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First published online January 5, 2005
Journal of Experimental Biology 208, 287-296 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01401

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3H-L-histidine and ⁶⁵Zn²⁺ are cotransported by a dipeptide transport system in intestine of lobster Homarus americanus

Erik M. Conrad and Gregory A. Ahearn^*

Department of Biology, 4567 St Johns Bluff Road, South, University of North Florida, Jacksonville, FL 32224, USA

View larger version (21K): [in a new window]   Fig. 1. Effects of 20 µmol l–1 zinc and 100 µmol l–1 L-leucine on the time course of transmural transport of 20 µmol l–1 3H-L-histidine. Values displayed are means ± S.E.M. of 3 replicates per time point. Slopes of the lines were calculated as regression lines using Sigma Plot software and the values on the figure (i.e. m-values) are the slopes obtained from these analyses at each condition used.

View larger version (11K): [in a new window]   Fig. 2. Effect of luminal 3H-L-histidine concentration (1–50 nmol l–1 µmol l–1) on the rate of mucosal-to-serosal transmural transport measured in the absence of mucosal zinc over 30 min time intervals at each luminal amino acid concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each amino acid concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (11K): [in a new window]   Fig. 3. Effect of varying luminal zinc concentration (1–50 µmol l–1) on transmural transport rate of 20 µmol l–1 3H-L-histidine over 30 min time intervals at each luminal zinc concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each zinc concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (17K): [in a new window]   Fig. 4. Effect of luminal zinc (20 µmol l–1) and L-leucine 100 µmol l–1) on the kinetics of transmural transport of 3H-L-histidine (1–50 µmol l–1) measured over 30 min time intervals at each luminal 3H-L-histidine concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each 3H-L-histidine concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (23K): [in a new window]   Fig. 5. Effect of 20 µmol l–1 L-histidine and 50 µmol l–1 Cu+ on the time course of transmural transport of 20 µmol l–1 65Zn2+. Values displayed are means ± S.E.M. of 3 replicates per time point. Slopes of the lines were calculated as regression lines using Sigma Plot software and the values on the figure (i.e. m-values) are the slopes obtained from these analyses at each condition used.

View larger version (12K): [in a new window]   Fig. 6. Effect of varying luminal L-histidine concentration (1–50 µmol l–1) on transmural transport rate of 20 µmol l–1 65Zn2+over 30 min time intervals at each luminal L-histidine concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each L-histidine concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (21K): [in a new window]   Fig. 7. Effect of luminal L-histidine (L-His; 20 µmol l–1) and both Cu+ and Cu2+ (50 µmol l–1) on the kinetics of transmural transport of 65Zn2+ (150–1000 µmol l–1) measured over 30 min time intervals at each luminal 65Zn2+ concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each 65Zn2+ concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (17K): [in a new window]   Fig. 8. Effect of zinc (20 µmol l–1) and glycyl-sarcosine (Gly-Sar; 100 µmol l–1) on the kinetics of transmural transport of 3H-L-histidine (1–50 µmol l–1) measured over 30 min time intervals at each luminal 3H-L-histidine concentration. Unidirectional fluxes were calculated from regression slopes determined over this time interval for each 3H-L-histidine concentration. Values displayed are means ± S.E.M. of 3 replicates per concentration.

View larger version (24K): [in a new window]   Fig. 9. Effect of luminal pH on the time course of 20 µmol l–1 3H-L-histidine (L-His) transport by perfused lobster intestine in the presence of 20 µmol l–1 zinc (control, pH 7.1; test, pH 6.1). Values displayed are means ± S.E.M. of 3 replicates per time point. Slopes of the lines were calculated as regression lines using Sigma Plot software and the values on the figure (i.e. m-values) are the slopes obtained from these analyses at each condition used.

View larger version (21K): [in a new window]   Fig. 10. Effect of 20 µmol l–1 L-histidine (L-His) and 20 µmol l–1 zinc on the time course of transmural transport of 100 µmol l–1 3H-glycyl-sarcosine. Values displayed are means ± S.E.M. of 3 replicates per time point. Slopes of the lines were calculated as regression lines using Sigma Plot software and the values on the figure (i.e. m-values) are the slopes obtained from these analyses at each condition used.

View larger version (20K): [in a new window]   Fig. 11. Effect of 20 µmol l–1 L-leucine (L-Leu) and 20 µmol l–1 zinc on the time course of transmural transport of 100 µmol l–1 3H-glycyl-sarcosine. Values displayed are means ± S.E.M. of 3 replicates per time point. Slopes of the lines were calculated as regression lines using Sigma Plot software, and the values on the figure (i.e. m-values) are the slopes obtained from these analyses at each condition used.

View larger version (21K): [in a new window]   Fig. 12. Working model of transmural mucosal-to-serosal transport of 3H-L-histidine (His) and 65Zn2+ across the perfused intestine of the American lobster Homarus americanus. The figure shows three mucosal membrane carrier proteins involved in the movement of these two solutes across the intestine. (1) A relatively specific L-histidine carrier that is not inhibited by L-leucine; (2) a relatively non-specific dipeptide transporter that accepts two histidine molecules (His) linked to a zinc ion in an apparent bis-complex; and (3) a relatively specific zinc transporter that is inhibited by luminal cupric ions (Cu2+). Luminal L-leucine (Leu) and glycyl-sarcosine (Gly-Sar) inhibit 3H-L-histidine transport by interacting with the dipeptide carrier in a mixed type inhibition. Luminal copper (Cu+ and Cu2+) inhibits 65Zn2+ transport by interacting with the dipeptide carrier in a mixed type inhibition. It is proposed that all interactions observed in this study occur on the brush border membrane (BBM) of intestinal epithelial cells and the mechanisms for efflux of both L-histidine and zinc from the cells to the blood across the basolateral membrane (BLM) are currently unclear.