Mathematical model of an avian urine concentrating mechanism.

A mathematical model was used to investigate how concentrated urine is produced within the medullary cones of the quail kidney. Model simulations were consistent with a concentrating mechanism based on single-solute countercurrent multiplication and on NaCl cycling from ascending to descending limbs of loops of Henle. The model predicted a urine-to-plasma (U/P) osmolality ratio of approximately 2.26, a value consistent with maximum avian U/P osmolality ratios. Active NaCl transport from descending limb prebend thick segments contributed 70% of concentrating capability. NaCl entry and water extraction provided 80 and 20%, respectively, of the concentrating effect in descending limb flow. Parameter studies indicated that urine osmolality is sensitive to the rate of fluid entry into descending limbs and collecting ducts at the cone base. Parameter studies also indicated that the energetic cost of concentrating urine is sensitive to loop of Henle population as a function of medullary depth: as the fraction of loops reaching the cone tip increased above anatomic values, urine osmolality increased only marginally, and, ultimately, urine osmolality decreased.

A leaf lipoxygenase of potato induced specifically by pathogen infection.

Lipoxygenase (LOX) activity has been identified consistently during pathogen-induced defense responses. Here we report the involvement of a specific leaf LOX gene of potato (Solanum tuberosum), designated POTLX-3 (GenBank/EMBL accession no. U60202), in defense responses against pathogens. The sequence of POTLX-3 does not match any other LOX genes of potato and has the greatest match to a tobacco LOX gene that contributes to a resistance mechanism against Phytophthora parasitica var nicotianae. POTLX-3 transcript accumulation was not detected in untreated, healthy potato organs or in wounded mature leaves. POTLX-3 mRNA accumulation was induced in potato leaves treated with ethylene or methyl jasmonate or infected with either virulent or avirulent strains of Phytophthora infestans, the causal agent of late blight. During the resistance response, POTLX-3 was induced within 6 hours, increased steadily through 24 hours, and its mRNA continued to accumulate for a week after inoculation. In contrast, when a plant was susceptible to P. infestans, induction of mRNA accumulation in response to inoculation was inconsistent and delayed. LOX activity assayed during an incompatible interaction in leaves peaked 3 days earlier than during a compatible interaction. POTLX-3 mRNA accumulation also was induced during hypersensitive response development caused by the incompatible pathogen Pseudomonas syringae pv phaseolicola. Our results show that POTLX-3 may be involved specifically in defense responses against pathogen infection.

Functional morphology of the avian medullary cone.

The organization of the renal medulla of the Gambel's quail, Callipepla gambelii, kidney was examined to determine the number of loops of Henle and collecting ducts and the surface area occupied by the different nephron segments as a function of distance down the medullary cones. Eleven medullary cones were dissected from the kidneys of four birds, and the tissue was processed and sectioned for light microscopy. In addition, individual nephrons were isolated on which total loop thin descending segment and thick prebend segment lengths were measured. The results show no correlation between the absolute number of loops of Henle and the length of the medullary cones. The number of thick and thin limbs of Henle and collecting ducts decrease exponentially with distance toward the apex of the cones and the rate of decrease is similar for cones of different lengths. Initially there is a rapid decrease in the number of thin limbs of Henle, indicating that most nephrons do not penetrate the cones a great distance. Thick descending limbs of Henle (prebend segment) ranged in length from 50 to 770 microm, and there was little correlation with the total length of the loop of Henle. However, the length of the thin limb of Henle correlated well with total loop length. The cell surface areas of the limbs of the loop of Henle and the collecting ducts decreased toward the apex of the cones.

Renal anatomy in sparrows from different environments.

