Taurine transport by isolated flounder renal tubules

Kidneys sans glomeruli

The evolution of the vertebrate kidney records three occasions, each separated by about 50 million years, when fish have abandoned glomeruli to produce urine by tubular mechanisms. The recurring dismissal of glomeruli suggests a mechanism of aglomerular urine formation intrinsic to renal tubules. Indeed, the transepithelial secretion of organic solutes and of inorganic solutes such as sulfate, phosphate, and magnesium can all drive secretory water flow in renal proximal tubules of fish. However, the secretion of NaCl via secondary active transport of Cl is the primary mover of secretory water flow in, surprisingly, proximal tubules of both glomerular and aglomerular fish. In filtering kidneys, the tubular secretion of solute and water is overshadowed by reabsorptive transport activities, but secretion progressively comes to light as glomerular filtration decreases. Thus the difference between glomerular and aglomerular urine formation is more a difference of degree than of kind. At low rates of glomerular filtration in seawater fish, NaCl-coupled water secretion serves to increase the renal excretory capacity by increasing the luminal volume into which waste, excess, and toxic solutes can be secreted. The reabsorption of NaCl and water in the distal nephron and urinary bladder concentrates unwanted solutes for excretion while minimizing renal water loss. In aglomerular fish, NaCl-coupled water secretion across proximal tubules replaces glomerular filtration to increase renal excretory capacity. A review of the literature suggests that tubular secretion of NaCl and water is an early function of the vertebrate proximal tubule that has been retained throughout evolution. Active transepithelial Cl secretion takes place in gall bladders studied as models of the mammalian proximal tubule and in proximal tubules of amphibians and apparently also of mammals. The tubular secretion of Cl is also observed in mammalian distal tubules. The evidence consistent with and for Cl secretion in, respectively, proximal and distal tubules of the mammalian kidney calls for a reexamination of basic assumptions in renal physiology that may lead to new opportunities for managing some forms of renal disease.

Assessment of tissue-level kidney functions with primary cultures

Primary cultures of renal proximal tubule have become important tools for examination of the mechanisms and control of transepithelial transport processes. The utility of the culture preparations for study of integrated tissue functions depends upon their accurate expression of in vivo transport processes. Maintenance of differentiation in culture is enhanced by contractible collagen substratum. Epithelial monolayer primary cultures of flounder and chicken proximal tubule, prepared by enzymatic and mechanical maceration with differential centrifugation, exhibit functional properties at the tissue level that generally resemble known properties of the freshly isolated or in vivo proximal tubule. Transepithelial electrical resistances and potential differences are very similar or identical to those of intact tubules. Na+-dependent glucose transport, a hallmark of proximal tubule function, has the same properties in culture as the tissue in vivo. Similarly, where appropriate comparisons are possible, amino acid, uric acid, and organic anion and cation transepithelial transport processes are qualitatively very similar in culture and in vivo. These two non-mammalian primary proximal tubule culture systems adequately reflect in vivo function, and thus provide opportunities for experimental manipulation otherwise not available.

Taurine secretion in primary monolayer cultures of flounder renal epithelium: stimulation by low osmolality

Transepithelial taurine fluxes determined in short-circuited monolayer cultures of flounder renal proximal cells in Ussing chambers revealed net taurine secretion. Both unidirectional secretory and reabsorptive taurine fluxes exhibited saturation kinetics contributed by two distinct saturable transepithelial taurine transport systems operating at different taurine concentration ranges. The taurine secretory system operating below 0. 5 mM had lower affinity but higher capacity than the reabsorptive system, whereas the one operating at high concentrations (0.5-3.0 mM) had higher affinity but the same capacity as the corresponding reabsorptive system. Exposure (2 h) of the cultures to hyposmotic medium in the presence of taurine increased taurine secretory flux twofold with no effect on the reabsorptive flux. The hyposmolality-induced increase in taurine secretion was associated with a decreased peritubular taurine efflux and a concurrent increased luminal taurine efflux; the latter occurred via a pathway that was not affected by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid but inhibited by probenecid. The culture response in hyposmotic medium mimics the in vivo response of the intact marine fish kidney to dilution.

