[Differentiation of the active and passive components of D-glucose transport in the proximal tubule of rat kidney]

Renal Tubular Handling of Glucose and Fructose in Health and Disease

The proximal tubule of the kidney is programmed to reabsorb all filtered glucose and fructose. Glucose is taken up by apical sodium-glucose cotransporters SGLT2 and SGLT1 whereas SGLT5 and potentially SGLT4 and GLUT5 have been implicated in apical fructose uptake. The glucose taken up by the proximal tubule is typically not metabolized but leaves via the basolateral facilitative glucose transporter GLUT2 and is returned to the systemic circulation or used as an energy source by distal tubular segments after basolateral uptake via GLUT1. The proximal tubule generates new glucose in metabolic acidosis and the postabsorptive phase, and fructose serves as an important substrate. In fact, under physiological conditions and intake, fructose taken up by proximal tubules is primarily utilized for gluconeogenesis. In the diabetic kidney, glucose is retained and gluconeogenesis enhanced, the latter in part driven by fructose. This is maladaptive as it sustains hyperglycemia. Moreover, renal glucose retention is coupled to sodium retention through SGLT2 and SGLT1, which induces secondary deleterious effects. SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing independent of kidney function and diabetes. Dietary excess of fructose also induces tubular injury. This can be magnified by kidney formation of fructose under pathological conditions. Fructose metabolism is linked to urate formation, which partially accounts for fructose-induced tubular injury, inflammation, and hemodynamic alterations. Fructose metabolism favors glycolysis over mitochondrial respiration as urate suppresses aconitase in the tricarboxylic acid cycle, and has been linked to potentially detrimental aerobic glycolysis (Warburg effect). © 2022 American Physiological Society. Compr Physiol 12:2995-3044, 2022.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

Pathophysiology of the kidney in rats with Heymann nephritis

Alterations in kidney function were assessed early in the course of Heymann nephritis that was induced in rats by immunization with Fx1A, an extract prepared from rat kidney cortex. Whole kidney and single nephron function were evaluated by clearance and micropuncture techniques. Kidney function was studied in stage 1 of Heymann nephritis, before the onset of proteinuria, and in stage 2, when antibodies are deposited along the brush border of proximal tubules. Although overall kidney function was similar in rats in stage 1 and normal controls, glucose reabsorption was somewhat depressed in the first part of the proximal convoluted tubule in stage 1. Both whole kidney and single nephron glomerular filtration rates were depressed in stage 2. Proteinuria in stage 2 was characterized by an increased albumin sieving coefficient, which resulted in an elevated excretion of albumin. Furthermore, several proximal tubule functions (glucose and fluid reabsorption and PAH extraction) were substantially depressed in stage 2. These findings demonstrate that immunological injury to the proximal tubules in stage 2 of Heymann nephritis produces a significant impairment of proximal function.

Contraluminal transport of hexoses in the proximal convolution of the rat kidney in situ

In order to study contraluminal hexose transport, concentration and time-dependent influx of 3H-2-deoxy-D-glucose from the interstitium into cortical tubular cells has been measured. The influx curves fit to a two parameter kinetics (Km 1.3 +/- 0.2 mmol/l, Jmax 0.67 +/- 0.16 pmol/s X cm) plus an additional diffusion term (with P = 6 X 10(-8) cm2/s) and a distribution ratio extracellular to intracellular amount of 2-deoxy-D-glucose of 1:0.6. Since the extracellular to intracellular free water space as estimated from morphological data was 1:2, one must conclude that glucose has only free access to 1/3 of the cell water. The intracellularly accessible space was augmented when the tubules were preperfused for 10 s with hypotonic saline. Thereby an increase of the compartment into which diffusion occurs was revealed and a final rupture of this intracellular compartment at 1/4 isotonic solutions was observed. Total replacement of ions in the peritubular perfusate by mannitol did not change 2-deoxy-D-glucose influx, indicating that it is Na+-independent. By adding isotonic concentrations of the respective sugars to the capillary perfusate, three degrees of inhibition of 2-deoxy-D-glucose influx could be revealed: strong inhibition by D-glucose, methyl-beta-D-glucoside, D-mannose, 3-O-methyl-D-glucose, 2-deoxy-D-galactose, methyl-beta-D-galactoside and 6-deoxy-D-glucose, moderate inhibition by D-galactose, L-glucose, L-mannose and D-fructose, no or borderline inhibition by methyl alpha-D-glucoside, 2-deoxy-methyl-alpha-D-galactoside, 1-thio-beta-D-glucose, 1-thio-beta-D-galactose, 5-thio-alpha-D-glucose, myo-inositol and mannitol.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular tubular balance in preterm and fullterm infants

