Is hexokinase present in the basal lateral membranes of rat kidney proximal tubular epithelial cells?

The sugar daddy: the role of the renal proximal tubule in glucose homeostasis

Renal blood flow represents >20% of total cardiac output and with this comes the great responsibility of maintaining homeostasis through the intricate regulation of solute handling. Through the processes of filtration, reabsorption, and secretion, the kidneys ensure that solutes and other small molecules are either returned to circulation, catabolized within renal epithelial cells, or excreted through the process of urination. Although this occurs throughout the renal nephron, one segment is tasked with the bulk of solute reabsorption-the proximal tubule. Among others, the renal proximal tubule is entirely responsible for the reabsorption of glucose, a critical source of energy that fuels the body. In addition, it is the only other site of gluconeogenesis outside of the liver. When these processes go awry, pathophysiological conditions such as diabetes and acidosis result. In this review, we highlight the recent advances made in understanding these processes that occur within the renal proximal tubule. We focus on the physiological mechanisms at play regarding glucose reabsorption and glucose metabolism, emphasize the conditions that occur under diseased states, and explore the emerging class of therapeutics that are responsible for restoring homeostasis.

"SLC-omics" of the kidney: Solute transporters along the nephron

The fluid in the 14 distinct segments of the renal tubule undergoes sequential transport processes that gradually convert the glomerular filtrate into the final urine. The solute carrier (SLC) family of proteins is responsible for much of the transport of ions and organic molecules along the renal tubule. In addition, some SLC family proteins mediate housekeeping functions by transporting substrates for metabolism. Here, we have developed a curated list of SLC family proteins. We used the list to produce resource webpages that map these proteins and their transcripts to specific segments along the renal tubule. The data were used to highlight some interesting features of expression along the renal tubule including sex-specific expression in the proximal tubule and the role of accessory proteins (β-subunit proteins) that are thought to be important for polarized targeting in renal tubule epithelia. Also, as an example of application of the data resource, we describe the patterns of acid-base transporter expression along the renal tubule.

Accumulation of some hexoses in adipocytes: properties of the system

Adipocytes accumulate 2-deoxy-D-glucose (2-DG) against significant chemical gradients (J. Foley and J. Gliemann. Biochim. Biophys. Acta 648: 100-106, 1981). The specificity of this accumulation process was examined. Isolated rat adipocytes incubated at 37 degrees C with 0.1 mM hexoses accumulated free 2-deoxy-D-galactose (2-DGal) or D-galactose (Gal) against steady-state gradients of 5.5 +/- 1.2 and 2.5 +/- 0.7, respectively. Like 2-DG, both 2-DGal and Gal are substrates for phosphorylation. Insulin and insulin-mimetic agents increased steady-state accumulations of Gal, 2-DGal, and free and phosphorylated 2-DG (100 nM insulin increased free 2-DG from 2.1 +/- 0.2 to 6.6 +/- 0.3 mM; external 2-DG = 0.1 mM). Removal of extracellular calcium or sodium or the presence of A23187 or ouabain failed to inhibit 2-DG accumulation. Plasma membrane permeabilization induced by either digitonin or high-voltage discharge produced a loss of cellular 2-DG and 2-DG-phosphate without affecting 3-O-methyl-D-glucose equilibration. The data indicate that neither transmembrane ionic gradients nor intracellular compartmentation suffice as explanations of the mechanism of the accumulation process. The possible role of phosphorylation in the process of hexose accumulation is discussed.

The accumulation of free and phosphorylated sugars in adipocytes based on a dynamic diffusion barrier

The simple theory of a dynamic diffusion barrier is described and it is shown how this could account for the accumulation, in adipocytes, of those free sugars which are also phosphorylated. The standing concentration gradient established by this mechanism depends on the recycling of free sugar and sugar phosphate in submembrane structures which start in juxtaposition to conventional membrane hexose transporters. Although a continual expenditure of metabolic energy is involved, there can be a net gain from the potential-energy store of accumulated substrates. The hypothesis leads to a series of simple equations which can be used as the basis for computer simulations of experimental procedures.

