Bailey Matthew A, Giebisch Gerhard, Abbiati Thecla, Aronson Peter S, Gawenis Lara R, Shull Gary E, Wang Tong
Cellular and Molecular Physiology, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06520, USA.
J Physiol. 2004 Dec 15;561(Pt 3):765-75. doi: 10.1113/jphysiol.2004.074716. Epub 2004 Oct 7.
NHE3(-/-) mice display a profound defect in proximal tubule bicarbonate reabsorption but are only mildly acidotic owing to reduced glomerular filtration rate and enhanced H(+) secretion in distal nephron segments. In vivo microperfusion of rat distal tubules suggests that a significant fraction of bicarbonate reabsorption in this nephron segment is mediated by NHE2. Two approaches were used to evaluate the role of distal tubule NHE2 in compensating for the proximal defect of H(+) secretion in NHE3(-/-) mice. First, renal clearance experiments were used to assess the impact of HOE694, an inhibitor with significant affinity for NHE2, on excretion of bicarbonate in NHE3(-/-) and NHE2(-/-) mice. Second, in vivo micropuncture and microperfusion were employed to measure the concentration of bicarbonate in early distal tubule fluid and to measure distal bicarbonate reabsorption during a constant bicarbonate load. Our data show that HOE694 had no effect on urinary bicarbonate excretion in NHE3(+/+) mice, whereas bicarbonate excretion was higher in NHE3(-/-) mice receiving HOE694. HOE694 induced a significant increase in bicarbonate excretion in mice given an acute bicarbonate load, but there was no effect during metabolic acidosis. Bicarbonate excretion was not affected by HOE694 in bicarbonate-loaded NHE2(-/-) mice. In vivo micropuncture revealed that early distal bicarbonate concentration was elevated in both bicarbonate-loaded and NHE3(-/-) mice. Further, microperfusion experiments showed that HOE694-sensitive bicarbonate reabsorption capacity was higher in acidotic and NHE3 null animals. We conclude that NHE2 contributes importantly to acidification in the distal tubule, and that it plays a major role in limiting urinary bicarbonate losses in states in which a high luminal bicarbonate load is presented to the distal tubule, such as in NHE3 null mice.
NHE3基因敲除（-/-）小鼠在近端小管碳酸氢盐重吸收方面表现出严重缺陷，但由于肾小球滤过率降低和远端肾单位节段氢离子分泌增加，仅表现为轻度酸中毒。对大鼠远端小管进行体内微灌注表明，该肾单位节段中相当一部分碳酸氢盐重吸收是由NHE2介导的。采用两种方法评估远端小管NHE2在补偿NHE3基因敲除（-/-）小鼠近端氢离子分泌缺陷中的作用。首先，利用肾脏清除实验评估对NHE2具有显著亲和力的抑制剂HOE694对NHE3基因敲除（-/-）和NHE2基因敲除（-/-）小鼠碳酸氢盐排泄的影响。其次，采用体内微穿刺和微灌注技术测量远端小管早期液体中碳酸氢盐的浓度，并在恒定碳酸氢盐负荷下测量远端碳酸氢盐重吸收情况。我们的数据显示，HOE694对NHE3基因敲除（+/+）小鼠的尿碳酸氢盐排泄没有影响，而接受HOE694的NHE3基因敲除（-/-）小鼠的碳酸氢盐排泄量更高。HOE694在给予急性碳酸氢盐负荷的小鼠中可显著增加碳酸氢盐排泄，但在代谢性酸中毒期间没有影响。在给予碳酸氢盐负荷的NHE2基因敲除（-/-）小鼠中，HOE694对碳酸氢盐排泄没有影响。体内微穿刺显示，在给予碳酸氢盐负荷的小鼠和NHE3基因敲除（-/-）小鼠中，远端小管早期碳酸氢盐浓度均升高。此外，微灌注实验表明，在酸中毒和NHE3基因敲除动物中，HOE694敏感的碳酸氢盐重吸收能力更高。我们得出结论，NHE2对远端小管的酸化起重要作用，并且在向远端小管呈现高管腔碳酸氢盐负荷的状态下，如在NHE3基因敲除小鼠中，它在限制尿碳酸氢盐损失方面起主要作用。
