KCl transport across an insect epithelium: II. Electrochemical potentials and electrophysiology

Summary

The cellular mechanism of K-stimulated Cl transport in locust hindgut was studied using double-barrelled ionsensitive microelectrodes and electrophysiological techniques. Steady-state net electrochemical potentials for Cl and K and the conductances of apical and basal membranes and paracellular pathway were determined under control conditions, during exposure to 1mm cAMP, and following ion substitutions. Under control open-circuit conditions, intracellular Cl activity (a ^c_Cl ) was 3.5 times that predicted for passive equilibrium across the apical membrane. The net electrochemical potential opposing Cl entry from the mucosal side$(\Delta \bar \mu _{Cl}^a /F)$ increased by 50% during cAMP stimulation of transepithelial Cl absorption whereas the net electrochemical potential favoring Cl exit across the basal membrane$( - \Delta \bar \mu _{Cl}^b /F)$ was unchanged. No correlation was observed between$\Delta \bar \mu _{Cl}^a /F$ and the net electrochemical potential across the apical membrane for Na. The net electrochemical potential favoring K entry across the apical membrane$( - \Delta \bar \mu _K^a /F)$ was negligible underI _sc conditions when Cl transport rate was approximately 10 μeq cm⁻² hr⁻¹. Locust rectal cells showed electrical and dye coupling. The results also indicate that most transepithelial diffusion of ions is transcellular and that epithelial tightness effectively increases during exposure to cAMP becauseR _a andR _b both decrease, by ≈80% whileR _j is unchanged. The cAMP-induced δR _b was abolished in Cl-free saline whereas δR _a was insensitive to Cl removal, but was blocked by removing K from the saline. Based on these findings, our model for Cl absorption in locust hindgut features i) an active entry step for Cl at the apical membrane which is stimulated by cAMP and by low levels of K on the mucosal side, but is not energized by$- \Delta \bar \mu _{Na}^a /F$ or$- \Delta \bar \mu _K^a /F$ a large cAMP-stimulated Cl conductance in the basal membrane and a similar cAMP-stimulated K conductance in the apical membrane. cAMP dose-response curves are similar for the stimulation of active Cl absorption and Cl-independent (i. e. K) conductance, indicating that cAMP exerts dual control over active Cl transport and counter-ion permeability.