Lanthanum permeability of tight junctions along the collecting duct of the rat

Loss of Residual Renal Function in Patients on Regular Haemodialysis

The literature offers scant data on loss of residual renal function in chronic haemodialysis patients. The present study was undertaken in 34 patients, to evaluate residual creatinine clearances (CCr) before the start of haemodialysis and after 3, 12 and 24 months. CCr progressively declined from 6.15± 2.61 (before) to 1.40± 1.29 ml.min–1 (after 24 months: p<0.01). The decrease was largest during the first three months of dialysis therapy (slope - 0.99± 1.01 ml.min–1. month-–1 for the first three months vs. – 0.23± 0.12 ml.min–1. month–1 for the entire 24-month period: p<0.01). The decline in Ccr during the first three months was significantly more pronounced in glomerular disease than in tubulo-interstitial disease (p<0.05). This could not be attributed to differences in blood pressure, body weight or hypotensive medications. Age and sex also had no influence. Our data indicate that there is a characteristic progressive loss of renal function in haemodialyzed patients and that the early decline is most pronounced in patients with glomerular disease. Regular assessment of residual renal function at least every three months is indicated in patients starting chronic haemodialysis treatment.

Comparison of two Methods for the Estimation of Urea Kinetics and Introduction of a Third Simplified Method

It has been claimed that computed urea kinetic (UK) modelling in hemodialysed patients, for the estimation of protein intake, leads to an overestimation of protein catabolic rate (PCR). In the present study, three different methods of kinetic modelling for the determination of PCR and Kt/V are compared in 24 patients. The first method was the direct quantification method (DDQ) based on the collection of all urea eliminated from the body. The first computed method (ICMI) was the urea kinetic modelling method as described by Sargent. Dialyzer clearances were measured directly and not estimated by theoretical extrapolation. The second computed method (ICMII) is based on the indirect calculation of urea distribution volume (Vu), according to Watson, and of dialyzer clearances from this Vu and from pre- and post-dialysis urea concentrations. All three methods resulted in PCR's that were not significantly different (DDQ: 1.03 ± 0.19; ICMI: 1.04 ± 0.22; ICMII: 1.08 ± 0.25 mg/Kg BW.24 hrs; p > 0.05). When the results were correlated, the following results were obtained: ICMI vs ICMII: r = 0.89, p < 0.001; ICMI vs DDQ: r = 0.68, p < 0.01; DDQ vs ICMII: r = 0.78, p < 0.001. Intermutual comparison of Kt/V values resulted in virtually identical results, especially when comparing ICMI and ICMII, where the regression line equalled the identity line.

In conclusion, all methods seem equally reliable in determining mean PCR and Kt/V. Our data, obtained with directly measured dialyzer urea clearances, do not confirm the earlier held opinion that computed modelling results in an overstimation of PCR. The newly developed third method (ICMII) appears to be as equally reliable as the two other methods, but is easier to perform and less labour intensive.

Clinical application of computer-aided diagnostic system for harmonious introduction of complementary dialysis therapy

In chronic peritoneal dialysis (PD) therapy, peritoneal permeability is gradually enhanced over the duration of the therapeutic course, leading to a grave decline in the therapeutic efficiency. In recent years, a novel therapy (CD therapy), which integrates PD therapy with hemodialysis therapy, is being applied to end-stage PD patients to complement the decline of therapeutic efficiency caused by the grave degeneration of the peritoneal tissue. To realize a harmonious introduction of the CD therapy, this study developed a useful index (KAu/c), which evaluates both therapeutic efficiency and degeneration of peritoneal tissue. Using a mathematical model and KAu/c, we were able to validate the therapeutic efficiency in the PD patients, and, in one case, propose a better prescription for the patient by employing the CD therapy. The clinical implementation of this methodology is indispensable with regard to expanding the therapeutic monitoring system for renal replacement therapy.

