Pathophysiological roles of arginine vasopressin and aquaporin-2 in impaired water excretion and hyponatremia

我们检测了精氨酸抗利尿素（AVP）依赖性低钠血症病理状态下水通道蛋白-2 （AQP-2）的变化。首先，通过给予液体饮食和皮下注射dDAVP制备sd大鼠SIADH模型。实验SIADH大鼠血清钠水平降至120 mmol/l以下，尿渗透压维持在800 ~ 900 mmol/kg，显著低于对照组的3000 ~ 3200 mmol/kg。因此，发现尿浓缩缺陷，称为AVP逃逸现象。AVP受体结合和AVP V_2受体mRNA表达均显著降低，但两组间差异无统计学意义。SIADH大鼠肾脏AQP-2 mRNA表达上调，与给予固体饮食和皮下注射dDAVP的过量大鼠相比，AQP-2 mRNA表达明显减弱。肾脏AQP-2蛋白的表达和尿中AQP-2的排泄结果相似。这些体内研究表明，细胞外体积扩张或低渗可能直接抑制肾脏AQP-2 mRNA表达，导致SIADH中AVP逃逸。随后进行体外研究，探讨渗透压是否直接调节AQP-2 5'-侧翼区的转录。获得AQP-2启动子基因（-9.5 kb），荧光素酶测定其活性。环AMP响应元件（CRE）存在至-1.1 kb。600 mmol/kg高渗压力使AQP-2启动子（-9.5 ~ -1.1 kb）荧光素酶活性提高约2倍。进一步研究确定了AQP-2启动子基因中两个渗透压响应位点（-9.5 ~ -7.7 kb和-6.0 ~ -4.2 kb）。

We examined the alteration in aquaporin-2 (AQP-2) water channel in pathological state of arginine vasopressin (AVP)-dependent hyponatremia. First, SIADH model was prepared in Sprague-Dawley rats by giving liquid diet and subcutaneous infusion of dDAVP. Serum Na level was decreased to below 120 mmol/l in the experimental SIADH rats, and urinary osmolality remained as low as 800-900 mmol/kg, that was significantly lower than that of 3,000-3,200 mmol/kg in the control rats. Thus, urinary concentrating defect was found, which is called as AVP escape phenomenon. There were marked reduction in AVP receptor binding and AVP V_2 receptor mRNA expression, but they were not different between the two groups. The expression of AQP-2 mRNA in kidney was upregulated in the SIADH rats, and its expression was significantly attenuated as compared to that in the dDAVP-excess rats, which were given solid diet and subcutaneous infusion of dDAVP. Similar results were obtained with kidney AQP-2 protein expression and urinary excretion of AQP-2. These in vivo studies indicate that either extracellular volume expansion or hypoosmolality may directly inhibit kidney AQP-2 mRNA expression, resulting in AVP escape in SIADH. Following in vitro study was carried out to examine whether osmolality directly regulate transcription of 5'-flanking region of AQP-2. The AQP-2 promoter gene (-9.5 kb) was obtained, and its activity was determined by luciferase assay. Cyclic AMP responsive element (CRE) was present until -1.1 kb. Hypertonic pressure of 600 mmol/kg increased the luciferase activity by approximately 2-fold in the AQP-2 promoter (-9.5〜-1.1 kb). Further study determined two osmolality responsive sites in the AQP-2 promoter gene (-9.5〜-7.7 kb and -6.0〜-4.2 kb).