Osmotically reactive plasma membrane vesicles prepared from rabbit kidney tubules by mild hypotonic lysis

Binding and transport of methotrexate and its derivative, MX-68, across the brush-border membrane in rat kidney

Binding and transport properties of methotrexate (MTX) and its novel derivative, MX-68, were examined in brush-border membrane vesicles (BBMVs) isolated from rat kidneys. The uptake of MTX, MX-68 and folic acid by BBMVs was stimulated by an inwardly-directed H(+) gradient. Such H(+)-dependent uptake of folic acid is compatible with a previous report (Bhandari et al., Biochim Biophys Acta 1988; 937: 211). The MTX uptake exhibits saturation with a K(m) of 0.834 microM. Although the uptake of these three compounds at optimal pH depended on the osmolarity of the medium, a substantial portion of the uptake was osmolarity-insensitive. By changing the medium osmolarity, the uptake by BBMVs could be separately discriminated as osmolarity-sensitive and insensitive portions, representing transport into the intravesicular space and binding to the surface of BBMVs, respectively. For all three compounds, the binding increased in a time-dependent manner, while the amount transported reached a maximum after a relatively short incubation period. The transport of folic acid, but not its binding, exhibited an overshoot phenomenon under conditions of an inward H(+) gradient. The present results suggest that reabsorption of MTX and MX-68 in the kidney is governed by both their binding and transport mechanisms, with a similar kinetic profile to that of folic acid. The involvement of a transport system seems to make a relatively small contribution to the reabsorption of MTX assessed in BBMVs compared with MX-68 and folic acid.

Binding of 125I-labelled albumin by isolated rat renal brush-border membrane vesicles. Evidence for uptake and internalization process

1. In the kidney, filtered proteins are rapidly reabsorbed by the proximal tubule via adsorptive endocytosis. This process starts with the protein binding to the luminal brush-border membrane. 2. The binding of 125I-labelled albumin to rat renal brush-border membrane vesicles and the effect of a low molecular weight protein lysozyme on that binding was assessed by the filtration method. 3. The Scatchard plot revealed a one-component binding-type curve with a dissociation constant Kd of 430.9 nM and 39.6 pmol/mg membrane protein for the number of binding sites. 4. Albumin binding was saturable and reversible, time and temperature dependent and the initial rate enhanced by increasing amounts of lysozyme. 5. The fact that association of albumin with the brush-border membrane vesicles was dependent upon the intravesicular space suggested a double process, binding of the ligand to the membrane surface and its internalization. These data suggest that albumin has a different binding site than that of a low-molecular weight protein lysozyme, with a constant affinity value near physiological loads. That specificity may confer selectivity upon the endocytic uptake process.

Folate binding and transport by rat kidney brush-border membrane vesicles

[3H]Pteroylglutamic acid (PteGlu) uptake was studied using brush-border membrane vesicles isolated from rat kidney. Results on the uptake of [3H]PteGlu by brush-border membrane vesicles incubated in media of increasing osmolarities demonstrated that uptake was contributed by two components, intravesicular transport and membrane binding. Both the components of the uptake exhibited similar pH dependence, with maxima at pH 5.6, and were found to be saturable mechanisms with Km values of 6.7.10(-7) and 11.2.10(-7) M, respectively. These studies show that PteGlu is transported by isolated rat kidney brush-border membrane vesicles in a manner consistent with a saturable system and that a binding component may be functionally associated with this.

Partial purification and characterization of rabbit-kidney brush-border (Ca2+ or Mg2+)-dependent adenosine triphosphatase

The ATPase activity of rabbit-kidney brush border can be activated almost equally well by Ca2+ and Mg2+ and, therefore, should be called (Ca2+ or Mg2+)-ATPase. This enzyme was solubilized and enriched 14-fold by the following steps: pretreatment with papain removed 69% of alkaline phosphatase without attacking a significant portion of the ATPase activity. Addition of 1% cholate removed 65% of the protein but no ATPase activity. The combination of cholate (0.5%) and deoxycholate (0.4%) solubilized most of the ATPase activity and most of the remaining protein. A column chromatography of the extract on Sepharose CL-2B resulted in an 6.5-fold increase of specific ATPase activity. A precipitation by ammonium sulfate (40% saturation) produced an additional 1.9-fold increase. The yield of this partial purification was 16%. Towards the nucleotides UTP and GTP the enzyme showed an activity slightly higher, and towards ITP and CTP an activity slightly lower than that with ATP. ADP was split about half as fast as ATP. AMP was not accepted by the enzyme. Replacing MgCl2 by CaCl2 resulted in an ATPase activity of 92% of that with MgCl2. Using calcium- and magnesium-ATP as substrates, apparent Km values of 0.22 and 0.33 mM, respectively, were obtained. The gel electrophoresis revealed the enrichment of a protein with an apparent Mr of 95000 and also that of microvillus actin.