Immunocytochemical localization of taurine in the developing retina of the lefteye flounder Paralichthys olivaceus

Light microscopic immunolocalization of taurine, a sulfur-containing free amino acid, was investigated in the developing retina of a lefteye flounder, Paralichthys olivaceus, which exhibits metamorphic changes with rod cell addition for 3-5 weeks after hatching. This immunocytochemical study of the developing retina revealed: 1) From 3 to 13 days after hatching, intense immunostaining was shifted from the surroundings of neural cells to the neural somata and processes in the inner retina. 2) Intense immunoreactivity appeared also in the outer and inner segments and basal processes (pedicles) of cone cells within 6 days or 13 days after hatching. 3) Lack of immunoreactivity was found in the outer segment of rod cells from their appearance during metamorphosis. These findings are discussed with the possible functional roles of taurine in the fish retina: 1) involvement in cell differentiation and/or development; 2) protection of the outer segments against light stimuli; and 3) regulation of neural transmission.

Changes of renal taurine transport after treatment with triiodothyronine or dexamethasone in amino acid loaded rats

In adult female anaesthetized rats, the influence of triiodothyronine or dexamethasone on renal amino acid (AA) handling was investigated in taurine (45 mg/100 g b.wt.) loaded animals. Bolus injections of taurine were followed by temporary increase in fractional excretion (FE(AA)) of taurine as well of the endogenous amino acids which were not administered. Under taurine load conditions, triiodothyronine treatment (20 microg/100 g b.wt. for 3 days, i.p. once daily) was followed by a slight stimulation of the renal taurine reabsorption: the increase in FE(taurine) after taurine load was lower than in untreated rats. Dexamethasone (60 microg/100 g b.wt. for 3 days, i.p. once daily) was without significant effect on FE(taurine) in taurine loaded rats. In non taurine loaded rats there was no hormone influence at all. Similarities and differences between the effects of bolus injections of taurine, glutamine, and leucine on the FE(AA) of these three amino acids were compared in detail to further clarify the reason for the increased amino acid reabsorption capacity after pretreatment with triiodothyronine or dexamethasone.

The hepato-renal syndrome: renal amino acid transport in bile duct ligated rats (DL)--influence of treatment with triiodothyronine or dexamethasone on renal amino acid handling in amino acid loaded rats

The influence of triiodothyronine or dexamethasone on renal amino acid handling was investigated in anaesthetized, bile duct-ligated (DL) adult female rats. 3 days after DL, glomerular filtration rate (GFR) was unchanged whereas urine flow was decreased. Plasma concentrations of 5 out of 16 amino acids were significantly enhanced after DL. On the other hand, the fractional excretion (FE) of 11 out of 16 amino acids was significantly reduced as a sign of improved reabsorption capacity. Bolus injections of leucine (20 mg/100 g b.wt.), glutamine (45 mg/100 g b.wt.), or taurine (45 mg/100 g b.wt.) were followed by a temporary increase in the FE of the administered amino acids as well of the endogenous amino acids which were not administered. This phenomenon was more pronounced in DL than in control rats. Under load conditions, dexamethasone (60 microg/100 g b.wt.) or triiodothyronine (20 microg/100 g b.wt.) treatment for 3 days, i.p. once daily, was followed by a stimulation of renal amino acid reabsorption in DL rats. The increase in fractional amino acid excretion after amino acid load was significantly lower than in untreated rats. This effect was also more pronounced in DL rats.

