Good D W, Di Mari J F, Watts B A
Deparments of Medicine and Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555, USA.
Am J Physiol Cell Physiol. 2000 Nov;279(5):C1443-54. doi: 10.1152/ajpcell.2000.279.5.C1443.
The signal transduction mechanisms that mediate osmotic regulation of Na(+)/H(+) exchange are not understood. Recently we demonstrated that hyposmolality increases HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through stimulation of the apical membrane Na(+)/H(+) exchanger NHE3. To investigate the mechanism of this stimulation, MTALs from rats were isolated and perfused in vitro with 25 mM HCO(3)(-)-containing solutions. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (100 nM) and LY-294002 (20 microM) blocked completely the stimulation of HCO(3)(-) absorption by hyposmolality. In tissue strips dissected from the inner stripe of the outer medulla, the region of the kidney highly enriched in MTALs, hyposmolality increased PI 3-K activity 2. 2-fold. Wortmannin blocked the hyposmolality-induced PI 3-K activation. Further studies examined the interaction between hyposmolality and vasopressin, which inhibits HCO(3)(-) absorption in the MTAL via cAMP and often is involved in the development of plasma hyposmolality in clinical disorders. Pretreatment with arginine vasopressin, forskolin, or 8-bromo-cAMP abolished hyposmotic stimulation of HCO(3)(-) absorption, due to an effect of cAMP to inhibit hyposmolality- induced activation of PI 3-K. In contrast to their effects to block stimulation by hyposmolality, PI 3-K inhibitors and vasopressin have no effect on inhibition of apical Na(+)/H(+) exchange (NHE3) and HCO(3)(-) absorption by hyperosmolality. These results indicate that hyposmolality increases NHE3 activity and HCO(3)(-) absorption in the MTAL through activation of a PI 3-K-dependent pathway that is inhibited by vasopressin and cAMP. Hyposmotic stimulation and hyperosmotic inhibition of NHE3 are mediated through different signal transduction mechanisms.
介导Na(+)/H(+)交换渗透调节的信号转导机制尚不清楚。最近我们证明，低渗通过刺激顶端膜Na(+)/H(+)交换体NHE3增加肾髓质厚升支（MTAL）中HCO(3)(-)的重吸收。为了研究这种刺激的机制，分离大鼠的MTAL并在体外用含25 mM HCO(3)(-)的溶液灌注。磷脂酰肌醇3激酶（PI 3-K）抑制剂渥曼青霉素（100 nM）和LY-294002（20 microM）完全阻断了低渗对HCO(3)(-)重吸收的刺激。在外髓质内带（肾脏中MTAL高度富集的区域）分离的组织条中，低渗使PI 3-K活性增加2.2倍。渥曼青霉素阻断了低渗诱导的PI 3-K激活。进一步的研究检测了低渗与血管加压素之间的相互作用，血管加压素通过cAMP抑制MTAL中HCO(3)(-)的重吸收，且在临床疾病中常参与血浆低渗的发生。用精氨酸血管加压素、福斯可林或8-溴-cAMP预处理消除了低渗对HCO(3)(-)重吸收的刺激，这是由于cAMP抑制低渗诱导的PI 3-K激活的作用。与它们阻断低渗刺激的作用相反，PI 3-K抑制剂和血管加压素对高渗抑制顶端Na(+)/H(+)交换（NHE3）和HCO(3)(-)重吸收没有影响。这些结果表明，低渗通过激活一条被血管加压素和cAMP抑制的PI 3-K依赖性途径增加MTAL中NHE3活性和HCO(3)(-)重吸收。NHE3的低渗刺激和高渗抑制是通过不同的信号转导机制介导的。