Standard Kt/V: Comparison of Calculation Methods

Standard Kt/V (stdKt/V) makes possible the comparison of the dialytic efficiency of different therapies, with different frequencies, whether intermittent, continuous or a mixture of both. To determine stdKt/V, the clearance of urea, a function of urea generation and blood concentration, is calculated over a period of one week and is normalized to total body water. For continuous processes, clearance per se is used. For intermittent therapy, two different equations have been developed by Gotch and by Leypoldt. Both equations allow the calculation of stdKt/V from the single-pool Kt/V (spKt/V). Gotch's method requires spKt/V and six other variables. Leypoldt's equation is simpler and limited to spKt/V plus two variables. The present study compared the nomograms created from both equations to determine their equivalence. The nomograms were superimposed for comparison. It also compared the stdKt/V obtained from random simulated patients with each method and the theoretic efficiency of continuous and intermittent clearances. The graphic comparison of the numeric values obtained appeared very similar in both methods, suggesting that either can be useful in clinical practice. A significant theoretic advantage was noted for continuous therapies over intermittent modalities. These results further support the use of frequent, daily, and continuous renal replacement therapies.

A mathematical analysis of a two-compartment model of urea kinetics

A model of urea distribution comprising well-mixed intracellular and extracellular compartments, with diffusive transfer of urea between compartments, is used to study blood urea variation during haemodialysis. Assuming that a typical adult patient (weight 70 kg and urea generation rate 5 mg min-1) is dialysed using a high-efficiency dialyser (urea clearance 0.2- 0.251 min-1) at an ultrafiltration rate of 10 ml min-1, it is shown that a perturbation analysis can account for the effects of ultrafiltration and urea generation. This permits an analytical solution of the equations which describe the variation in solute concentration in each of the compartments, which may be compared with the solution obtained when urea generation and ultrafiltration are neglected. For a typical adult patient with urea distribution volume in the range 25-40 l undergoing high-efficiency haemodialysis, the analysis suggests that the contribution of ultrafiltration to the variation in urea concentration during dialysis is similar in magnitude to the experimental errors in measuring the blood concentration of urea and that a constant volume model will be sufficiently accurate to describe urea clearance in many patients.

An evaluation of twelve nested models of transperitoneal transport of urea: the one-compartment assumption is valid

Models of transperitoneal urea transport are generally based on the one-compartment assumption, i.e. that the plasma water urea concentration in the peritoneal capillary bed is equal to the plasma water urea concentration in the peripheral veins. The aim of this study was to investigate the mechanism(s) of transperitoneal urea transport and to test the one-compartment assumption for urea. A total of 12 nested models were formulated and validated on the basis of experimental results obtained from 23 non-diabetic patients undergoing peritoneal dialysis. The validation procedure demonstrated that transperitoneal transport of urea probably involves diffusion, non-lymphatic convection and lymphatic convection. It was furthermore demonstrated that the inclusion of lymphatic convection changes the mass transfer area coefficient considerably. Finally, no deviation from the one-compartment assumption was demonstrated by our results.

Clinical Pharmacy Care in Continuous Ambulatory Peritoneal Dialysis Patients

Continuous ambulatory peritoneal dialysis (CAPD) continues to grow as a treatment modality for end-stage renal disease (ESRD). The high cost of care, multiplicity of drugs used by each patient, high cost of individual drugs, and high incidence of complications, make this patient population a challenging area for clinical pharmacy input. This article discusses the mechanics of CAPD, together with patient selection criteria, monitoring of the dialysis prescription, and infectious complications. The potential for research involving clinical pharmacists is discussed. Apart from studies on the pharmacokinetic disposition of drugs administered to CAPD patients, there remains a relative lack of information on many aspects of drug therapy. Recommendations for areas of continued study include the optimization of antibiotic therapy for peritonitis and exit-site infections, use of the intraperitoneal route for systemic effects of drugs, quality-of-life investigations, and stability studies of drugs in dialysate.

An analysis of the single-pool urea kinetic model and estimation of errors

The single-pool urea kinetic model assumes that urea is distributed within the body in a volume Vf at a uniform concentration. It may be used to describe the clearance of urea during and following haemodialysis, and to determine the value of the urea concentration at the end of dialysis Cf or the urea generation rate G, and the urea distribution volume Vf. The protein catabolic rate (PCR) is obtained using the ratio G/Vf. The sensitivity of the predictions of the model to small errors in the experimentally estimated model parameters is evaluated and suggests that the model prediction of Cf is relatively insensitive to errors in the estimate of G, but more sensitive to errors in Vf and dialyser clearance K. The determination of G from urea concentration measurements made during dialysis alone is very sensitive to errors. The accurate estimation of G requires the use of concentration measurements made between dialyses. The calculation of the PCR is very sensitive to errors in G and Vf since these are not independent and must be used with caution, particularly when used to compare values between patients.