Electro-osmosis and the reabsorption of fluid in renal proximal tubules

The lateral intercellular spaces (LIS) are believed to be the final common pathway for fluid reabsorption from the renal proximal tubule. We postulate that electrogenic sodium pumps in the lateral membranes produce an electrical potential within the LIS, that the lateral membranes bear a net negative charge, and that fluid moves parallel to these membranes because of Helmholtz-type electro-osmosis, the field-induced movement of fluid adjacent to a charged surface. Our theoretical analysis indicates that the sodium pumps produce a longitudinal electric field of the order of 1 V/cm in the LIS. Our experimental measurements demonstrate that the electrophoretic mobility of rat renal basolateral membrane vesicles is 1 micron/s per V/cm, which is also the electro-osmotic fluid velocity in the LIS produced by a unit electric field. Thus, the fluid velocity in the LIS due to electro-osmosis should be of the order of 1 micron/s, which is sufficient to account for the observed reabsorption of fluid from renal proximal tubules. Several experimentally testable predictions emerge from our model. First, the pressure in the LIS need not increase when fluid is transported. Thus, the LIS of mammalian proximal tubules need not swell during fluid transport, a prediction consistent with the observations of Burg and Grantham (1971, Membranes and Ion Transport, pp. 49-77). Second, the reabsorption of fluid is predicted to cease when the lumen is clamped to a negative voltage. Our analysis predicts that a voltage of -15 mV will cause fluid to be secreted into the Necturus proximal tubule, a prediction consistent with the observations of Spring and Paganelli (1972, J. Gen. Physiol., 60:181).

Asymmetry in the transport of lactate by basolateral and brush border membranes of rat kidney cortex

The uptake of L(+)lactate into rat renal cortical brush border (BBV) and basolateral (BLV) membrane vesicles, isolated through differential centrifugation and free flow electrophoresis, were studied using a rapid filtration technique. In contrast to the lactate transport into the BBV, that into the BLV: 1) was found to proceed only towards equilibrium, 2) showed Na+ -independent coupling of the influx of L(+)lactate and the efflux of L(+) but not to the efflux of D(-)lactate, 3) was not inhibited by D(-)lactate, 2-thiolactate or 3-phenyl-lactate, but 4) was inhibited by 3-thiolactate and alpha-hydroxybutyrate and 5) was accelerated by changes in inwardly directed ionic gradients or by increases in cation conductance both of which led to increased intravesicular positivity. The latter changes had the opposite effect on the uptake of L(+)lactate by BBV. Thus, while the L(+)lactate transport system present in BBV showed the characteristics of Na-dependent electrogenic cotransport system, that in the BLV was consistent with a carrier mediated Na-dependent, facilitated diffusion system.

Transport of p-aminohippuric acid, uric acid and glucose in highly purified rabbit renal brush border membranes

A procedure for preparing highly purified brush border membranes from rabbit kidney cortex using differential and density gradient centrifugation is described. Brush border membranes prepared by this procedure were substantially free of basal-lateral membranes, mitochondria, endoplasmic reticulum and nuclear material as evidenced by an enrichment factor of less than 0.3 for (Na+ + K+)-ATPase, succinate dehydrogenase, NADPH-cytochrome c reductase and DNA. Alkaline phosphatase was enriched ten fold indicating that the membranes were enriched at least 30 fold with respect to other cellular organelles. The yield of brush border membranes was 20%. Transport of D-glucose by the membranes was identical to that previously reported except that the Arrhenius plot for temperature dependence of transport was curvilinear (EA = 11.3--37.6 kcal/mol) rather than biphasic. Transport of p-aminohippuric acid and uric acid were increased by the presence of NaCl, either gradient or preequilibrated. However, no overshoot was obtained in the presence of a NaCl gradient, and KCl and LiCl also produced equivalent stimulation of transport suggesting a nonspecific ionic strength effect. Uptakes of p-aminohippuric acid and uric acid were not saturable, and were increased markedly by reducing the pH from 7.5 to 5.6. Probenecid (1 mM) reduced p-aminohippuric acid and uric acid (50 muM) uptake by 49% and 21%, respectively. We conclude that the uptake of uric acid and p-aminohippuric acid by renal brush border membranes of the rabbit occurs primarily by a simple solubility-diffusion mechanism.