1. The maintenance of in vitro transepithelial potential difference (PD) across the larval midgut of Manduca sexta is dependent upon the presence of metabolic substrates. Deletion of exogenous substrates from the bathing saline results in a rapid decline in PD. The PD can be restored by the bilateral addition of a saline containing physiological levels of amino acids and sugars. This stimulation is largely dependent upon the presence of potassium. 2. The following substrates stimulate the PD more than twofold: citrate, succinate, malate, fructose, sucrose, trehalose, glucose, hexanoate and octanoate. Acetate and some amino acids stimulate the PD, but to a lesser extent. Of the substrates tested, only proline, glycine and 3-methyl-O-glucose fail to stimulate the PD. 3. To determine if the observed stimulation of PD is due to absorption of anions, negatively charged substrates were added to either the haemolymph or luminal side of the tissue and the PD was monitored. The results indicate that diffusion of anions cannot account for the stimulation of PD. 4. These results are discussed with regard to midgut oxidative metabolism and the metabolic support of active potassium secretion.
Parallel effects of VX-770 on transepithelial potential difference in vitro and in vivo
