Hanrahan J, Phillips J E
Am J Physiol. 1983 Feb;244(2):R131-42. doi: 10.1152/ajpregu.1983.244.2.R131.
The rectum is the main reabsorptive site in the excretory system of locusts. The primary urine entering this organ from the Malpighian tubules is rich in K+ (140 mM) and Cl- (90 mM), and most of this fluid is normally reabsorbed. Fluid and active Cl- reabsorption in the rectum are regulated by neuropeptide hormones from the corpus cardiacum. We have studied the mechanism of KCl reabsorption using voltage clamp, tracers, double-barreled ion-sensitive microelectrodes, and ion substitutions. Locust Cl- absorption differs from vertebrate systems in that it is not dependent on Na+ or HCO-3/CO2, and it is insensitive to normal inhibitors of Cl- transport. Entry of Cl- into rectal cells is active, electrogenic, and stimulated by luminal K+. This cation substantially increases the electrochemical gradient across the apical membrane against which Cl- is pumped; therefore K+ does not act solely and indirectly by electrical coupling. Kinetic studies also suggest that K+ activates the Cl- pump. Consequently at least two levels of control are exerted during cAMP stimulation; K+ permeability of the epithelium and the transepithelial potential generated by active Cl- transport both increase. The enhanced net K+ absorption from the lumen side after stimulation is largely passive, being electrically coupled to Cl- transport. However, this general increase in KCl absorption is "fine tuned" by K+ itself, through its direct effect on the Cl- pump.
直肠是蝗虫排泄系统中的主要重吸收部位。从马氏管进入该器官的原尿富含K⁺（140 mM）和Cl⁻（90 mM），且大部分这种液体通常会被重吸收。直肠中的液体和Cl⁻的主动重吸收受来自心侧体的神经肽激素调节。我们使用电压钳、示踪剂、双管离子敏感微电极和离子替代法研究了KCl重吸收的机制。蝗虫的Cl⁻吸收与脊椎动物系统不同，它不依赖于Na⁺或HCO₃⁻/CO₂，并且对正常的Cl⁻转运抑制剂不敏感。Cl⁻进入直肠细胞是主动的、生电的，并受到管腔K⁺的刺激。这种阳离子显著增加了Cl⁻被泵出时跨顶端膜的电化学梯度；因此K⁺并非仅通过电偶联起间接作用。动力学研究还表明K⁺激活了Cl⁻泵。因此，在cAMP刺激过程中至少施加了两个控制水平；上皮细胞的K⁺通透性和由主动Cl⁻转运产生的跨上皮电位均增加。刺激后从管腔侧增强的净K⁺吸收在很大程度上是被动的，与Cl⁻转运电偶联。然而，KCl吸收的这种总体增加通过K⁺对Cl⁻泵的直接作用而被K⁺自身“微调”。
