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

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.

Prior exercise enhances passive absorption of intraduodenal glucose

The purpose of this study was to assess whether a prior bout of exercise enhances passive gut glucose absorption. Mongrel dogs had sampling catheters, infusion catheters, and a portal vein flow probe implanted 17 days before an experiment. Protocols consisted of either 150 min of exercise (n = 8) or rest (n = 7) followed by basal (-30 to 0 min) and a primed (150 mg/kg) intraduodenal glucose infusion [8.0 mg x kg-1x min-1, time (t) = 0-90 min] periods. 3-O-[3H]methylglucose (absorbed actively, facilitatively, and passively) and l-[14C]glucose (absorbed passively) were injected into the duodenum at t = 20 and 80 min. Phloridzin, an inhibitor of the active sodium glucose cotransporter-1 (SGLT-1), was infused (0.1 mg x kg-1 x min-1) into the duodenum from t = 60-90 min with a peripheral venous isoglycemic clamp. Duodenal, arterial, and portal vein samples were taken every 10 min during the glucose infusion, as well as every minute after each tracer bolus injection. Net gut glucose output in exercised dogs increased compared with that in the sedentary group (5.34 +/- 0.47 and 4.02 +/- 0.53 mg x kg-1x min-1). Passive gut glucose absorption increased approximately 100% after exercise (0.93 +/- 0.06 and 0.45 +/- 0.07 mg x kg-1 x min-1). Transport-mediated glucose absorption increased by approximately 20%, but the change was not significant. The infusion of phloridzin eliminated the appearance of both glucose tracers in sedentary and exercised dogs, suggesting that passive transport required SGLT-1-mediated glucose uptake. This study shows 1). that prior exercise enhances passive absorption of intraduodenal glucose into the portal vein and 2). that basal and the added passive gut glucose absorption after exercise is dependent on initial transport of glucose via SGLT-1.

Pyruvate attenuates myoglobin in vitro toxicity

Myoglobinuria is a complication of crush injury as well as substance abuse. This study examined whether pyruvate modified myoglobin in vitro renal toxicity. Renal slices from Fischer-344 rats were incubated for 120 min with 0-12 mg/ml myoglobin. In an initial study, gluconeogenesis was stimulated by the addition of 10 mM pyruvate during the final 30 min. In all other studies, renal slices were incubated with myoglobin in the presence of 0 or 10 mM pyruvate for 120 min. Myoglobin increased lactate dehydrogenase (LDH) release and this was not modified by the presence of pyruvate for the last 30 min of the incubation. Myoglobin toxicity was reduced by coincubation of myoglobin with pyruvate for 120 min. LDH leakage was increased 1.2-, 1.7-, and 1.8-fold above control by 4, 10, and 12 mg/ml myoglobin, compared to 1.2, 1.3, and 1.3 fold in slices coincubated with 10 mM pyruvate, respectively. Myoglobin diminished adenosine triphosphate (ATP) levels but pyruvate maintained a 5x higher level of ATP within the slices. Glucose (10 mM) provided protection only for the low concentration (4 mg/ml) of myoglobin. Myoglobin induced oxidative stress while pyruvate prevented the rise in lipid peroxidation and glutathione disulfides by myoglobin. Myoglobin diminished total glutathione levels in pyruvate-treated tissue, but glutathione levels remained higher than tissues incubated in the absence of pyruvate. These results indicate that pyruvate reduced toxicity by preventing oxidative stress and via a supply of an energy substrate.

Tubular reabsorption of myo-inositol vs. that of D-glucose in rat kidney in vivo et situ