Taurine efflux is a cell volume regulatory process in proximal renal tubules from the teleost Carassius auratus

The potential role of taurine transport associated with volume regulation in renal tissue and isolated proximal renal tubules was studied in the teleost Carassius auratus (goldfish). The cellular taurine content in renal tissue fragments incubated in isosmotic solution (290 mOsm) (7.8 +/- 0.9 (SD) micromol g wet wt(-1)) decreased by 60% following exposure to hyposmotic medium (100 mOsm). The rate coefficient for [14C]taurine efflux in renal tissue and in isolated proximal renal tubules was strongly stimulated following hyposmotically or urea-activated cellular swelling. The stimulated basolateral taurine efflux pathway exhibited channel-like functional characteristics since (a) [14C]taurine influx was stimulated in parallel with the osmolality-dependent taurine efflux and (b) this efflux could not be stimulated by high medium taurine concentrations (40 mM) applied 10 min following the osmolality reduction (trans-stimulation test). Administration of the 5-lipoxygenase inhibitor ETH 615-139 (20 microM) during hyposmotic stimulation inhibited regulatory volume decreases but had no effect on taurine efflux. In addition, hyposmotically induced taurine efflux was slightly but significantly inhibited by the cyclooxygenase inhibitor indomethacin (10 microM). The taurine efflux was also dependent on both extra- and intracellular Ca2+. It is concluded that taurine is likely to coparticipate with KCl as an osmoeffector during RVD in Carassius proximal renal tubule cells. Cellular swelling seems to activate a basolateral taurine transport pathway with functional properties of a channel. This efflux mechanism appears to be partly regulated by Ca2+. Such a transport pathway could play a role in the cell volume regulatory mechanisms participating during transepithelial solute and water transport.

Sodium-taurine cotransport in reptilian renal brush-border membrane vesicles

The coupled transport of Na+ with taurine into snake renal brush-border membrane vesicles (BBMV) was studied using 5-s uptake conditions. Taurine transport into snake renal BBMV involved two parallel processes, one saturable (Na(+)-dependent) and one (Na(+)-independent) that behaved like passive diffusion. Below 1 mM taurine concentration, the Na(+)-dependent system accounted for 60% of total taurine uptake. Over both low (0.001-0.80 mM) and high (0.8-5.0 mM) taurine concentration ranges, the Na(+)-dependent taurine uptake within each range showed Michaelis-Menten kinetics, suggesting the presence of two independent saturable Na(+)-dependent transport systems for taurine. The high-affinity, low-capacity system saturated above 100 microM with a Km of 71.4 +/- 45.7 microM and a maximum velocity (Vmax) of 21.9 +/- 3.77 pmol (mg protein)-1 (5 s)-1. The low-affinity, high-capacity system saturated above 1 mM, with a Km of 1.11 +/- 0.63 mM and a Vmax of 252 +/- 47 pmol (mg protein)-1 (5 s)-1. The stoichiometric relationship between external Na+ concentration and taurine uptake (at 10 microM) by the high-affinity BBMV transport system was examined by the activation method under short-circuited conditions. The 5-s rate of taurine transport was a sigmoid function of increasing extravesicular Na+ concentration. Kinetic analysis of the interaction of Na+ with the high-affinity taurine transport system suggested that 3 Na+ ions (3.2 +/- 0.7) may be involved with 1 taurine molecule in the transport event.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of taurine transport in cultured renal epithelial cell lines: asymmetric polarity of proximal and distal cell lines

Taurine transport was determined in two continuous, renal epithelial cell lines: LLC-PK1 derived from the proximal tubule of the pig, and the Madin-Darby canine kidney cell (MDCK) from the distal tubule of the dog. In LLC-PK1, taurine transport is maximal at the apical surface, whereas in MDCK cells, transport is greatest at the basolateral surface. Transport is highly dependent on both sodium and chloride in the external medium, and is specific for beta-amino acids. The apical and basolateral surfaces of both cell lines show an adaptive response to extracellular taurine concentration, but only the basolateral surface of the MDCK cell responds to hyperosomolality by increased taurine accumulation. Thus, differential control of the beta-amino acid transport system by substrate and external tonicity exists. The role of the beta-amino acid transport system may differ according to the origin of the cell: in the proximal renal tubular cell, net transepithelial reabsorption of filtered taurine increases the body pool. By contrast, taurine accumulation by distal tubular cells may form a mechanism of cell volume regulation in response to osmotic stress.