Continuous ambulatory peritoneal dialysis: a review of its mechanics, advantages, complications, and areas of controversy

OBJECTIVE

The primary objective of this article is to review the mechanics, advantages, complications, pharmacokinetics, and future trends of continuous ambulatory peritoneal dialysis (CAPD) as they pertain to pharmacotherapy.

DATA SOURCES

Pertinent articles were obtained from an English-language literature search using MEDLINE (1980-1991), Index Medicus (1987-1990), and bibliographic reviews of review articles. Indexing terms included peritoneal dialysis, pharmacokinetics, peritonitis, vancomycin, and fluoroquinolones.

DATA SYNTHESIS

All clinical studies comparing organism recovery methods and treatment of peritonitis have methodologic limitations (e.g., comparison of disparate patient groups, different definitions of peritonitis, lack of follow-up, lack of control for sterile cultures) that may affect the reported results.

CONCLUSIONS

CAPD is an alternative to hemodialysis for the treatment of endstage renal disease and has many complications, leading to significant morbidity. This indicates that CAPD is not appropriate for all patients. Using blood-culturing techniques to culture for dialysate is most productive, but also the most costly. There are few data to indicate exactly the drugs, doses, and durations of choice for peritonitis. Both intraperitoneal and oral administration appear to be appropriate.

Paediatric haemodialysis: estimation of treatment efficiency in the presence of urea rebound

The 2-pool urea kinetic model has been developed analytically and applied to the description of the observed increase in blood levels of urea following dialysis (urea rebound), assuming that the dialyser urea clearance K less than 0.4X where X is the urea mass transfer coefficient between the intracellular and extracellular pools (volumes V1, V2 respectively). Urea generation was also neglected. Measurements were made in a group of six children suffering from chronic renal failure. From the model X, the efficiency of dialysis, and the equilibrium urea concentration C infinity were estimated in the presence of urea rebound using a blood urea measurement taken 90 min following start of dialysis, in addition to the conventional samples taken immediately pre- and post-dialysis. In three of the patients agreement between the experimental value of X derived from a multi-blood-sample technique post-dialysis, and the model value, was within 10%, for the range V1 = 0.4 W - 0.38 W, V2 = 0.2 W - 0.238 W, (W = patient's weight). Experimental values of X were in the range 93 - 300 ml min-1. Model estimates of C infinity were accurate to within 10%. An approximate technique was also developed which permitted an estimate of C infinity which was independent of V1, V2, K. The results indicated that C infinity was estimated to within 10% of the true equilibrium urea concentration. The error in the estimate of dialysis efficiency based on a single pool model was reduced by at least 50% using the model. The model may be applied clinically to the estimation of dialysis efficiency in the presence of significant urea rebound.

Evaluation of hemodialysis efficiency by an edema fluid model

The concentrations of urea, creatinine, and uric acid were measured in edema fluid and plasma during hemodialysis and 18 h after hemodialysis. The concentrations of these solutes in plasma were 15-17% lower than in the edema fluid after hemodialysis. Eighteen hours after hemodialysis, however, the concentrations in plasma were almost the same as those in edema fluid. These data suggest that the removal of these solutes from the extracellular space is delayed during hemodialysis. The plasma concentrations obtained at 18 h after hemodialysis are better indicators of hemodialysis efficiency.