Filtered myo-inositol, an important renal intracellular organic osmolyte, is almost completely reabsorbed. To examine tubule sites and specificity and, thus possible mechanism of this reabsorption, we microinfused myo-[(3)H]inositol or D-[(3)H]glucose into early proximal (EP), late proximal (LP), or early distal tubule sections of superficial nephrons and into long loops of Henle (LLH) of juxtamedullary nephrons and papillary vasa recta in rats in vivo et situ and determined urinary fractional recovery of the (3)H label compared with comicroinfused [(14)C]inulin. To determine the extent to which the proximal convoluted tubule (PCT) alone contributes to myo-inositol reabsorption, we also microperfused this tubule segment between EP and LP puncture sites. We examined specificity of reabsorptive carrier(s) by adding high concentrations of other polyols and monosaccharides to the infusate. The results show that >60% of the physiological glomerular load of myo-inositol can be reabsorbed in the PCT and >90% in the short loop of Henle (SLH) by a saturable, phloridzin-sensitive process. myo-Inositol can also be reabsorbed in the ascending limb of LLH and can move from papillary vasa recta blood into ipsilateral tubular structures. Essentially no reabsorption occurred in nephron segments beyond the SLH or in collecting ducts. Specificity studies indicate that reabsorption probably occurs via a luminal Na(+)-myo-inositol cotransporter.

3-O-methylglucose as probe of cytoplasmic volumes

The intracellular volume of distribution (V) of 3-O-methylglucose (3OM), as calculated from mean artery-to-vein transit times (t)in the rabbit kidney, has been proposed as measure of cytoplasmic volume in renal cortex. This hypothesis requires that the non-concentrative uptake of the sugar be flow-dependent rather than diffusion-limited, and that total cell H2O (VHOH) exceed V3OM by a volume presumably reflecting that of 3OM-impermeable organelles. Diffusion-dependence of the calculated V3OM could be excluded by the finding that lengthening of t does not alter 3OM uptake. Determination of VHOH in kidney is complicated by presence of the luminal fluid compartment. VHOH and V3OM were therefore compared in leucocytes freshly prepared from rat blood; V3OM was found to equal 40% of VHOH. There is no evidence to suggest concentrative 3OM uptake by these cells; 3OM-impermeable organelles therefore occupy 60% of VHOH. These findings support the equation of V3OM with the cytoplasmic volume. As volume control is a basic function of intact cells, convenient determination of VHOH and V3OM on small numbers of readily available nucleated cells may prove useful in detecting early cytotoxic effects in vivo or in vitro.

Sugar transport in isolated rat kidney papillary collecting duct cells

D-Glucose is an important substrate of energy metabolism and osmolyte synthesis in the renal papillary collecting duct. In order to characterize the cellular entry of D-glucose in this tubular segment, collecting duct cells were isolated from rat kidney papilla and the rate of D-glucose uptake was measured indirectly by monitoring the D-glucose-dependent O2 uptake in the presence of the uncoupler CCCP. D-Glucose uptake was found to be sodium-independent and not sensitive to phlorizin even at a concentration of 10(-3) M. Uptake was, however, completely inhibited by 10(-5) M cytochalasin B and 10(-4) M phloretin. The apparent Ki for cytochalasin B was 1.5 x 10(-6) M and for phloretin 2.0 x 10(-5) M. Studies on the substrate specificity revealed that at 1 mM D-mannose is taken up and metabolized to the same extent as D-glucose. A 50-fold higher concentration of 2-deoxy-D-glucose and 2-amino-2-deoxy-D-glucose inhibited D-glucose uptake completely whereas alpha-methyl-D-glucoside, D-allose, and D-galactose were without effect. Under conditions where D-glucose utilization was maximally stimulated an apparent Km of 1.2 mM and a Vmax of 1 mmol D-glucose/g protein.hour was found for D-glucose uptake. These results indicate that the D-glucose uptake into papillary collecting duct cells is probably mediated by a transport system similar to the one found in basal-lateral membranes of polarized renal, intestinal, and liver cells as well as in nonpolarized fat cells and erythrocytes.

