Cheng H F, Becker B N, Burns K D, Harris R C
Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA.
J Clin Invest. 1995 May;95(5):2012-9. doi: 10.1172/JCI117886.
Angiotensin II (Ang II) is an important regulator of proximal tubule salt and water reabsorption. Recent studies indicate that rabbit proximal tubule angiotensin II receptors are the type-1 (AT1R) subtype. We studied the effect of Ang II on proximal tubule receptor expression. Rabbits were treated with either angiotensin converting enzyme inhibitors or a low salt diet to modulate endogenous Ang II levels. In captopril-treated rabbits, liver and glomerular AT1R mRNA levels increased 242 +/- 125 and 141 +/- 60%, respectively (n = 6-7; P < 0.05), as determined by quantitative PCR. In contrast, proximal tubule AT1R mRNA levels decreased 40 +/- 11% (n = 6; P < 0.05). Binding of 125I Ang II to renal cortical basolateral membranes of captopril-treated rabbits decreased from 2.9 +/- 0.55 to 1.4 +/- 0.17 fmol/mg protein (n = 6; P < 0.025). In rabbits fed a sodium chloride-deficient diet for 4 wk, AT1R mRNA levels decreased 52 +/- 11% in liver and 43 +/- 7% in glomeruli (n = 4-5; P < 0.05), whereas they increased 141 +/- 85% (n = 5; P < 0.05) in proximal tubule. In basolateral membranes from rabbits on the sodium chloride-deficient diet, specific binding of 125I Ang II increased from 2.1 +/- 0.2 to 4.3 +/- 1.1 fmol/mg protein (n = 7; P < 0.05). To determine whether Ang II directly regulates expression of proximal tubule AT1 receptors, further studies were performed in cultured proximal tubule cells grown from microdissected S1 segments of rabbit proximal tubules and immortalized by transfection with a replication-defective SV40 vector. Incubation of these cells with Ang II (10(-11) to 10(-7) M) led to concentration-dependent increases in both AT1R mRNA levels and specific 125I Ang II binding. Pretreatment with pertussis toxin inhibited Ang II stimulation of AT1R mRNA. AT1R mRNA expression was decreased by either forskolin or a nonhydrolyzable cAMP analogue (dibutryl cAMP). Simultaneous Ang II administration overcame the inhibitory effect of forskolin but not dibutryl cAMP. These results indicate that proximal tubule AT1R expression is regulated by ambient Ang II levels, and Ang II increases AT1R mRNA at least in part by decreasing proximal tubule cAMP generation through a pertussis toxin-sensitive mechanism. Upregulation of proximal tubule AT1R by Ang II may be important in mediating enhanced proximal tubule sodium reabsorption in states of elevated systemic or intrarenal Ang II.
血管紧张素II（Ang II）是近端小管盐和水重吸收的重要调节因子。最近的研究表明，兔近端小管血管紧张素II受体为1型（AT1R）亚型。我们研究了Ang II对近端小管受体表达的影响。用血管紧张素转换酶抑制剂或低盐饮食处理兔子以调节内源性Ang II水平。通过定量PCR测定，在卡托普利处理的兔子中，肝脏和肾小球AT1R mRNA水平分别增加了242±125%和141±60%（n = 6 - 7；P < 0.05）。相比之下，近端小管AT1R mRNA水平下降了40±11%（n = 6；P < 0.05）。125I Ang II与卡托普利处理的兔子肾皮质基底外侧膜的结合从2.9±0.55降至1.4±0.17 fmol/mg蛋白质（n = 6；P < 0.025）。在喂食缺盐饮食4周的兔子中，肝脏中AT1R mRNA水平下降了52±11%，肾小球中下降了43±7%（n = 4 - 5；P < 0.05），而近端小管中则增加了141±85%（n = 5；P < 0.05）。在缺盐饮食兔子的基底外侧膜中，125I Ang II的特异性结合从2.1±0.2增加到4.3±1.1 fmol/mg蛋白质（n = 7；P < 0.05）。为了确定Ang II是否直接调节近端小管AT1受体的表达，我们用从兔近端小管微切割的S1段培养并通过用复制缺陷型SV-40载体转染而永生化的近端小管细胞进行了进一步研究。用Ang II（10^(-11)至10^(-7) M）孵育这些细胞导致AT1R mRNA水平和特异性125I Ang II结合呈浓度依赖性增加。百日咳毒素预处理抑制了Ang II对AT1R mRNA的刺激。福斯可林或不可水解的cAMP类似物（二丁酰cAMP）降低了AT1R mRNA表达。同时给予Ang II克服了福斯可林的抑制作用，但未克服二丁酰cAMP 的抑制作用。这些结果表明，近端小管AT1R表达受周围Ang II水平调节，并且Ang II至少部分通过百日咳毒素敏感机制降低近端小管cAMP生成来增加AT1R mRNA。Ang II对近端小管AT1R的上调在介导全身或肾内Ang II升高状态下近端小管钠重吸收增强方面可能很重要。