Taurine behaves as an osmolyte in Madin-Darby canine kidney cells. Protection by polarized, regulated transport of taurine

Using a clonal growth assay, we demonstrated that taurine, a nonperturbing osmolyte accumulated in kidney medulla, brain, and some other tissues of hypertonic experimental animals can function as a nonperturbing osmolyte in Madin-Darby canine kidney (MDCK) cells. The taurine content of hypertonic MDCK cells is twice that of isotonic MDCK cells (isotonic 160 nmol/mg protein; hypertonic 320 nmol/mg protein). Therefore we studied taurine transport in MDCK cells grown on porous supports and then studied the effect of hypertonicity which is known to elicit increased uptake of some other nonperturbing osmolytes by MDCK cells. Basal uptake exceeded apical uptake, with Km and Vmax of 56 microM and 933 pmol/min.mg protein on the basal surface and 10 microM and 50 pmol/min.mg protein on the apical surface. On both surfaces, virtually all taurine uptake was Na+ and Cl- dependent. 24 h after cells were shifted to hypertonic medium (500 mosmol/kg), taurine uptake doubled on the basolateral surface without change on the apical surface. The response to hypertonicity was the result of an increase in Vmax without change in Km. There was no change in taurine efflux when cells were shifted from isotonic to hypertonic medium. When cells adapted to hypertonic medium were shifted to isotonic medium, a large transient basolateral efflux of taurine occurred within 10 min. We conclude that taurine can function as a nonperturbing osmolyte in MDCK cells and that tonicity-regulated taurine transport is a basolateral function in MDCK cells.

Sodium-coupled amino acid transport in renal tubule

Amino acids are reabsorbed from the tubular lumen by a saturable, carrier-mediated, concentrative transport mechanism driven by a Na+ electrochemical gradient across the luminal membrane. This process is followed by efflux mainly via carrier-mediated, Na+-independent facilitated diffusion across the basolateral membrane. Individual amino acids may have two or more Na+-dependent transport systems with different kinetic characteristics along the luminal membrane of the proximal tubule, thereby enabling very efficient amino acid reabsorption. Dual Na+-coupled transport pathways for some amino acids located in both the luminal and the peritubular membranes may operate in concert to provide the tubular epithelial cell with essential nutrients. One or more Na+ ions, H+, Cl- and in the case of acidic amino acids, K+ ion, may be involved in the translocation of the carrier complex. For most amino acids this process is electrogenic positive, favored by a negative cell interior. At least seven distinct, but largely interacting, Na+-dependent amino acid transport systems have been identified in the brush border membrane. A diet-induced adaptation in Na+-coupled taurine transport and acidosis-induced adaptive response in Na+-dependent glutamine transport are expressed at the luminal and the basolateral membrane surfaces, respectively. The aminoaciduria of early life may be related to a rapid dissipation of the Na+ electrochemical gradient necessary for amino acid reabsorption.

Na- and Cl-dependent glycine transport in human red blood cells and ghosts. A study of the binding of substrates to the outward-facing carrier

