Active chloride transport in isolated posterior midgut of tobacco hornworm (Manduca sexta).

The short-circuited posterior midgut of larval tobacco hornworm (Manduca sexta) actively transports Cl- from lumen to hemolymph as measured by unidirectional fluxes of 36Cl-. Potentials and Cl- activities in cytosol and goblet cavity were measured using double-barreled Cl--selective microelectrodes. In the short-circuited tissue, the goblet cavity was electrically positive to the bathing solution, and Cl- activity was below electrochemical equilibrium with luminal fluid. The cytosol was electrically negative to the bathing solution, and Cl- activity was above electrochemical equilibrium. Thus Cl- is pumped from goblet cavity into cell. Although the Cl- that is pumped into the cell can cross the basal membrane, its Cl- conductance is quite low. The Cl- conductance is also quite low in apical membranes of columnar cells. Depression of intracellular Cl- after exposure of the luminal side to high HCO-3 suggested that these membranes have a Cl- for HCO-3 exchange mechanism. The paracellular pathway for Cl- comprises approximately 10% of the total transepithelial conductance.