The renal anatomy of three species of sparrows, two from mesic areas, the House Sparrow (Passer domesticus) and Song Sparrow (Melospiza melodia), and one salt marsh species, the Savannah Sparrow (Passerculus sandwichensis) was examined. Electron microscopy was used to describe the ultrastructure of the nephron. In addition, stereology was used to quantify the volumes of cortex, medulla, and major vasculature of the kidneys, and the volumes and surface areas occupied by individual nephron components. There appeared to be no differences in the ultrastructural anatomy of the nephrons among the sparrows. Proximal tubules contained both narrow and wide intercellular spaces filled with interdigitations of the basolateral membrane. The thin limbs of Henle contained very wide intercellular spaces which were absent in the thick limbs of Henle. The distal tubule cells contained short, apical microvilli and infoldings of the basolateral membrane. In cross section, the medullary cones of all birds display an outer ring of thick limbs of Henle which surround an inner ring of collecting ducts, which in turn surround a central core of thin limbs of Henle. The Savannah Sparrow has a significantly higher volume of medulla compared to the two more mesic species. Within the cortex, the Savannah Sparrow also has a significantly higher volume of proximal tubules but a significantly lower volume of distal tubules than the other species. Within the medulla, the Savannah Sparrow has a significantly higher volume and surface area of capillaries, and a significantly higher surface area of thick limbs of Henle and collecting ducts than the mesic species. These data suggest that the salt marsh Savannah Sparrow has the renal morphology necessary to produce a more highly concentrated urine than the mesic zone species.

Integration of renal and gastrointestinal function.

In birds, the kidney does not independently regulate the composition of the extracellular fluid, as is the case for mammals. The urine of birds enters the cloaca and is moved by a reverse peristaltic action into the colon. In the colon, the urine comes in contact with an epithelium that modifies its composition. If the osmotic potential of the urine is significantly higher than that of the plasma, the urine will not be refluxed into the colon. The composition of the ureteral urine is sensed in the cloaca which in turn modulates the refluxing activity. It appears to be the large contraction waves of the colon that are modified by changes in the composition of the urine, although radiographic evidence indicates that the small contraction waves of the colon do the actual refluxing. It is necessary for the urine to be moved into the colon. This may be driven by the need to recover some, if not all, of the protein in the urine. This protein maintains uric acid in a colloidal suspension that prevents the formation of uric acid crystals which would block the renal tubules. Thus, the kidneys and lower gastrointestinal tract must function in concert in the regulation of the composition of the extracellular fluid.

Response to 45 degrees head-down tilt as measured by organ weight/body weight ratios and spiral computed tomography.

BACKGROUND: Exposure to microgravity or simulated microgravity causes significant shifts in body fluids which may initiate physiological adaptations to the microgravity stressor. It is imperative to understand the physiological adaptations to microgravity in order to develop appropriate countermeasures to the deleterious aspects (i.e., muscle and bone wasting) of long-term spaceflights. HYPOTHESIS: The significant shifts in body fluids by 45 degrees head-down tilt can be measured by changes in organ weight/body weight (OW/BW) ratios and non-invasively by spiral computed tomography. METHODS: In a previous study (14), rats were weighed and exposed to either 45 degrees head-down tilt (45HDT) or a prone control position for one of the following experimental times: 0.5 h, 1 h, 2 h, 4 h, 8 h, or 24 h. A radioactive tracer was injected intramuscularly immediately prior to the start of the experimental time periods. At the end of the experiment, the major organs were harvested, weighed, and measured for gamma radiation levels. We used the organ weights from this previous study to calculate OW/BW ratios for the present study. Additionally, in the present study, rats in the 14-d experimental groups were weighed, lightly anesthetized to facilitate placement in the 45HDT position, and placed in a specially designed 45HDT cage (45HDT group) or left unrestrained in the cages (control group). At the end of the 14-d experimental time period, the rats were anesthetized and their lung densities measured with spiral computed tomography. RESULTS: The OW/BW ratios for the liver, kidneys, and spleen of 24 h 45HDT rats were significantly lower (p<0.05) than control values while at 1 h the 45HDT rats had a higher kidney OW/BW ratio. Lung density from the 14-d 45HDT rats was 24.4% greater than control rats' values. CONCLUSIONS: The physiological change due to the 45HDT position to simulate microgravity begins as early as 1 h, and the kidney appears to be the first organ affected. Spiral computed tomography may offer a viable method of non-invasively measuring organ densities in the 45HDT model. The OW/BW data generated in the present study does not correlate with the changes in radioactive tracer distribution data from our previous study.

Substrate specificity of organic cation/H+ exchange in avian renal brush-border membranes.