Na- and Cl-dependent glycine transport was investigated in human red blood cells. The effects of the carrier substrates (Na, Cl, and glycine) on the glycine transport kinetics were studied with the goal of learning more about the mechanism of transport. The K1/2-gly was 100 microM and the Vmax-gly was 109 mumol/kg Hb.h. When cis Na was lowered (50 mM) the K1/2-gly increased and the Vmax-gly decreased, which was consistent with a preferred order of rapid equilibrium loading of glycine before Na. Na-dependent glycine influx as a function of Na concentration was sigmoidal, and direct measurement of glycine and Na uptake indicated a stoichiometry of 2 Na:1 glycine transported. The sigmoidal response of glycine influx to Na concentration was best fit by a model with ordered binding of Na, the first Na with a high K1/2 (greater than 250 mM), and the second Na with a low K1/2 (less than 10.3 mM). In the presence of low Cl (cis and trans 5 mM), the K1/2-gly increased and the Vmax-gly increased. The Cl dependence displayed Michaelis-Menten kinetics with a K1/2-Cl of 9.5 mM. At low Cl (5 mM Cl balanced with NO3), the glycine influx as a function of Na showed the same stoichiometry and Vmax-Na but a decreased affinity of the carrier for Na. These data suggested that Cl binds to the carrier before Na. Experiments comparing influx and efflux rates of transport using red blood cell ghosts indicated a functional asymmetry of the transporter. Under the same gradient conditions, Na- and Cl-dependent glycine transport functioned in both directions across the membrane but rates of efflux were 50% greater than rates of influx. In addition, the presence of trans substrates modified influx and efflux differently. Trans glycine largely inhibited glycine efflux in the absence or presence of trans Na; trans Na largely inhibited glycine influx and this inhibition was partially reversed when trans glycine was also present. A model for the binding of these substrates to the outward-facing carrier is presented.

Taurine and cell volume maintenance in the shark rectal gland: cellular fluxes and kinetics

Tissue slices of shark rectal gland are studied to examine the kinetics of the cellular fluxes of taurine, a major intracellular osmolyte in this organ. Maintenance of high steady-state cell taurine (50 mM) is achieved by a ouabain-sensitive active Na+-dependent uptake process and a relatively slow efflux. Uptake kinetics are described by two saturable taurine transport components (high-affinity, Km 60 microM; and low-affinity, Km 9 mM). [14C]Taurine uptake is enhanced by external Cl-, inhibited by beta-alanine and unaffected by inhibitors of the Na+/K+/2Cl- co-transport system. Two cellular efflux components of taurine are documented. Incubation of slices in p-chloromercuribenzene sulfonate (1 mM) reduces taurine uptake, increases efflux of taurine and induces cell swelling. Studies of efflux in isotonic media with various cation and anion substitutions demonstrate that high-K+ markedly enhances taurine efflux irrespective of cell volume changes (i.e. membrane stretching is not involved). Moreover, iso-osmotic cell swelling induced in media containing propionate is not associated with enhanced efflux of taurine from the cells. It is suggested that external K+ exerts a specific effect on the cytoplasmic membrane to increase its permeability to taurine.

Effects of dietary taurine on tissue taurine and free amino acid levels of the chum salmon, Oncorhynchus keta, reared in freshwater and seawater environments

1. Young chum salmon were fed on the basal and taurine-supplemented diets for 30 days in freshwater (FW) and for 25 days in seawater (SW). Levels of taurine, major free amino acids (FAA) and non-protein nitrogen (NPN) in various tissues were determined. 2. Tissue taurine levels were higher when fish were fed on the taurine-supplemented diets. All tissues of the SW fish did not contain higher taurine levels than those of the FW. 3. Levels of major FAA in the tissues differed little between fish fed on the basal and taurine-supplemented diets and also between the FW and SW fish. 4. No difference was observed in tissue NPN levels between fish ingesting the basal and taurine-supplemented diets; the levels were slightly higher in the SW fish.

