Watts B A, George T, Good D W
Departments of Medicine and Physiology & Biophysics, University of Texas Medical Branch, Galveston, Texas 77555, USA.
J Biol Chem. 1999 Mar 19;274(12):7841-7. doi: 10.1074/jbc.274.12.7841.
Nerve growth factor (NGF) inhibits transepithelial HCO3- absorption in the rat medullary thick ascending limb (MTAL). To investigate the mechanism of this inhibition, MTALs were perfused in vitro in Na+-free solutions, and apical and basolateral membrane Na+/H+ exchange activities were determined from rates of pHi recovery after lumen or bath Na+ addition. NGF (0.7 nM in the bath) had no effect on apical Na+/H+ exchange activity, but inhibited basolateral Na+/H+ exchange activity by 50%. Inhibition of basolateral Na+/H+ exchange activity with ethylisopropyl amiloride (EIPA) secondarily reduces apical Na+/H+ exchange activity and HCO3- absorption in the MTAL (Good, D. W., George, T., and Watts, B. A., III (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 12525-12529). To determine whether a similar mechanism could explain inhibition of HCO3- absorption by NGF, apical Na+/H+ exchange activity was assessed in physiological solutions (146 mM Na+) by measurement of the initial rate of cell acidification after lumen EIPA addition. Under these conditions, in which basolateral Na+/H+ exchange activity is present, NGF inhibited apical Na+/H+ exchange activity. Inhibition of HCO3- absorption by NGF was eliminated in the presence of bath EIPA or in the absence of bath Na+. Also, NGF blocked inhibition of HCO3- absorption by bath EIPA. We conclude that NGF inhibits basolateral Na+/H+ exchange activity in the MTAL, an effect opposite from the stimulation of Na+/H+ exchange by growth factors in other systems. NGF inhibits transepithelial HCO3- absorption through inhibition of basolateral Na+/H+ exchange, most likely as the result of functional coupling in which primary inhibition of basolateral Na+/H+ exchange activity results secondarily in inhibition of apical Na+/H+ exchange activity. These findings establish a role for basolateral Na+/H+ exchange in the regulation of renal tubule HCO3- absorption.
神经生长因子（NGF）可抑制大鼠髓袢升支粗段（MTAL）的跨上皮HCO₃⁻重吸收。为研究这种抑制作用的机制，在无钠溶液中对MTAL进行体外灌注，并通过管腔或浴液添加钠后pH恢复率来测定顶端和基底外侧膜的Na⁺/H⁺交换活性。NGF（浴液中浓度为0.7 nM）对顶端Na⁺/H⁺交换活性无影响，但可使基底外侧Na⁺/H⁺交换活性降低50%。用乙基异丙基氨氯吡脒（EIPA）抑制基底外侧Na⁺/H⁺交换活性会继而降低MTAL中顶端Na⁺/H⁺交换活性和HCO₃⁻重吸收（古德，D.W.，乔治，T.，以及瓦茨，B.A.，三世（1995年）《美国国家科学院院刊》92，12525 - 12529）。为确定类似机制是否能解释NGF对HCO₃⁻重吸收的抑制作用，通过测量管腔添加EIPA后细胞酸化的初始速率，在生理溶液（146 mM Na⁺）中评估顶端Na⁺/H⁺交换活性。在存在基底外侧Na⁺/H⁺交换活性的这些条件下，NGF抑制顶端Na⁺/H⁺交换活性。在浴液中存在EIPA或无浴液钠的情况下，NGF对HCO₃⁻重吸收的抑制作用消失。此外，NGF可阻断浴液EIPA对HCO₃⁻重吸收的抑制作用。我们得出结论，NGF抑制MTAL中的基底外侧Na⁺/H⁺交换活性，这一作用与其他系统中生长因子对Na⁺/H⁺交换的刺激作用相反。NGF通过抑制基底外侧Na⁺/H⁺交换来抑制跨上皮HCO₃⁻重吸收，这很可能是由于功能偶联，即基底外侧Na⁺/H⁺交换活性的原发性抑制继而导致顶端Na⁺/H⁺交换活性的抑制。这些发现确立了基底外侧Na⁺/H⁺交换在肾小管HCO₃⁻重吸收调节中的作用。