Contraluminal transport of small aliphatic carboxylates in the proximal tubule of the rat kidney in situ

In order to study the characteristic of contraluminal transport of hydrophylic small fatty acids the in situ stopped flow microperfusion technique [12] has been applied. By measuring with 4 s contact time the decrease in the contraluminal concentration of the respective radiolabelled substances the concentration dependence of the influx into the cortical cells was tested. The 4 s decrease in contraluminal concentration of chloroacetate, L-lactate, D-lactate, 3-hydroxybutyrate and acetoacetate was between 26% and 31%. For each substance the percent decrease was the same, no matter whether it was offered in a concentration of 0.1 or 10 mmol/l. Contraluminal disappearance of 0.1 mmol/l L-lactate was not influenced by 5 mmol/l H2DIDS, probenecid, phloretin, mersalyl or cyanocinnamate, but it was significantly (37%) inhibited by 5-nitro-2-(phenyl-propyl-amino) benzoate, a blocker of the nonspecific anion channel. The percent decrease in propionate uptake was somewhat larger - between 36% and 39% - but again not different at 0.01, 0.1, 1.0 and 10 mmol/l. With pyruvate the contraluminal decrease was 20% at 0.1 mmol/l and 31% at 10 mmol/l. The percent disappearance of the aromatic pyrazinoate was 38% and 34% at 0.1 and 10 mmol/l and for nicotinate 42% and 22%, respectively. The disappearance of nicotinate (0.1 mmol/l) was significantly inhibited by 10 mmol/l pyrazinoate and paraaminohippurate (PAH). The data are in agreement with the hypothesis that the hydrophilic small fatty acids traverse the contraluminal cell side by simple diffusion, possibly via the unspecific anion channel [14], pyruvate via the dicarboxylic acid pathway in a cooperative manner and pyrazinoate, as well as nicotinate, via the PAH pathway.

Inositol 1,4,5-trisphosphate releases Ca2+ from a nonmitochondrial store site in permeabilized rat cortical kidney cells

We have recently shown that inositol 1,4,5-trisphosphate (IP3) releases Ca2+ from the endoplasmic reticulum of pancreatic acinar cells and suggested that IP3 may function as a second messenger of hormonal receptors to mobilize Ca2+ from intracellular stores (Streb et al, 1983, Streb et al, 1984). In rat kidney cortical tubules and microdissected mouse proximal tubules, an increased turnover of polyphosphoinositide metabolism following hormonal stimulation with angiotensin II-amide and phenylephrine has been reported (Wirthensohn et al, 1984; Wirthensohn et al, 1985). This suggests that IP3, one of their hydrolysis products, increases during hormonal stimulation. We therefore investigated the effect of angiotensin II-amide and IP3 on intracellular Ca2 stores in saponin-treated cells and homogenate from rat kidney cortex. Saponin-treated isolated cortical kidney cells or homogenate was incubated in a high K+ buffer in the presence of MgATP and respiratory substrates. Ca2+ uptake was determined by measuring the free Ca2+ concentration of the surrounding medium with a Ca2+ specific macroelectrode. Addition of cells or homogenate to the incubation medium resulted in a decrease of the medium free Ca2+ concentration until a steady-state concentration of 5.7 +/- 0.2 X 10(-7) mole/l was obtained. In the presence of mitochondrial inhibitors Ca2+ uptake rate was reduced, whereas the steady-state concentration was unchanged. In contrast, in the presence of the CA2+-ATPase inhibitor vanadate mitochondrial uptake proceeded at the same rate as the control, but the steady-state concentration was higher (6.9 +/- 0.2 X 10(-7) mole/l).(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Contraluminal phosphate transport in the proximal tubule of the rat kidney

In order to study the characteristics of contraluminal phosphate transport the stopped flow microperfusion technique [13] has been applied. By measuring the time-dependent decrease of interstitial 33Pi concentration at different starting concentrations a simple diffusion kinetics with a permeability coefficient of 7.5 +/- 1.0 X 10(-8) cm2 s-1 was found. Such a kinetic was so far only observed with 2-deoxy-D-glucose. This substance, however, is transported in addition by facilitated diffusion as was seen by paraaminohippurate, methylsuccinate and sulfate. The contraluminal transport of phosphate was inhibited by H2-DIDS (5 mmol/l). It was, however, not influenced by omission of Na+ from the perfusates, by addition of sulfate (150 mmol/l), methylsuccinate (50 mmol/l), arsenate (50 mmol/l), the Hg-compound mersalyl (5 mmol/l), high and low phosphate diet and pH changes between 6.0 and 8.0. The data indicate that phosphate, which is reabsorbed from the lumen by a Na+-dependent transport system, leaves the cell by a rather unspecific contraluminal diffusion pathway.