Urea kinetic modelling: comparison of three methods

It has been claimed that computed urea kinetic (UK) modelling in haemodialysed patients for the estimation of protein intake and of the relation between total dialyser urea clearance and distribution volume (Kt/V) leads to an overestimation of protein catabolic rate (PCR). In the present study three different methods of kinetic modelling for the determination of PCR and Kt/V are compared in 15 patients. The first method (MI) is the direct quantification method based on the collection of all urea eliminated from the body. The two other methods are based on an iterative computed calculation. The second method (MII) is the urea kinetic modelling method as described by Sargent. Dialyser clearances were measured directly and not estimated by theoretical extrapolation. The third method described here (MIII), is based on the indirect calculation of urea distribution volume (Vw) according to Watson and of dialyser clearances from this Vw and from pre- and post-dialysis urea concentrations. All three methods result in PCRs that are not significantly different (MI: 1.04 +/- 0.29; MII: 1.07 +/- 0.28; MIII: 1.05 +/- 0.24 mg/kg BW per 24 h; p greater than 0.05). When the results are correlated, the following results are obtained: MI vs MII: r = 0.76, p less than 0.001; MI vs MIII: r = 0.78, p less than 0.001; MII vs MIII: r = 0.90, p less than 0.001. For Kt/V virtually identical results were obtained for each of the methods under study. In conclusion, all methods under study seem equally reliable in determining mean PCR.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide and furosemide effects on hydraulic pressure in the renal papilla

Atrial peptides (ANP) have been shown to preferentially increase blood flow to juxtamedullary nephrons and to augment vasa recta blood flow. To determine the effect of this alteration in intrarenal blood flow distribution on pressure relationships in inner medullary structures and their significance as a determinant of ANP-induced natriuresis, we measured hydraulic pressures in vascular and tubule elements of the renal papilla exposed in Munich-Wistar rats in vivo during an euvolemic baseline period and again during an experimental period. Rats in Group 1 received intravenous infusion of rANP administered as a 4 micrograms/kg prime and 0.5 microgram/kg/min continuous infusion, and were maintained euvolemic by plasma replacement. Infusion of ANP resulted in significant natriuresis, diuresis and increase in inulin clearance. Within 90 seconds of initiation of this systemic infusion, vasa recta hydraulic pressures were markedly increased and exceeded the small pressure increment occurring in loops of Henle and collecting ducts. Infusion of furosemide in Group 2 rats at a dosage which reproduced the increase in urine flow in Group 1 was associated with small and equivalent increases in both vascular and tubule elements, indicating that the differential pressure response observed in Group 1 was not due to increased tubule fluid flow rates, but was rather a specific ANP-induced vascular effect. Group 3 rats received an infusion of ANP in a setting where its whole kidney hemodynamic effects were prevented. This resulted in a marked blunting of natriuresis and diuresis, and obliteration of the pressure differential between vasa recta and tubules observed in Group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Net protein catabolic rate after kidney transplantation: impact of corticosteroid immunosuppression

Protein catabolic rate (PCR) was measured during the first 3 to 7 weeks after kidney transplantation in nine nondiabetic patients (16-52 yr old) who were receiving azathioprine and corticosteroid immunosuppression. Five of the patients received additional amounts of corticosteroids to treat rejection episodes. The PCR paralleled the amounts of corticosteroids administered initially as prophylactic immunosuppression and later as treatment for rejection episodes. In spite of a diet containing 1.2 g of protein per kg ideal body weight, net urea generation reflected severe lean body mass breakdown, roughly equivalent in magnitude to a 50% body surface area full-thickness burn. This catabolism was accompanied by a fall in serum albumin without significant weight loss. Hypercatabolism in the early posttransplant period is severe. After transplantation diet should be modified to try to satisfy these nitrogen demands.

Water balance in the Brattleboro rat: single or multiple defects?

The total restoration of urinary concentrating ability of the DI rat given daily injections of vasopressin takes several weeks, although complete osmotic equilibrium across the collecting duct is manifest within hours. This suggests that there may be other deficiencies of the renal concentrating mechanism that, if corrected by vasopressin treatment, are corrected more slowly. I have focussed on just three possibilities. First, the morphology of the medullary interstitium is different from normal rats. Perhaps associated with this finding are alterations in the levels of medullary glycosaminoglycans which may have a role to play in water balance. Functional and morphological changes in the juxtamedullary nephrons are also evident. Second, the possibility exists that the countercurrent multiplier of the DI rat operates less efficiently than in the normal animal. Finally, reduced synthesis of PGs in the renal medulla of DI rats may also influence the concentrating mechanism, although in a favorable direction. While most (if not all) of these differences are secondary to the lack of vasopressin, in some instances it appears that it is the high water turnover (possibly the altered chemical composition of the medullary interstitium) that is the primary culprit. While the DI rat remains an excellent model for the study of water balance and the action of vasopressin, the presence of multiple defects within the system should be borne in mind. This is particularly true when comparing data obtained following acute treatment with vasopressin versus that following chronic treatment.

