Comparative physiology of dipeptide transport in lower vertebrates (fishes) and invertebrates (lobster)

Abstract

In my study I propose to undertake an investigation to characterize the brush border uptake and basolateral efflux mechanisms of a biologically stable dipeptide, glycylsarcosine in an herbivorous teleost (tilapia, Oreochromis mossambicus.). This will be the first study to characterize dieptide uptake and efflux processes of a single dipeptide in any animal. In order to extend our understanding of such a unique system, I would like to compare the characteristics of brush border uptake in the herbivorous tilapia to those of a carnivorous teleost (rock fish, Sebastes caurinus) and an omnivorous invertebrate (lobster, Homarus americanus). The lobster hepatopancreas is a diverticulum of the pyloric stomach. Over the past few years a number of studies have focused on the mechanism of sugar and amino acid transport by hepatopancreatic BBMV (4,5). These investigations showed that the hepatopancreas plays a major role in the absorption of nutrients in this animal. A novel feature of these diverticula is that the luminal pH at times of feeding may drop to as low as 4 (18). A number of studies have shown that a drop in external pH stimulates sugar and amino acid transport into hepatopancreatic BBMV. The observed stimulation was attributed either to protonation of amino acids with the protonated form being the preferred substrate, or protonation of the carrier resulting in an increase in the binding affinity for the sugars. The acidic nature of these diverticula at the absorptive site, markedly affecting nutrient transport, make this an ideal animal model since the solutes under investigation (dipeptides) are known to be coupled to protons in other types of animals. It will be of interest to investigate: (1) whether such a proton coupled dipeptide mechanism exists in the brush border membrane of lobster hepatopancreas, (2) If so, are the affinities and transport capacities of these dipeptide transporters any different than those described for mammals and fishes, (3) Does the binding affinity of this transporter show any variation at different pH values?, and (4) Is the specificity of this transporter any different from those exhibited by vertebrates?