Adaptation of renal function to hypotonic medium in the winter flounder (Pseudopleuronectes americanus)

The kidneys of winter flounders transferred to hypotonic medium were investigated for glomerular and tubular handling of fluid and electrolytes and for the urinary excretion of proteins. Media were sea water (925 mosm X kg-1) and brackish water (70 mosm X kg-1). In sea water, the urine was hypertonic to the plasma in 7 fish of this study. Urine flow rate was correlated with the GFR. After adaptation to brackish water a delay of 1 to 3 days was observed until the kidneys switched from fluid retention to the excretion of large amounts of dilute urine. GFR and urine flow rate were increased from 0.61 +/- 0.08 to 1.58 +/- 0.29 ml X h-1 X kg-1 and from 0.14 +/- 0.02 to 0.68 +/- 0.08 ml X h-1 X kg-1, respectively (mean +/- SEM). With increased filtered load the tubular reabsorption of fluid decreased from 74 +/- 2.4% to 45 +/- 11.2%. The excretion rates of sodium and potassium were increased due to decreased fractional sodium and potassium reabsorption. The urinary excretion of divalent cations, however, was reduced because the net tubular reabsorption of calcium was increased and the net secretion of magnesium was diminished. Both the urinary total protein concentration and the protein pattern showed no significant change, but the rate of protein excretion was increased from 0.21 +/- 0.04 to 0.60 +/- 0.05 mg X h-1 X kg-1. The comparison of protein patterns obtained from urine and serum samples revealed that high molecular weight (HMW) proteins prevail in the serum whereas low molecular weight (LMW) proteins dominate in the urine. The diminished quantity of the HMW-protein fraction in the urine thus may reflect size selectivity of the glomerular filtration barrier for serum proteins also in the winter flounder.

Renal handling of taurine, L-alanine, L-glutamate and D-glucose in Opsanus tau: studies on isolated brush border membrane vesicles

Renal brush border membrane vesicles (bbmv) from the aglomerular toadfish (Opsanus tau), isolated by differential precipitation, were tested for their ability to actively translocate (i) taurine, known to be secreted by the kidney of several marine teleosts, and (ii) L-alanine, L-glutamic acid, and D-glucose, solutes that are normally reabsorbed in the filtering nephron. Vesicular taurine uptake displayed a Na+ dependence. Transport was greatest under conditions of an inward-directed Na+ gradient, but a significant stimulation by Na+ over K+ could also be observed in the absence of a salt gradient. At high extravesicular K+, the addition of valinomycin reduced taurine uptake. Na+-dependent 3H-taurine flux was almost completely inhibited by non-labeled taurine (tracer replacement) or beta-alanine, but was unaffected by L-alanine. Replacement of medium chloride by SCN- or NO3- in the presence of Na+ resulted in significantly lower uptake rates under both anion gradient and anion equilibrium conditions, whereas Br- could almost fully substitute for the stimulatory Cl- action. These results indicate the presence of an electrogenic Na+-cotransport mechanism with specificity for beta-amino acids in the toadfish renal brush border. Whether the system under physiological conditions mediates reabsorption or secretion of taurine remains to be determined. Toadfish bbmv also translocated L-alanine and L-glutamic acid in a Na+-dependent manner. Possible roles for these most likely reabsorptive transport systems in a non-filtering kidney are discussed. D-glucose uptake, however, appeared to occur via Na+-independent pathways, since it was not affected by phlorizin in the presence of Na+, or by Na+ replacement.

Taurine transport by the flounder (Pseudopleuronectes americanus) intestine

Taurine transport by the flounder gut was characterized in isolated strips of intestine mounted between Ringer's solutions. Taurine was transported into the cell, against its concentration gradient, by a sodium-dependent system present in both the mucosal and serosal membranes. This system appears to be specific for beta-amino acids and is regulated by cyclic nucleotides (cGMP and cAMP). Kinetic analyses indicated that under physiological conditions the magnitudes of the bidirectional taurine uptake rates would favor net absorption of taurine from lumen to blood.

Factors affecting the transport of beta-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the beta-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na+ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN- and NO3-) or less permeant (SO4(2-)), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl- stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl- in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other beta-amino acids and in a competitive fashion. D-Glucose and p-aminohippurate at high concentrations (greater than 10(-3) M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of D-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal beta-amino acid transport system in brush-border vesicles and indicate a role for external Cl- in this uptake system.