Sodium-dependent glucose transport by cultured proximal tubule cells

Characterization of glucose transport by cultured rabbit kidney proximal convoluted and proximal straight tubule cells

Rabbit kidney proximal convoluted tubule (RPCT) and proximal straight tubule (RPST) cells were independently isolated and cultured. The kinetics of the sodium-dependent glucose transport was characterized by determining the uptake of the glucose analog alpha-methylglucopyranoside. Cell culture and assay conditions used in these experiments were based on previous experiments conducted on the renal cell line derived from the whole kidney of the Yorkshire pig (LLC-PK1). Results indicated the presence of two distinct sodium-dependent glucose transporters in rabbit renal cells: a relatively high-capacity, low-affinity transporter (V(max) = 2.28 +/- 0.099 nmoles/mg protein min, Km = 4.1 +/- 0.27 mM) in RPCT cells and a low-capacity, high-affinity transporter (V(max) = 0.45 +/- 0.076 nmoles/mg protein min, K(m) = 1.7 +/- 0.43 mM) in RPST cells. A relatively high-capacity, low-affinity transporter (V(max) = 1.68 +/- 0.215 nmoles/mg protein min, Km = 4.9 +/- 0.23 mM) was characterized in LLC-PK1 cells. Phlorizin inhibited the uptake of alpha-methylglucopyranoside in proximal convoluted, proximal straight, and LLC-PK1 cells by 90, 50, and 90%, respectively. Sodium-dependent glucose transport in all three cell types was specific for hexoses. These data are consistent with the kinetic heterogeneity of sodium-dependent glucose transport in the S1-S2 and S3 segments of the mammalian renal proximal tubule. The RPCT-RPST cultured cell model is novel, and this is the first report of sodium-dependent glucose transport characterization in primary cultures of proximal straight tubule cells. Our results support the use of cultured monolayers of RPCT and RPST cells as a model system to evaluate segment-specific differences in these renal cell types.

Normal rat kidney proximal tubule cells in primary and multiple subcultures

An in vitro model to establish primary and subcultures of rat kidney proximal tubule (RPT) cells is described. After excising the kidneys and separating the cortex, the cortical tissue is digested with the enzymes DNAse-collagenase (Type I) resulting in a high yield of viable RPT Cells. The isolated RPT cells are then seeded onto rat tail collagen-coated surfaces and grown to confluency in a serum-free, hormonally defined medium. The cell yield can be increased by transferring the conditioned medium on Day 1 to more rat tail collagen-coated surfaces. RPT cell attachment and morphology was better on rat tail collagen-coated surfaces than on bovine collagen Type I coated surfaces. The culture medium was a 1:1 mixture of Ham's F-12 and Dulbecco's modified Eagle's medium supplemented with bovine serum albumin, insulin, transferrin, selenium, hydrocortisone, triiodothyronine, epidermal growth factor, and glutamine. The RPT cells became confluent in 7-10 d, at which point they could be subcultured by trypsinizing and growth in the same medium. In some studies, 10 ng/ml cholera toxin was added to the culture medium. We could passage the RPT cells up to 14 times in the presence of cholera toxin. The cells were investigated for activity of several markers. The cells were histochemically positive for alkaline phosphatase and gamma-glutamyl transpeptidase activity and synthesized the intermediate filament pankeratin. The RPT cells displayed apically directed sodium-dependent active glucose transport in culture. Hence, the RPT cells retain structural and functional characteristics of transporting renal epithelia in culture. This rat cell culture model will be a valuable tool for substrate uptake and nephrotoxicity studies.

Glucose-induced excitation in molluscan central neurons producing insulin-related peptides

The light green cells (LGCs) are a group of identified central neurons in the pond snail, Lymnaea stagnalis, that produce a number of insulin-related peptides. Freshly dissociated LGCs are activated by physiological concentrations of extracellular glucose. The response to glucose consists of a slow depolarization, which, at concentrations of 1 mM or more, rapidly induces regular spiking activity. The response persists during prolonged application of glucose but is completely reversed upon washing. The threshold concentration is 0.2 mM; the maximal effect occurs at 5 mM. In LGCs in central nervous system preparations kept in organ culture for 16-24 h, glucose causes a similar depolarization, which may lead to spiking activity. In freshly isolated preparations, which have very inexcitable LGCs, no direct response to glucose was seen. The response is specific to the LGCs; no other central neurons in Lymnaea showed consistent responses. The glucose response is evoked by D-glucose and the non-metabolized analogue 2-deoxy-D-glucose, but not by related hexoses, including L-glucose, nor pentoses or disaccharides. The response is not affected by interfering with the glucose metabolism, nor is the response mimicked by the metabolite D-glyceraldehyde or by injection of glucose. This suggests that glucose metabolites are not involved in the response. The glucose response depends on the presence of extracellular Na+ and is blocked by phlorizin, which specifically inhibits Na(+)-coupled glucose transport. This suggests that the response is due to activation of an electrogenic Na(+)-coupled glucose transporter.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Endogenous D-glucose transport in oocytes of Xenopus laevis

