| المستخلص: |
The renal H+/peptide cotransporters (PEPT1 and PEPT2) play important roles in the reabsorption of di- and tri- peptides. Based on structural similarity, PEPT1 and PEPT2 are also capable of transporting a broad range of peptidomimetic drugs. These cotransporters differ from one another with respect to their affinity and capacity, as PEPT1 is a high capacity / low affinity transporter, whilst PEPT2 is a low capacity / high affinity transporter. They are also expressed in different regions of the nephron with PEPT1 predominately found in cortex and PEPT2 mostly expressed in the outer medulla. In the present study, the effect of ageing on the activity and enrichment of Alkaline phosphatase and leucineaminopeptidase and on L-glycyl-L-glutamine transport was measured in brush border membrane vesicles (BBMV) isolated from the superficial cortex and outer medulla of two groups of male Wister rats, age approximately 6-7 weeks (young) and 9 months (middle-aged). The specific enzyme activity of both enzymes was higher in the cortex and the outer medulla of young compared to middle-aged rats. This difference was statistically significant in the outer medulla for both enzymes, outer medulla-young and middle-aged for leucine aminopeptidase 42.14 ± 17.98 and 0.2 ±0.03 mU/min/mg protein respectively (n=7-10 ± SE, p<0.05); for alkaline phosphatase 17.49 ± 8.31 and 0.35 ± 0.23 mU/min/mg protein respectively (n=7-10 ± SE, p<0.05). When BBMV, pre-equilibrated at pH 7.4, were added to a cuvette containing L-glycyl-L-glutamine (gly-gln) in a medium at pH 6.0, there was a sharp increase in the fluorescence of the probe 3,3-Dipropylthiacarbocyanine iodide (Dis-C3-(3)). The increase in fluorescence was proportional to the concentration of gly-gln applied, suggesting the entrance into the vesicles of gly-gln, cotransported with H+, lead to a depolarization of the membrane potential. When the effect of different concentrations of gly-gln were investigated in BBMV isolated from the cortex and outer medulla, a larger increase in fluorescence suggesting a greater uptake of gly-gln was measured in BBMV isolated from young rats. In addition, at the same concentration of gly-gln, the increase in fluorescence measured in BBMV isolated from the outer medulla was greater than in BBMV isolated from the superficial cortex suggesting a larger uptake of gly-gln. These results suggest that alterations to the renal brush border membrane with ageing may lead to differences in dipeptide uptake that may reflect a change in the activity of PEPT1 and PEPT2.
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