The substrate specificity of the avian renal organic cation exchanger was examined in isolated renal brush-border membrane vesicles. Endobiotic and xenobiotic organic cations (OCs) were tested at a concentration of 100 microM for cis-inhibition of 14C-tetraethylammonium (TEA)/H+ exchange and at 1 mM for trans-stimulation of 14C-TEA efflux. The xenobiotic cations amiloride, cimetidine, mepiperphenidol, procainamide, quinidine, quinine, and ranitidine cis-inhibited TEA uptake >/= 80%; isoproterenol and unlabeled TEA inhibited uptake at least 30%. In contrast, the endogenous cations acetylcholine, choline, and guanidine did not inhibit TEA uptake; however, epinephrine, N1-methylnicotinamide, serotonin, and thiamine inhibited uptake as much as 60%. Each endogenous cation, except thiamine, trans-stimulated TEA efflux, and xenobiotic cations, excluding isoproterenol and TEA, trans-inhibited TEA efflux. The data suggest that the avian renal tubule luminal OC exchanger has greater affinity for xenobiotic cations than for endobiotic cations, but greater transport capacity for endobiotics than for xenobiotics.

Identification, characterization, and mapping of Ecm1, a locus affecting extracellular matrix production and lesion size in Cochliobolus heterostrophus.

A mutant of Cochliobolus heterostrophus lacking the outer layer of extracellular matrix around its germ tubes and hyphae was obtained by mutagenizing protoplasts. The mutant not only lacks the outer matrix, but also produces much smaller lesions on corn leaves than nonmutant strains; the area of mutant lesions averages 0.6 mm2 compared with 5.8 mm2 for nonmutant lesions. Genetic analysis demonstrated that the failure to produce the outer matrix cosegregates with the reduced lesion size, indicating that the two traits are controlled by the same locus, designated Ecm1 (Extracellular Matrix Deficient). The mutant retains normal growth on media and normal abilities to germinate, form appressoria, and penetrate corn leaves. This indicates that the outer matrix is not necessary for infection prior to entrance of the fungus into the leaf. It also indicates that the pathogenicity defect in this mutant is manifested after penetration. To facilitate future tests of whether the pathogenicity defect is caused by the lack of the outer matrix, Ecm1 was mapped. Seven markers linked to Ecm1 were found by analysis of amplified fragment length polymorphisms. Ecm1 maps to chromosome 4; the closest markers to Ecm1 are 5 cM distant, which is estimated to represent about 115 kb.

Ingress of Clavibacter michiganensis subsp. michiganensis into Tomato Leaves Through Hydathodes.

ABSTRACT Hydathodes of tomato leaves served as extremely efficient infection courts for the bacterial canker pathogen, Clavibacter michiganensis subsp. michiganensis. Chlorotic lesions developed at the tips of leaflet lobes about 2 weeks after inoculation of guttation droplets. Lesions expanded along the leaflet margins and became necrotic. Movement of C. michiganensis subsp. michiganensis from the inoculated leaflet into the rachis was slow and erratic. Histological observations revealed that pathogen populations first developed within large intercellular spaces lying beneath the stomata, which serve as water pores in tomato hydathodes. Bacteria were first observed within vessels of the large marginal fimbriate veins 7 days after inoculation. By 14 days after inoculation, large populations could be seen within the vessels; and by 21 days after inoculation, tissue collapse was widespread and masses of bacteria could be seen in the intercellular spaces and within necrotic cells.

Gastric Na+K+ATPase activity and intestinal urea hydrolysis of the common vampire bat, Desmodus rotundus.

The blood diet of the vampire bat represents an extraordinarily high ratio of protein to other nutrients and the highest water consumption per body weight of any other mammal. This bat has a unique gastrointestinal morphology that is characterized by a reduced small intestine, absence of a large intestine and intestinal cecum and the presence of a water-absorptive gastric fundus. The present study demonstrates that the gastric fundus has a greater Na+K+ATPase activity for active ion transport compared with other equally sized mammals. This activity is believed to be necessary to establish a gradient favoring water absorption across what would otherwise be an osmotic disequilibrium. The absence of a large intestine and intestinal cecum may reflect a reduced urea hydrolysis by the vampire bat. The present study demonstrated that the vampire bat does not hydrolyze urea as does an equally sized non-sanguinivorous mammal. These data suggest that the blood diet and the relocation of water-absorptive tissue from the lower intestinal tract to the stomach is associated with an active ion transport mechanism in the gastric tissue and a reduced capacity for ureolytic microbes to hydrolyze urea in the intestine. Both processes are specializations for a diet high in protein and water.

Changes in radioactive tracer distribution in rats after 24 hours of 45 degrees hind limb unweighting.

