Anomalous glutamate/alkali cation symport in larval Manduca sexta midgut

Ammonia transport by terrestrial and aquatic insects

Ammonia, an end product from amino acid and nucleic acid metabolism, is highly toxic for most animals. This review will provide an update on nitrogen metabolism in terrestrial and aquatic insects with emphasis on ammonia generation and transport. Aspects that will be discussed include metabolic pathways of nitrogenous compounds, the origin of ammonia and other nitrogenous waste products, ammonia toxicity, putative ammonia transporters as well as ammonia transport processes known in insects. Ammonia transport mechanisms in the mosquito Aedes aegypti, the tobacco hornworm Manduca sexta and the locust Schistocerca gregaria will be discussed in detail while providing additional, novel data.

Cloning and characterization of a potassium-coupled amino acid transporter

Active solute uptake in bacteria, fungi, plants, and animals is known to be mediated by cotransporters that are driven by Na+ or H+ gradients. The present work extends the Na+ and H+ dogma by including the H+ and K+ paradigm. Lepidopteran insect larvae have a high K+ and a low Na+ content, and their midgut cells lack Na+/K+ ATPase. Instead, an H+ translocating, vacuolar-type ATPase generates a voltage of approximately -240 mV across the apical plasma membrane of so-called goblet cells, which drives H+ back into the cells in exchange for K+, resulting in net K+ secretion into the lumen. The resulting inwardly directed K+ electrochemical gradient serves as a driving force for active amino acid uptake into adjacent columnar cells. By using expression cloning with Xenopus laevis oocytes, we have isolated a cDNA that encodes a K+-coupled amino acid transporter (KAAT1). We have cloned this protein from a larval lepidopteran midgut (Manduca sexta) cDNA library. KAAT1 is expressed in absorptive columnar cells of the midgut and in labial glands. When expressed in Xenopus oocytes, KAAT1 induced electrogenic transport of neutral amino acids but excludes alpha-(methylamino)isobutyric acid and charged amino acids resembling the mammalian system B. K+, Na+, and to a lesser extent Li+ were accepted as cotransported ions, but K+ is the principal cation, by far, in living caterpillars. Moreover, uptake was Cl(-)-dependent, and the K+/Na+ selectivity increased with hyperpolarization of oocytes, reflecting the increased K+/Na+ selectivity with hyperpolarization observed in midgut tissue. KAAT1 has 634 amino acid residues with 12 putative membrane spanning domains and shows a low level of identity with members of the Na+ and Cl(-)-coupled neurotransmitter transporter family.

Cationic lysine uptake by System R+ and zwitterionic lysine uptake by System B in brush border membrane vesicles from larval Manduca sexta midgut

Lysine uptake was studied at pH 7.4 and 10.0 by rapid filtration methods in brush border membrane vesicles from fifth instar larvae of a model insect, the tobacco hornworm, Manduca sexta (Lepidoptera, sphingidae). At both pH values the uptake was mediated by K+ coupled symport. The uptake rate increased between pH 5.5 and 10, especially so in the alkaline range. The total lysine uptake could be divided into two components based upon lysine's ionic form as a function of pH. Lysine uptake at pH 7.4 was strongly cis-inhibited by arginine but at pH 10 was cis-inhibited and trans-stimulated by many neutral amino acids, e.g. leucine, but not by arginine. Lysine uptake by the arginine-inhibitable component paralleled the titration curve of cationic lysine whereas uptake by the leucine-inhibitable component paralleled that of zwitterionic lysine. Evidently, the brush border membrane contains at least two separate, K(+)-dependent amino acid symporters (co-transporters) that mediate lysine uptake. A cationic amino acid:K+ symporter selects cationic lysine and arginine but not histidine and other amino acids. One or more zwitterionic amino acid:K+ symporters select zwitterionic lysine, possibly arginine, histidine and neutral amino acids. Based upon these substrate repertoires, the zwitterionic symporters are B-type systems whereas the cationic symporter is identical with System R+, which partially resembles System y+. Arginine uptake in vivo is likely to be mediated mainly by System R+ whereas lysine uptake is likely to be mediated by System B-type symporters.