The use of isolated membrane vesicles to study epithelial transport processes

Epithelia are multicompartment and multicomponent systems performing transcellular and paracellular transport in a very complex manner. One way to get a deeper understanding of the function of such a complex system is to dissect it into the single components and then, after having defined the components under well-controlled conditions, to try to describe the behavior of the whole system on the basis of the properties of the single components. This article deals with the analysis of isolated plasma membranes derived from the luminal and contraluminal face of epithelial cells, predominantly renal proximal tubular and small intestinal cells. It is aimed to give an overview of methods used to isolate and separate plasma membranes, to study their transport properties as membrane vesicles, and also to address the question of how information gained with the isolated membranes corresponds to observations made in the intact cell using other, notably electrophysiological, measurements. The review also critically evaluates the limitations of the approach and thereby tries to set the work on isolated membranes in the proper perspective within the field of transport physiology.

Glycosaminoglycans of the plasma membranes of renal tubule and liver cells

Glycosaminoglycans were isolated from plasma membranes of hepatic and renal tubule cells of guinea pig. Plasmalemma of renal tubule cells contained more total glycosaminoglycans, hyaluronic acid, chondroitin-4 sulfates and chondroitin-6 sulfates, and less dermatan sulfates and heparin sulfates than liver plasma membranes. These glycocalyx components, owing to their polyanionic properties, may have a role in the transport of water, ions, and macromolecules across the cell membrane.

myo-Inositol binding and transport in brush border membranes of rat kidney

Using hypotonically treated brush border membranes, binding and transport of myo-inositol were examined. By hypotonic treatment, both total and non-specific uptake decreased significantly, but specific uptake was not affected. myo-Inositol release from membranes preloaded by incubation for 2 min was very rapid and about 98% of preloaded myo-inositol was released in 5 min of incubation. However, myo-inositol release from membranes preloaded by incubation for 20 min was fairly slow and 50% of myo-inositol remained in the membranes even after 10 min of incubation. Uptake of myo-inositol decreased by the increase of osmolarity in the medium. However, effect of osmolarity on the uptake was less significant when myo-inositol concentration was lower. Under conditions in which mainly binding occurred, myo-inositol binding to the membranes was measured. Two binding systems were demonstrated and high affinity site could bind 22 pmol/mg protein at most and the apparent Km value was 8.3 muM. Both binding and transport processes were dependent on Na+ and enhanced by Na+-gradient.

Carrier-mediated transfer of D-glucose in brush border vesicles derived from rabbit renal tubules. Na+-dependent versus Na+-independent transfer

A brush border preparation from rabbit renal tubules containing a high yield of vesicles has been used to study the transfer of D-glucose through the brush border membrane. In the presence of an Na+ gradient across the vesicular membrane, the vesicles could concentrate D-glucose to a factor of 1.5, whereas in the absence of an Na+ gradient, only equilibrium with the medium was achieved. Two types of transfer could be distinguished by their requirement of Na+, their sensitivity to phlorizin and their pH optimum. The Na+-independent transfer was about 100 times less sensitive to phlorizin than the Na+-dependent path and exhibited a pH optimum between 7 and 8, whereas the Na+-dependent transfer was highest at a pH between 8 and 9. The brush border preparation could be freed of most of the contaminating material derived from the basal and lateral tubular cell membrane by a discontinuous density gradient centrifugation. It still showed both forms of transfer to a similar extent, indicating that both are located in the brush border membrane. A study of the sensitivity of D-glucose transfer to phlorizin, in the presence and absence of Na+ at different temperature, suggests a single carrier species functioning in two interchangeable conformational states with different affinities for phlorizin rather than two transfer systems working independently.