INTRODUCTION: Changes in radioactive tracer distribution were examined in rats after exposure to a simulated microgravity model of 45 degrees head down tilt (45HDT) or 45 degrees hind limb unweighting (45HU) for up to 24 h. METHODS: Rats were randomly assigned to either 45HDT (or 45HU) experimental groups or control groups for each time point of 0.5 h, 1 h, 2 h, 4 h, 8 h, or 24 h. The 0.5-h through 8-h experimental rats were anesthetized and placed head-down on a ramp at 45 degrees, while control rats were placed in a prone position. Non-anesthetized rats in the 24-h experimental group were tail-suspended at 45 degrees, while control rats were allowed unrestrained movement. Technetium-labeled diethylenetriamine pentaacetate (99mTcDTPA, physical half-life of 6.02 h, MW = 492 amu) and indium-labeled diethylenetriamine pentaacetate (111In DTPA, physical half-life of 3.5 d, MW = 545 amu) were used to measure body organ distributions of the radioactive tracers at the 0.5-h-8-h and 24-h time points, respectively. Major organs were harvested after each time period and measured for radioactive counts. Light and electron micrographs were examined. RESULTS: Mean 111InDTPA counts for the lungs, kidneys, and brains of the 24 h 45HU groups were significantly higher than control counts. Light and electron microscopy demonstrated the development of pulmonary edema in the alveolar septal areas after 2 h of 45HDT, and a shift in edema to the pulmonary airways and pulmonary arteries after 24 h of 45HU. CONCLUSIONS: Pulmonary edema development, accompanied by a significant increase in 111InDTPA lung, kidney, and brain counts in the 24-h 45HU groups, suggests vascular injury in the microcirculation of these organs.

Urinary protein excretion in red jungle fowl (Gallus gallus).

Urinary protein excretion by red jungle fowl (Gallus gallus) was examined by measuring total protein concentrations in the ureteral urine and by comparing the proteins in the urine with plasma proteins. Protein concentration in the ureteral urine did not differ between males and females, and averaged 2.01 mg/ml. Gel electrophoresis showed many plasma proteins (30-149 kD) also present in the urine. Serum albumin is the most abundant protein, comprising approximately 50% of the total protein concentration in the plasma and 60% of the total protein concentration in the urine. Urinary protein, and particularly serum albumin, may be important in packaging uric acid in spheres, which facilitates excretion of uric acid without formation of large crystals that could block renal tubules.

Functional morphology of the glomerular filtration barrier of Gallus gallus.

The anionic charge barrier and the endothelial and epithelial pore sizes on the glomerular filtration barrier (GFB) were examined in white leghorn chickens (Gallus gallus). Ruthenium red was used to stain anionic charge sites on the GFB. The tissue was treated by normal dehydration and freeze substitution dehydration for transmission electron microscopy (TEM). In addition, the basal lamina was isolated for study. The results of our study indicate that G. gallus possess a thick, negatively charged glycocalyx surrounding the podocytes and slit diaphragm and on the endothelium. However, in all cases, little anionic charge is present in the basal lamina. The pores on the endothelium are elliptical and have mean dimensions of 148 x 110 nm. This is in contrast to mammals, which have smaller, round pores. The epithelial pores in G. gallus measure approximately 35 nm in length, approximately 4 times larger than those found in mammals. These results indicate that the avian glomerulus may allow the filtration of larger molecules from the plasma than occurs in mammals and that the charge on the molecule may not be as restrictive a filtration characteristic as in mammals.

Renal compensation for cecal loss in Gambel's quail (Callipepla gambelii).

Some studies have implicated the avian digestive cecae as important sites of water and solute reclamation working in concert with the lower intestine and the kidneys as part of an integrated osmoregulatory system. In Gambel's quail (Callipepla gambelii), we studied compensatory adjustments in renal function on days 6-7 and 16-17 following ligation of cecae. Plasma osmolality (Posm) varied significantly between groups with sham-operated birds (Cs), with an average (Posm) of 348 mOsm/kg H2O and quail with ligated cecae (Cx) having a (Posm) of 355 mOsm/kg H2O. We detected no change in the rate of glomerular filtration (GFR) between experimental and control groups either shortly after cecectomy or after 16-17 d following surgery. Regression analysis of GFR and urine flow rate (V) showed that Cx birds had a significantly lower V at a given GFR than did controls, evidence that Cx quail absorbed more fluid in their renal tubules. Increased fluid reabsorption was apparently driven by an enhanced reabsorption of sodium. Indeed, sodium excretion was lower in Cx quail as compared to sham-operated birds. On days 6-7, Cx quail drank more water than Cs birds, but by days 16-17 drinking rates were similar. At the end of the experiments, Cx quail showed a proliferation of microvilli along the apical membrane of the rectum, an adjustment consistent with the idea that the rectum alters its absorption capacity to adjust for the loss of cecal function.

