ATP-dependent transport of organic anions into isolated basolateral membrane vesicles from rat intestine

The mechanism for the cellular extrusion of organic anions across the intestinal basolateral membrane was examined using isolated membrane vesicles from rat jejunum, ileum, and colon. It was found that 17β-estradiol 17β-d-glucuronide (E217βG) is taken up in an ATP-dependent manner into the basolateral membrane vesicles (BLMVs) but not into the brush-border or microsomal counterparts. The ATP-dependent uptake of E217βG into BLMVs from jejunum and ileum was described by a single component with a Km value of 23.5 and 8.31 μM, respectively, whereas that into the BLMVs from colon was described by assuming the presence of high ( Km = 0.82 μM)- and low-affinity ( Km = 35.4 μM) components. Taurocholate, 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole glucuronide and taurolithocholate sulfate, but not leukotriene C4, were significantly taken up by the BLMVs. In addition to such substrate specificity, the inhibitor sensitivity of the ATP-dependent transport in BLMVs was similar to that of rat multidrug resistance-associated protein 3 (Mrp3), which is located on the basolateral membrane of enterocytes. Together with the fact that the rank order of the extent of the expression of Mrp3 (jejunum < ileum << colon) is in parallel with that of the extent of the transport of ligands, these results suggest that the ATP-dependent uptake of organic anions into isolated intestinal BLMVs is at least partly mediated by Mrp3.

Urate/α-ketoglutarate exchange in avian basolateral membrane vesicles

Membrane transport pathways for transcellular secretion of urate across the proximal tubule were investigated in avian kidney. The presence of coupled urate/α-ketoglutarate exchange was investigated in basolateral membrane vesicles (BLMV) by [14C]urate and [α-3H]ketoglutarate flux measurements. An inward Na gradient induced accumulation of α-ketoglutarate of sufficient magnitude to suggest a Na-dicarboxylate cotransporter. An inward Na gradient also induced concentrative accumulation of urate in the presence of α-ketoglutarate but not in its absence, suggesting urate/α-ketoglutarate exchange. α-Ketoglutarate-dependent stimulation of urate uptake was not observed in brush-border membrane vesicles. An outward urate gradient induced concentrative accumulation of α-ketoglutarate. α-Ketoglutarate-coupled urate uptake was specific for α-ketoglutarate, Cl dependent, and insensitive to membrane potential. α-Ketoglutarate-coupled urate uptake was inhibited by increasing p-aminohippurate (PAH) concentrations, and α-ketoglutarate-coupled PAH uptake was observed. α-Ketoglutarate-coupled PAH uptake was inhibited by increasing urate concentrations, and an outward urate gradient induced concentrative accumulation of PAH. These results suggest a Cl-dependent, α-ketoglutarate-coupled anion exchange mechanism as a pathway for active urate uptake across the basolateral membrane of urate-secreting proximal tubule cells.

Mechanism of n-butyrate uptake in the human proximal colonic basolateral membranes

Current studies were undertaken to characterize the mechanism of short-chain fatty acid (SCFA) transport in isolated human proximal colonic basolateral membrane vesicles (BLMV) utilizing a rapid-filtration n-[14C]butyrate uptake technique. Human colonic tissues were obtained from mucosal scrapings from organ donor proximal colons. Our results, consistent with the existence of a HCO[Formula: see text]/SCFA exchanger in these membranes, are summarized as follows: 1) n-[14C]butyrate influx was significantly stimulated into the vesicles in the presence of an outwardly directed HCO[Formula: see text] and an inwardly directed pH gradient; 2) n-[14C]butyrate uptake was markedly inhibited (∼40%) by anion exchange inhibitor niflumic acid (1 mM), but SITS and DIDS (5 mM) had no effect; 3) structural analogs e.g., acetate and propionate, significantly inhibited uptake of HCO[Formula: see text] and pH-gradient-driven n-[14C]butyrate; 4) n-[14C]butyrate uptake was saturable with a Kmfor butyrate of 17.5 ± 4.5 mM and a Vmaxof 20.9 ± 1.2 nmol · mg protein−1· 5 s−1; 5) n-[14C]butyrate influx into the vesicles demonstrated a transstimulation phenomenon; and 6) intravesicular or extravesicular Cl−did not alter the anion-stimulated n-[14C]butyrate uptake. Our results indicate the presence of a carrier-mediated HCO[Formula: see text]/SCFA exchanger on the human colonic basolateral membrane, which appears to be distinct from the previously described anion exchangers in the membranes of colonic epithelia.