The development of glomerular and tubular function was studied in preterm and fullterm infants of varying gestational and postnatal age. The results indicate that glomerular functional development precedes tubular functional development until the 34th postmenstrual week. After the 34th week the tubular transport capacity seems to be more vulnerable than the glomerular filtration rate in states of disease. The release of a postulated renal vasoconstriction could account for the rapid changes in renal function after birth. The purpose of such a vasoconstriction could be to protect the tubules from an overload.

Electrophysiological analysis of rat renal sugar and amino acid transport. I. Basic phenomena

The electrical events associated with the absorption of D-glucose or L-amino acids in renal proximal tubules were studied in microperfusion experiments on rat kidneys in vivo. Intratubular application of these substrates led concomitantly to: 1) a shift of the transepithelial potential into lumen negative direction, 2) a partial depolarization of the tubular cell membranes and 3) a reduction of the electrical resistance of the brushborder membrane. By means of rapid perfusion experiments it was possible to discern two phases in the potential response to substrate perfusion, a fast initial response which reflects a substrate-induced Na+ ion current from lumen to cell, and a slower secondary response which reflects the relaxation of the intracellular ion and substrate concentrations towards new steady states. A quantitative analysis of the data yielded estimates of 1) the apical (Ra) and basal (Rb) cell membrane resistances and of the shunt resistance, Rs, of rat proximal tubule of approximately Ra = 255 omega cm2, Rb = 92 omega cm2 and Rs = 5 omega cm2 (all referred to the quasi macroscopic surface area of the tubular lumen), 2) the conductance of the Na+ and glucose cotransport pathway and 3) the driving forces acting on the cotransport mechanism in the brushborder membrane. The latter were found to be a) the electrical cell membrane potential of -74 mV, b) the Na+ ion concentration gradient between the tubular lumen (clumNa = 145 mmol/l) and the cytoplasm (ccellNa approximately 33 mmol/l) which corresponds to an additional equivalent potential of 51 mV and c) the substrate concentration gradient, which varies according to the experimental conditions. Moreover the analysis provided a quantitative estimate of the relationship between the substrate-induced changes in transepithelial potential or short circuit current and the actual cotransport current in the brushborder membrane. Based on this analysis it is concluded that the stoichiometry of Na+ and glucose flux coupling in the brushborder membrane of rat proximal tubule is close to 1.0.

Na+-dependent sugar transport in a cultured epithelial cell line from pig kidney

A Na+-dependent hexose transport system with similar characteristics that observed in the kidney is retained in a cultured epithelial cell line from pig kidney (LLC-PK1). The active transport of oc methyl-D-glucoside (oc MGP), a nonmetabolizable sugar, which shares the glucose-galactose transport system in kidney cells is mediated through a Na+-dependent, substrate-saturable process. The kinetic analysis of the effect of Na+ on the uptake of ocMGP indicated that the Na+-sugar cotransport system is an affinity type system in which the binding of either sugar or Na+ carrier increases the affinity for the other ligand without affecting the Vmax. The sequence of selectivity for different sugars studied by the inhibition produced in the uptake of ocMGP is very similar to that reported in rat kidney, rabbit kidney cortex slices, and rabbit renal brush border membrane vesicles. Phlorizin, even at very low concentration, almost completely inhibits ocMGP uptake. Conversely, phloretin at the same low concentration stimulated the sugar accumulation by inhibition of efflux, probably at the level of the basolateral membrane. Sulfhydryl group inhibitors also blocked the ocMGP uptake, suggesting that these groups were required for normal functioning of the sugar carrier system. This sugar transport system is an important functional marker to study the molecular events associated with the development of polarization in epithelial cells.