Characterization of organic cation transport by avian renal brush-border membrane vesicles.

Organic cations are actively secreted by the renal proximal tubule. Studies on perfused tubules and isolated membranes from mammals and reptiles have demonstrated that organic cations (OC) are transported across the luminal (brush-border) membrane by OC/H+ exchange. Our objective was to determine whether a similar mechanism was present in the avian kidney. Uptake of [14C]tetraethylammonium (TEA) was assayed under various ionic conditions by rapid filtration in brush-border membrane vesicles (BBMV) isolated from chicken kidney (Gallus domesticus). An outwardly directed proton gradient (pHin = 6.0: pHout = 7.5) stimulated concentrative TEA uptake. TEA/H+ exchange was saturable, having a maximal rate of uptake of approximately 25 nmol.mg protein-1.min-1 and a Michaelis constant for TEA of approximately 500 microM. TEA transport could be indirectly coupled to sodium transport. Unlabeled TEA, N'-methylnicotinamide (NMN), choline, cimetidine, mepiperphenidol, quinidine, quinine, and ranitidine markedly cis-inhibited uptake of [14C]TEA. However, the organic anions probenecid and p-aminohippurate poorly inhibited uptake. Unlabeled TEA and NMN also trans-stimulated [14C]TEA uptake. Thus, in avian renal BBMV, organic cations are transported by an OC/H+ exchange mechanism qualitatively similar to that present in mammals.

Renal response of roosters with diabetes insipidus to infusions of arginine vasotocin.

In a strain of white Leghorn chicken with symptoms of hereditary diabetes insipidus (DI) and with elevated circulating levels of the antidiuretic hormone arginine vasotocin (AVT), we investigated whether the defect is based on nonfunctional endogenous AVT or on a diminished renal response to AVT. DI chickens responded to hyposmotic infusions with a higher urine flow and lower glomerular filtration rate (GFR) than healthy control birds. Urine osmolality, fractional excretion, and clearance of sodium were lower in DI birds while potassium values were similar in both groups. Hence, the high urine flow rates of the DI birds were predominantly based on a water diuresis. When infused with AVT, the GFR and the urine flow of the control birds decreased and urine osmolality increased, showing both glomerular and tubular effects of AVT. During AVT infusion, the GFR of DI birds increased, urine flow decreased only moderately, and urine osmolality was half that of the control birds. In control birds, the clearance of sodium was unchanged, whereas that of potassium decreased. In DI birds, which experienced a strong natriuresis, the clearance of both sodium and potassium increased. These data suggest that the sensitivity of the DI birds to AVT is reduced at the tubular level. The AVT-induced increase in GFR may be related to vascular effects of high AVT doses, added to the already high basal AVT level of the DI birds, in combination with an imbalance in the function of V1 and V2 receptor subtypes in these chickens.

Structure of the glomerular capillaries of the domestic chicken and desert quail.

The glomerular capillary architecture of nephrons that include a loop of Henle (looped) and those that lack the loop (loopless) nephrons was examined qualitatively and quantitatively by electron microscopy in Gallus gallus and Callipepla gambelii. The glomerular capillaries of looped nephrons form a dichotomously branched network, while those of loopless nephrons are arranged loosely, and the majority are unbranched. There was no significant difference in the diameter of the glomerular capillaries between looped and loopless nephrons; however, in all cases the diameter of the afferent arteriole was significantly larger than that of the efferent arteriole. Based on size alone, the predicted blood flow rate in the efferent arteriole is 20% that of the afferent arteriole in G.gallus and 7% that of the afferent arteriole in C.gambelii. There was no significant difference in the volume density (Vv) of the glomerular capillaries between looped and loopless nephrons. However, the surface area density (Sv) of the glomerular capillaries in loopless nephrons of C.gambelii was significantly larger than for the looped nephrons, and for the loopless nephrons in G. gallus. This suggests that there may be a decrease in blood flow rate along the glomerular capillaries of the loopless nephrons in C. gambelii. Overall, the results indicate that the avian glomerular capillaries are less complex than those of mammals.

