The mechanisms of transepithelial absorption of dietary sucrose in the American lobster, Homarus americanus, were investigated in this study to determine whether sugars can be transported across an animal gut intact or as monosaccharides following hydrolysis. Lobster intestine was isolated and mounted in a perfusion chamber to characterize the mechanisms of mucosal to serosal (MS) 14C -sucrose transport across the intestine MS fluxes were measured by adding varying concentrations of 14C-sucrose to the perfusate which resulted in a hyperbolic curve following Michaelis-Menten kinetics. The kinetic constants of the proposed sucrose transporter were KM = 15.84 ± 1.81 µM and Jmax = 2.32 ± 0.07 ρmol cm-2min-1. The accumulation of 14C-sucrose in the bath in the presence of inhibitors, phloretin, phloridzin, and trehalose was observed. Inhibitory analysis showed that phloridzin, an inhibitor of Na+-dependent mucosal glucose transport, decreased MS 14C-sucrose transport suggesting that MS 14C-sucrose radioactive flux may partially involve an SGLT-1-like transporter. Phloretin, a known inhibitor of Na+-independent basolateral glucose transport, decreased MS 14C-sucrose transport, suggesting that some 14C-sucrose radioactivity may be transported to the blood by a GLUT 2-like carrier. Decreased MS 14C-sucrose transport was also observed in the presence of trehalose, a disaccharide containing D-glucose moieties. Thin-layer chromatography (TLC) was used to identify the chemical nature of radioactively labeled sugars in the bath following transport. TLC revealed 14C-sucrose was transported across the intestine largely as an intact molecule with no 14C-glucose or 14C-fructose appearing in the serosal bath or luminal perfusate. Bath samples evaporated to dryness and resuspended disclosed only 15% volatile metabolites. Results of this study strongly suggest that disaccharide sugars can be transported intact across animal intestine and provide support for the occurrence of a disaccharide membrane transporter that has not previously been functionally characterized.

Previous nutritional physiology research using L-histidine and zinc in American lobster intestine (Homarus americanus) has suggested that these solutes can be co-transported as complexes (Histidine-Zinc-Histidine) across the intestine using a peptide transporter. Furthermore, transport of L-leucine was shown to be inhibited by high calcium concentrations. Dipeptide and bis-complex transport and the role of calcium were investigated in the perfused intestines of lobster and Atlantic white shrimp (Litopenaeus setiferus). Following trans-intestinal transport, serosal medium was analyzed for amino acid composition by gas chromatography. In lobster, the transport of glycylsarcosine (Gly-Sar) from mucosa to serosa was stimulated two-fold with luminal pH 8.5, compared to the pH 5.5 control. Mucosa to serosa and serosa to mucosa fluxes of Gly-Sar were measured; the dipeptide was transported intact in both directions, but the net flux was from mucosa to serosa. The use of 0.5mM calcium chloride stimulated Gly-Sar transport two-fold, compared to 25 mM. In shrimp, the addition of 50 µM zinc chloride increased the rate of L-histidine transport, while Gly-Sar inhibited histidine transport in the presence of zinc. The rate of histidine transport was significantly higher with 1mM calcium chloride than with 25mM. These results suggest that shrimp transport bis-complexes in a manner similar to lobster. High calcium concentration had an inhibitory effect on both amino acid and dipeptide transport. Proposed mechanisms accounting for the effects of metals and calcium on trans-intestinal transports of both amino acids and dipeptides by lobster and shrimp digestive tracts are discussed.