 mM Li. Li activated only from the basolateral side where its effect was inhibited by amiloride, presumably because Li entered the cells through a basolateral Na-H exchanger. Li and IBMX, which also weakly activated urea flux, greatly augmented each others' stimulatory effect on urea flux. However, cellular cAMP levels did not rise commensurately with urea fluxes, and even though Li augments the activation by forskolin, it greatly inhibits the forskolin-induced formation of cAMP. These results suggest that the effect of Li in this MDCK model of renal cells does not involve cAMP or at least utilizes an additional signaling pathway independent of cAMP.
跨上皮示踪尿素通量通过永久表达尿素转运体UT - A1的Madin - Darby犬肾（MDCK）细胞时，会受到激活cAMP信号通路的物质（如血管加压素或福斯可林）的刺激，从而模拟在血管加压素存在时髓质内集合管中尿素通透性的激活。在此，我们报告，通过将细胞暴露于含LiCl的培养基中，UT - A1介导的尿素通量也会在背景基础上激活两到三倍。这与关于皮质和髓质集合管小管的报道形成对比，在这些报道中，急性和慢性暴露于锂（Li）会抑制渗透水通透性，而渗透水通透性也受cAMP水平调节。尿素通量激活的Li浓度依赖性在高达150 mM Li时呈线性。Li仅从基底外侧激活，其作用被氨氯地平抑制，推测是因为Li通过基底外侧的Na - H交换体进入细胞。Li和IBMX（也能微弱激活尿素通量）极大地增强了彼此对尿素通量的刺激作用。然而，细胞内cAMP水平并没有与尿素通量相应升高，并且即使Li增强了福斯可林的激活作用，但它极大地抑制了福斯可林诱导的cAMP形成。这些结果表明，在这个肾细胞的MDCK模型中，Li的作用不涉及cAMP，或者至少利用了一条独立于cAMP的额外信号通路。