Freeze-fracture and morphometric analysis of occluding junctions in rectal glands of elasmobranch fish

The structure of occluding junctions in secretory and ductal epithelium of salt-secreting rectal glands from two species of elasmobranch fish, the spiny dogfish Squalus acanthias and the stingray Dasyatis sabina, was examined by thin-section and freeze-fracture electron microscopy. In both species, occluding junctions between secretory cells are shallow in their apical to basal extent and are characterized by closely juxtaposed parallel strands. Average strand number in the dogfish was 3.5 +/- 0.2. with a mean depth of 56 +/- 5 nm; in the stingray a mean of 2.0 +/- 0.2 strands encompassed an average depth of 18 +/- 3 nm. In contrast, the linear extent of these junctions was remarkably large due to the intermeshing of the narrow apices of the secretory cells to form the tubular lumen. Morphometric analysis gave values of 66. 8 +/- 2.5 and 74.9 +/- 4.6 m/cm2 for the length of junction per unit of luminal surface area in the dogfish and stingray, respectively. This junctional morphology is similar to that generally described for "leaky" epithelia. In comparison, the stratified ductal epithelium which carries the NaCl-rich secretion to the intestine is characterized by extensive occluding junctions which extend 0.6-0.8 mum in depth and consist of a mean of 12 strands arranged in an anastomosing network, an architectural pattern typical of "tight" epithelia. The length density of these junctions in the dogfish rectal gland was 7.6 +/- 0.1 m/cm2. The junctional architecture of the rectal gland secretory epithelium (few strands, large junctional length densities) is similar to that described for several other hypertonic secretory epithelia [20, 34] and is compatible with the recent model for salt secretion in rectal glands [39] and in other C1- secretory epithelia which posits a conductive paracellular pathway for trans-epithelial Na+ secretion from intercellular space to the lumen to form the NaCl-rich secretory product.

Occluding junctions in a cultured transporting epithelium: structural and functional heterogeneity

MDCK cells (epithelioid of renal origin) form monolayers which are structurally and functionally similar to transporting epithelia. One of these similarities is the ability to form occluding junctions and act as permeability barriers. This article studies the junctions of MDCK monolayers formed on a permeable and transparent support (a disk of nylon cloth coated with collagen) by combining two different approaches: (i) Scanning of the electric field: the disk is mounted as a flat sheet between two Lucite chambers and pulses of 20--50 microA cm-2 are passed across. The apical surface of the monolayer is then scanned with a microelectrode to detect those points where the current is flowing. This shows that the occluding junctions of this preparation are not homogeneous, but contain long segments of high resistance, intercalated with sites of high conductance. (ii) Freeze fracture electron microscopy: the junctions are composed of regions of eight to ten strands intercalated with others where the strands are reduced to one or two ridges. The sites of high conductance may correspond to those segments where the number of junctional strands is reduced to 1 or 2. It is concluded that the occluding junctions of MDCK monolayers are functionally and morphologically heterogeneous, with "tight" regions intermixed with "leaky" ones.

Structural simplicity of the zonula occludens in the electrolyte secreting epithelium of the avian salt gland