Endogenous glucose uptake by the oocytes of Xenopus laevis consists of two distinct components: one that is independent of extracellular Na+, and the other one that represents Na+-glucose cotransport. The latter shows similar characteristics as 2 Na+-1 glucose cotransport of epithelial cells: The similarities include the dependencies on external concentrations of Na+, glucose, and phlorizin, and on pH. As in epithelial cells, the glucose uptake in oocytes can also be stimulated by lanthanides. Both the electrogenic cotransport and the inhibition by phlorizin are voltage-dependent; the data are compatible with the assumption that the membrane potential acts as a driving force for the reaction cycle of the transport process. In particular, hyperpolarization seems to stimulate transport by recruitment of substrate binding sites to the outer membrane surface. The results described pertain to oocytes arrested in the prophase of the first meiotic division; maturation of the oocytes leads to a downregulation of both the Na+-independent and the Na+-dependent transport systems. The effect on the Na+-dependent cotransport is the consequence of a change of driving force due to membrane depolarization associated with the maturation process.

Primary cultures of rabbit renal proximal tubule cells: I. Growth and biochemical characteristics

Before the usefulness of a new in vitro model can be ascertained, the model must be properly defined and characterized. This study presents the growth rate and biochemical characteristics of rabbit renal proximal tubule cells in primary culture over a 2-wk culture period. When grown in a hormonally defined, antibiotic-free medium these cells form confluent monolayer cultures within 7 d after plating. Multicellular dome formation, an indicator of transepithelial solute transport, was expressed after confluent cultures were formed. The activity of the cytosolic enzyme, lactate dehydrogenase, and the lysosomal enzyme, N-acetyl-glucosaminidase, increased 14- and 2-fold during the first 8 d of culture, respectively. In contrast, the activity of a brush border enzyme, alkaline phosphatase, decreased 85% within the first 8 d of culture. Release of these enzyme markers into the culture medium, which are routinely used to measure cytotoxicity, stabilized after 8 d in culture. The ratio of cellular protein to DNA changed according to the state of cellular growth. Values rose from 0.035 mg protein/micrograms DNA in preconfluent cultures to 0.059 mg protein/micrograms DNA in confluent cultures. These results document the characteristics of a primary proximal tubule cell culture system for future studies in in vitro toxicology.

Characteristics of Na+-dependent hexose transport in OK, an established renal epithelial cell line

The characteristics of Na+-dependent hexose uptake were determined for monolayers of OK, an established renal epithelial cell line derived from an opossum kidney. A comparison is made with other cultured cells, particularly LLC-PK1. The capacity to accumulate alpha-methyl D-glucoside (AMG) in OK cells develops with time, reaching a maximum level of 18 nmol/mg protein per h, 3 days after confluency. In contrast to LLC-PK1, this level is not influenced by the medium D-glucose concentration. AMG uptake in OK cells was characterized by an apparent Km of 2.9 mM and a Vmax of 17.1 nmol/mg protein per min. For Na+-dependent phlorisin binding, a KD of 0.025 microM and a Bmax of 1.5 pmol/mg protein were found. A turnover frequency of 158/s was derived from our data. The hexose carrier of OK shares with the carrier of LLC-PK1 a high level of expression, its substrate specificity and turnover frequency. It differs however with respect to the substrate binding site. The affinity for AMG and D-glucose is 3- and 10-fold lower, whereas the affinity for phlorizin is 3-times higher in OK than in LLC-PK1. The Na+ dependence of AMG uptake was also different for both cell lines and suggested for OK cells a 1:1, Na+:substrate stoichiometry. In OK cells, the phlorizin-sensitive uptake rate of D-glucose is much lower than the one for AMG. Nevertheless, D-glucose interacts with the AMG binding site in a competitive way and with an affinity similar to AMG. This could indicate a malfunction of the carrier with D-glucose as a substrate at the level of the translocation step.