Mechanism and control of salt absorption in locust rectum.

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.