Intestinal amino acid absorption in tobacco hornworm larvae is stimulated by potassium and sodium but not rubidium or lithium

We used rapid filtration assays to determine the ion selectivity of ion gradientdriven phenylalanine uptake by brush border membrane vesicles prepared from the larval midgut of the tobacco hornworm (Manduca sexta). Phenylalanine uptake by these vesicles is stimulated by both potassium and sodium. Phenylalanine uptake by larval M. sexta midgut brush border membrane vesicles is voltage sensitive and shows little selectivity for potassium over sodium. However, phenylalanine uptake by these vesicles is stimulated by neither rubidium nor lithium.