title: Regulation of transmural transport of amino acid/metal conjugates by dietary calcium in crustacean digestive tract.
abstract: Effects of luminal Ca(2+) and Mn(2+) on transmural mucosal to serosal (MS) transport of (3) H-L-leucine were characterized in the isolated and perfused intestine of the American lobster, Homarus americanus. (3) H-L-leucine MS transport in the presence of 20 µM Mn(2+) was a sigmoidal function of luminal amino acid concentration, following the Hill equation for multisite cooperative, carrier-mediated, transport. Luminal Ca(2+) was a non-competitive inhibitor of Mn(2+) -stimulated (3) H-L-leucine MS flux. Amino acid transport was hyperbolically stimulated by luminal Ca(2+) or Mn(2+). During 20 µM Mn(2+) -stimulation of (3) H-L-leucine MS flux, addition of 25 mM Ca(2+) strongly reduced amino acid transport Jmax , without affecting amino acid binding properties. Hyperbolic luminal Mn(2+) stimulation of 20 µM (3) H-L-leucine MS flux was also strongly inhibited by 25 mM luminal Ca(2+) , significantly reducing 20 µM (3) H-L-leucine Jmax . Increasing the luminal concentration of verapamil, a calcium channel blocker, significantly increased MS transport of 20 µM (3) H-L-leucine in the presence of 100 nM Mn(2+) by reducing diffusional Ca(2+) uptake into intestinal epithelial cells through verapamil-sensitive channels. A model is proposed supporting the concept of molecular mimicry, whereby (3) H-L-leucine enters lobster intestinal epithelial cells by one or more amino acid-specific transporters and by a dipeptide-like transporter that is capable of binding and transporting peptide molecular mimics (bis-complexes) between Ca(2+) or Mn(2+) and (3) H-L-leucine using the membrane potential as a major driving force for the transport event. According to the model, Ca(2+) entry through apical Ca(2+) channels regulates the magnitude of the membrane potential and therefore the size of the driving force for bis-complex uptake.
authors.affiliation: Department of Biology, University of North Florida, Jacksonville, Florida.
authors.forename: Rania
authors.initials: R
authors.lastname: Abdel-Malak
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Ahearn
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002136:Calcium, Dietary / Q000494:pharmacology*; D041981:Gastrointestinal Tract / Q000378:metabolism*; D007930:Leucine / Q000378:metabolism*; D008297:Male; D008345:Manganese / Q000378:metabolism*; D008954:Models, Biological; D018716:Molecular Mimicry / Q000502:physiology*; D008121:Nephropidae / Q000378:metabolism*; D014316:Tritium; D014700:Verapamil
chemical_list: D002136:Calcium, Dietary; D014316:Tritium; D008345:Manganese; D014700:Verapamil; D007930:Leucine