Acid–Base Transport and Control in Locust Hindgut: Artefacts Caused by Short-Circuit Current

The commonly used method of passing short-circuit current (I_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.

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.

When salt bridges are used to pass short-circuit current across locust hindgut, acid secretion (J_H) into the lumen equals alkaline movement (J_OH) to the haemocoel side, and J_H is similar under both open- and short-circuit conditions. J_H is similar (1.5_μequiv cm^_2h^_1) in recta and ilea.

Addition of cyclic AMP inhibits J_H across the rectum by 42–66 %, but has no effect on the ileum when salt bridges are used.

Electrical parameters (I_sc, V_t, R_t) reflecting hindgut Cl⁻ transport (J_C1) 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 J_cI and J_H / J_OH.