Acid-base Transport and Control in Locust Hindgut: Artefacts Caused by Short-circuit Current

1. The commonly used method of passing short-circuit current (/sc) across insect epithelia through Ag-AgCl electrodes, without the use of salt bridges, leads to significant OH~ production at the cathode (lumen side) when high currents are applied. 2. The alkalization of the lumen previously reported when cyclic AMP was added to short-circuited locust hindgut is a result of this phenomenon rather than cyclic-AMP-mediated stimulation of acid-base transport in the hindgut. 3. When salt bridges are used to pass short-circuit current across locust hindgut, acid secretion (7H) into the lumen equals alkaline movement (7OH) to the haemocoel side, and JH is similar under both openand short-circuit conditions. 7H is similar (1.5^equivcm~h~') in recta and ilea. 4. Addition of cyclic AMP inhibits Ju across the rectum by 42-66 %, but has no effect on the ileum when salt bridges are used. 5. Electrical parameters (/sc, Vt, Rt) reflecting hindgut Cl~ transport (7a) before and after stimulation with cyclic AMP are the same whether or not salt bridges are used. We found no evidence of any coupling between 7C1 and JH/JQH-