Tang Lieqi, Fang Xiefan, Winesett Steven P, Cheng Catherine Y, Binder Henry J, Rivkees Scott A, Cheng Sam X
Department of Pediatrics, University of Florida, Gainesville, FL, United States of America.
Department of Internal Medicine, Yale School of Medicine, New Haven, CT, United States of America.
PLoS One. 2017 Feb 2;12(2):e0171045. doi: 10.1371/journal.pone.0171045. eCollection 2017.
Mammalian colonic epithelia consist of cells that are capable of both absorbing and secreting Cl-. The present studies employing Ussing chamber technique identified two opposing short-circuit current (Isc) responses to basolateral bumetanide in rat distal colon. Apart from the transepithelial Cl--secretory Isc in early distal colon that was inhibited by bumetanide, bumetanide also stimulated Isc in late distal colon that had not previously been identified. Since bumetanide inhibits basolateral Na+-K+-2Cl- cotransporter (NKCC) in crypt cells and basolateral K+-Cl- cotransporter (KCC) in surface epithelium, we proposed this stimulatory Isc could represent a KCC-mediated Cl- absorptive current. In support of this hypothesis, ion substitution experiments established Cl- dependency of this absorptive Isc and transport inhibitor studies demonstrated the involvement of an apical Cl- conductance. Current distribution and RNA sequencing analyses revealed that this Cl- absorptive Isc is closely associated with epithelial Na+ channel (ENaC) but is not dependent on ENaC activity. Thus, inhibition of ENaC by 10 μM amiloride or benzamil neither altered the direction nor its activity. Physiological studies suggested that this Cl- absorptive Isc senses dietary Cl- content; thus when dietary Cl- was low, Cl- absorptive Isc was up-regulated. In contrast, when dietary Cl- was increased, Cl- absorptive Isc was down-regulated. We conclude that an active Cl- extrusion mechanism exists in ENaC-expressing late distal colon and likely operates in parallel with ENaC to facilitate NaCl absorption.
哺乳动物结肠上皮由既能吸收又能分泌氯离子的细胞组成。本研究采用尤斯灌流室技术，在大鼠远端结肠中发现了对基底侧布美他尼的两种相反的短路电流（Isc）反应。除了早期远端结肠中被布美他尼抑制的跨上皮氯离子分泌Isc外，布美他尼还刺激了此前未被发现的晚期远端结肠中的Isc。由于布美他尼抑制隐窝细胞中的基底侧钠-钾-2氯共转运体（NKCC）和表面上皮中的基底侧钾-氯共转运体（KCC），我们推测这种刺激性Isc可能代表一种KCC介导的氯离子吸收电流。为支持这一假设，离子替代实验确定了这种吸收性Isc对氯离子的依赖性，转运抑制剂研究证明了顶端氯离子电导的参与。电流分布和RNA测序分析表明，这种氯离子吸收性Isc与上皮钠通道（ENaC）密切相关，但不依赖于ENaC的活性。因此，10 μM氨氯地平或苯甲酰咪对ENaC的抑制既未改变其方向也未改变其活性。生理学研究表明，这种氯离子吸收性Isc能感知饮食中的氯离子含量；因此，当饮食中的氯离子含量较低时，氯离子吸收性Isc会上调。相反，当饮食中的氯离子增加时，氯离子吸收性Isc会下调。我们得出结论，在表达ENaC的晚期远端结肠中存在一种活跃的氯离子外排机制，可能与ENaC并行运作以促进氯化钠的吸收。