Effect of salt and water balance on colonic motility of white leghorn roosters.

The motility of the lower gastrointestinal (GI) tract of white leghorn roosters was observed by radiographic methods. Intracolonic pressure changes were recorded, and the mechanical activity of the colon was quantified using strain gauges that were attached to the serosa. Two types of retrograde contraction waves were observed: small fast peristaltic contractions (15 min-1) and large slow contractions. The small fast contractions are the actual vehicle of the retrograde movement, but they seemed to be linked to respiratory movements in the fashion of coupled oscillators during most of their activity time (94%). This makes them an unlikely target for regulation. The large slow waves (3 min-1) probably reflect contractions of the longitudinal musculature. They are retrograde and may permit or modulate the refluxing of urine. The effects of water deprivation, intravenous volume, and salt loads on colonic motility were analyzed. All manipulations that increased urine flow rates without changing urine osmolality resulted in increased propagation speed of the large contraction waves. All manipulations that increased urine osmolality resulted in decreases in activity time of the large-wave pattern. These results suggest a hydration state-related control of the retrograde colonic motility in birds. Direct stimulation of central osmoreceptors had no such effects, indicating that local volume and osmolality receptors of the cloaca are more important than central receptors for this adjustment of colonic motility.

Kidney structure and function of obligate and facultative hibernators: the white-tailed prairie dog (Cynomys leucurus) and the black-tailed prairie dog (Cynomys ludovicianus).

The white-tailed prairie dog is an obligate hibernator that enters a heterothermic phase when maintained in the cold with low intensity light and ad libitum food and water. The black-tailed prairie dog (a facultative hibernator) will not hibernate under similar conditions. It has been suggested that the black tailed prairie dog remains active during the winter because it can conserve water more effectively due to a more efficient kidney. The present study revealed no significant differences between the species in renal morphology: relative medullary thickness, nephron heterogeneity, renal vasculature, or fornix dimensions, all of which are structures associated with the urinary concentrating mechanism. In addition, there was no difference in number of nephrons between the two species. The black-tailed prairie dog does produce a more concentrated urine when food and water deprived. However, this difference was not observed when the animals were salt loaded. The water-deprivation and salt-loading experiments suggest that the higher urine osmolality produced by the back-tailed prairie dog during fasting is a result of a higher urea load due to a greater protein catabolism and not because of a differential capacity to concentrate urine.

Transport of PAH, urate, TEA, and fluid by isolated perfused and nonperfused avian renal proximal tubules.

Transport of organic anions [p-aminohippurate (PAH) and urate] and organic cations [tetraethylammonium (TEA)] and reabsorption of fluid were studied for the first time in individual renal proximal tubules isolated from avian kidneys. In isolated nonperfused tubules, PAH and urate uptake occurred against electrochemical gradients, whereas TEA uptake appeared to result from the electrical gradient. Radiolabeled PAH uptake and radiolabeled urate uptake were inhibited to an equal extent by high concentrations of unlabeled PAH and probenecid, suggesting that they might share the same transport system. However, the rate of uptake of radiolabeled PAH was significantly stimulated by preloading with alpha-ketoglutarate (alpha-KG), suggesting PAH/alpha-KG countertransport as in mammals and reptiles, whereas uptake of radiolabeled urate was not clearly stimulated. In isolated perfused tubules, net fluid reabsorption averaged approximately 2 nl.min-1.mm-1 and was inhibited by ouabain with or without bicarbonate in the perfusate and bathing medium. In these perfused tubules, the unidirectional bath-to-lumen fluxes of PAH and urate exceeded the unidirectional lumen-to-bath fluxes, indicating net secretion of both compounds. During the bath-to-lumen fluxes the uptake across the basolateral membrane was against an electrochemical gradient for both compounds. However, for PAH the steady-state intracellular concentration was about half that observed in nonperfused tubules, as generally expected during net secretion, whereas for urate the steady-state intracellular concentration was about twice that observed in nonperfused tubules, suggesting stimulation of uptake during net secretion. During the PAH lumen-to-bath flux, the steady-state intracellular concentration was significantly above that in the perfusate, suggesting that this flux involved transport into the cells from the lumen against an electrochemical gradient. However, during the urate lumen-to-bath flux, there was no urate in the cells, suggesting that this flux, as in reptiles, occurred by a paracellular route.