KCl transport across and insect epithelium: I. Tracer fluxes and the effects of ion substitutions

Summary Models for active Cl transport across epithelia are often assumed to be universal although they are based on detailed studies of a relatively small number of epithelia from vertebrate animals. Epithelial Cl transport is also important in many invertebrates, but little is known regarding its cellular mechanisms. We used short-circuit current, tracer fluxes and ion substitutions to investigate the basic properties of Cl absorption by locust hindgut, an epithelium which is ideally suited for transport studies. Serosal addition of 1mm adenosine 35-cyclic monophosphate (cAMP), a known stimulant of Cl transport in this tissue, increased short-circuit current (I _sc) and net reabsorptive³⁶Cl flux (J _net ^Cl ) by 1000%. Cl absorption did not exhibit an exchange diffusion component and was highly selective over all anions tested except Br. Several predictions of Na- and HCO₃-coupled models for Cl transport were tested: Cl-dependentI _sc was not affected by sodium removal (<0.05mm) during the first 75 min. Also, a large stimulation ofJ _net ^Cl was elicited by cAMP when recta were bathed for 6 hr in nominally Na-free saline (<0.001 to 0.2mm) and there was no correlation between Cl transport rate and the presence of micromolar quantities of Na contamination. Increased unidirectional influx of³⁶Cl into rectal tissue during cAMP-stimulation was not accompanied by a comparable uptake of²²Na.J _net ^Cl was independent of exogenous CO₂ and HCO₃, but was strongly dependent on the presence of K. These results suggest that the major fraction of Cl transport across this insect epithelium occurs by an unusual K-dependent mechanism that does not directly require Na or HCO₃.