Effects of dietary protein concentration on L-proline transport by Manduca sexta midgut

Foliar protein concentrations vary widely. Phytophagous insects may respond to this variation by regulating the rate of absorption of protein breakdown products. Using fifth-instar Manduca sexta reared on artificial diets, we examined whether the rate of L-proline transport across the posterior midgut was affected by dietary protein concentration. Caterpillars were reared to the fifth instar on a low- or high-protein diet and then transferred to the same or opposite diet for 24 h prior to the flux measurements. Posterior midguts were mounted in Ussing chambers, and both potential difference and (14)C-proline flux across the tissue were measured during two-hour experiments. Midguts transported proline from lumen to hemolymph at about 6.5 &mgr;mol/cm(2)/h and from hemolymph to lumen at less than 0.43 &mgr;mol/cm(2)/h. Only a small fraction was oxidized (0.58 &mgr;mol/cm(2)/h), and proline was not converted to another compound while crossing the midgut epithelium. Neither proline transport rates nor potential differences across the epithelium differed significantly among treatment groups, suggesting that larval M. sexta do not regulate proline transport in response to dietary protein variation.