Inhibition of midgut ion transport by allatotropin (Mas-AT) and Manduca FLRFamides in the tobacco hornworm Manduca sexta

Short-circuit current (I(sc)) across the posterior midgut of day 2 fifth-instar tobacco hornworms (Manduca sexta) is inhibited by Manduca allatotropin (Mas-AT) and two Manduca FLRFamides (F7G and F7D). Another FLRFamide, F10, and the related molluscan tetrapeptide FMRFamide elicited only a modest inhibition of I(sc). MasAT, F7G and F7D inhibited the I(sc) in a dose-dependent manner. Maximal inhibition of I(sc) by Mas-AT was achieved at a concentration of 50 nmol l^-1. At 100 nmol l^-1, Mas-AT produced a significantly greater inhibition of I(sc) than did F7G and F7D. The inhibition caused by Mas-AT was reversed by removing the peptide. Furthermore, the action of Mas-AT could be blocked by preabsorption with its antiserum. When F7G or F7 was added after the I(sc) had been inhibited by Mas-AT, a further reduction in the I(sc) was observed. No additive effects were seen when F7G and F7D were added together. In comparison with the anterior and middle regions, the posterior section of the midgut was the most sensitive to these three peptides. The I(sc) of midguts dissected from feeding fourth instars was inhibited by MasAT, F7D and F7G, whereas the I(sc) of midguts dissected from pharate fifth instars or wandering fifth instars was virtually unaffected by these peptides. Active ion transport across the posterior midgut of the silk moths Hyalophora cecropia and Bombyx mori was unaffected by these peptides.