Effects of glucose on water and sodium reabsorption in the proximal convoluted tubule of rat kidney

1. The effects of glucose on sodium and water reabsorption by rat renal proximal tubules was investigated by in situ microperfusion of segments of proximal tubules with solutions containing glucose or no glucose, with and without phlorizin. 2. Absence of glucose did not significantly alter net water flux. Sodium flux was reduced by about 10% but this was not statistically significant. 3. In the absence of glucose in the perfusion fluid net secretion of glucose occurred. 4. Phlorizin reduced either net reabsorption or net secretion of glucose; and net water flux. 5. The data suggest that in later parts of the proximal convoluted tubule some sodium may be co-transported with glucose, but that this represents only a small fraction of the total sodium reabsorption. 6. It is suggested that the glucose carrier is reversible and in appropriate circumstances could cause glucose secretion. 7. Although phlorizin alters net water flux the underlying mechanisms are not clear. 8. The calculated osmolality of the reabsorbate was significantly greater than the perfusate osmolality and greater than plasma osmolality although this was not quite significant statistically.

The role of brush border enzymes in tubular reabsorption of disaccharides: a microperfusion study in rat kidney

Renal tubular reabsorption of maltose, sucrose and lactose were studied in vivo et situ by continuous microperfusion of single proximal convolutions of rat kidney. The 14C-label of maltose (2.5 mmol/l) was removed from the lumen of the proximal tubule at about the same rate as found for glucose. Maltose reabsorption was completely inhibited in presence of 30 mmol/l glucose or of 0.1 mmol/l phlorizin. Chemical analysis of the samples showed a complete conversion of maltose into glucose within a perfusion distance of 2 mm. It is concluded from these results that within the tubular lumen maltose is split very rapidly by a brush border glucosidase. The short half time of this process permits the breakdown product glucose to be almost completely reabsorbed subsequently within the proximal tubule. In contrast, sucrose and lactose were neither split nor reabsorbed by the tubule brush border.

Amino acid reabsorption in the proximal tubule of rat kidney: stereospecificity and passive diffusion studied by continuous microperfusion

Renal tubular reabsorption of glycine and of the L- and D-isomers of histidine, serine, phenyl-alanine, methionine, proline and cystine was investigated in vivo et situ by continuous microperfusion of single proximal convolutions of the rat kidney. In the case of glycine and the L-isomers, tubular reabsorption is saturable to a great extent. The D-amino acids are reabsorbed much more slowly than the respective L-forms. Furthermore in the case of methionine and perhaps also of proline, serine and phenylalanine, the fractional reabsorption decreases in the presence of high concentrations of the L-form. This indicates that the D-isomers also have a measurable affinity for the reabsorption mechanisms of the renal tubule. The very poor reabsorption of D-amino acids in the presence of their L-isomers indicates that simple passive diffusion plays only a relatively small role in tubular amino acid reabsorption. Permeability coefficients estimated from these findings are in the range from 1--5 X 10(-7) cm2 - s-1. These values are very similar to those found for other organic molecules of comparable molecular weights.

Renal phosphate transport: inhomogeneity of local proximal transport rates and sodium dependence

The standing droplet method has been used in combination with the peritibular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (Piota) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate Piota. At zero time both the intraluminal and the contraluminal Piota concentration was 2 mM. The time dependent decrease of the intraluminal Piota concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM Piota was achieved in the early part. In the late part the intraluminal Piota concentration approached a steady statevalue of 0.54 mM at 123 sec. When sodium free solutions were used the intaluminal Piota concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. the rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phospate reabsorption might occur as co-transport with Na+ ions.

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.