The structure of the zonula occludens in the secretory epithelium of the salt gland of the domestic duck was determined by thin section and freeze-fracture electron microscopy. These glands secrete an effluent with a NaCl concentration four times that of plasma, and thus maintain a steep ionic gradient across their secretory epithelium. Freeze-fracture replicas from salt stressed ducks demonstrate that the zonula occludens is surprisingly shallow in depth (20-25 nm) and generally consists of two parallel junctional strands which are juxaposed along their entire length. In addition to the simplicity of the junction separating mucosal and serosal compartments, the ratio of junctional length to apical surface area is large since luminal surfaces of secretory cells are narrow and intermesh with one another. The zonula occludens in nonsecreting fresh water-adapted birds is similar to the salt stressed group except that two sets of double strand junctions are seen in addition to junctions consisting of a single set. Based on previous ultrastructural, cytochemical and physiological studies in salt glands and in other epithelia, a model for salt secretion was suggested in which intercellular space Na+, generated by basolateral ouabain-sensitive Na+ pumps, reaches the lumen via a paracellular route (Ernst & Mills, 1977, J. Cell Biol. 75:74). The simplicity of the morphological appearance of the zonula occludens in the salt gland, which resembles that described for several epithelia known to be leaky to ions, is consistent with this hypothesis.

Structural-functional heterogeneity along the rabbit collecting tubule

The possibility that the rabbit collecting tubule is heterogeneous with respect to electrical and morphologic properties was examined in isolated segments. Collecting tubules from rabbits treated with desoxycorticosterone acetate were dissected from various locations within the cortex and outer medulla, and the transepithelial potential was measured. The cortical collecting tubule always displayed a negative potential. In contrast, collecting tubules dissected from the inner stripe of the outer medulla displayed a positive potential. That this change is a function of position within the kidney is supported by the observation that some tubules generated a negative potential at the superficial (cortical) end and a positive potential at the deep (medullary) end of the same tubule. Histologic evaluation of tubules after perfusion resulted in a striking correlation between the existence of a negative potential and the presence of dark cells. The results of this study provide evidence that the collecting tubule is heterogeneous with respect to active transport processes and suggest that these functional differences may correlate with morphologic differences between the cortical segment and the medullary segment of the collecting tubule.

Effect of hypertonic urea and mannitol on distal nephron permeability

The paracellular pathway permeability is known to increase in perfused amphibian kidneys if the luminal fluid is made hyperosmotic with mannitol or urea. To investigate whether luminal hypertonicity increases paracellular pathway permeability in the mammalian nephron, early rat distal tubules were micropunctured and perfused through one micropipette with either isosmotic saline (IS), hyperosmotic urea (HU) or hyperosmotic mannitol (HM) solutions. A second micropipette was placed down-stream in the same tubule and test solutions of 30 nl of a mixture of 14C-inulin and 3H-mannitol or of 3H-inulin and 14C-urea were injected. Similar intratubular injections of tracers were performed in a second group of rats undergoing diuresis induced either by infusing intravenously saline alone (VS) or receiving saline plus 0.4 M urea (VU). In the latter group (VU) luminal urea concentration was increased without the tubular lumen being made hyperosmotic to its peritubular fluid. Urinary unulin recovery was essentially complete and unaffected by experimental procedures. Difference between mannitol recoveries in isosmotic saline and hyperosmotic urea perfusions IS-HU was 2.6 +/- 0.8% (P less than 0.001). Difference in urea recoveries IS-HM was 4.1 +/- 5.1% (P greater than 0.40), IS-HU was 13.9 +/- 5.3% (P equal to 0.015) and, VS-VU equal to 17.0 +/- 3.4 (P less than 0.001). Therefore, elevated luminal urea concentration increased tracer mannitol and also tracer urea permeability, both in the presence and absence of tubular hyperosmolarity. Electron microscopic observations showed changes in geometry of tubular junctional complexes compatible with the observed increase in permeability.

Bidirectional sodium ion movements via the paracellular and transcellular routes across short-circuited rabbit ileum

1. It has been confirmed that the agent 2,3,6-triaminopyrimidine decreases Na+ conductance in the paracellular pathway of rabbit ileum. 2. Triaminopyrimidine has been used as a means of measuring transcellular bidirectional Na+ flux, and also, of assessing the contribution of the paracellular pathway to transepithlial Na+ flux. 3. Reduction of Ringer [Na+] to 25 mM or incubation with 0.1 mM ouabain reduces paracellular Na+ permeability. This effect may be due to lateral space collapse. Ringer galactose increases serosa to mucosa Na+ flux by a stimulating reflux through the tight junctions. A proportion of net Na+ flux in control tissues is due to asymmetry generated in the paracellular pathway. It is likely that this passive asymmetry results from an osmotic pressure gradient across the tight-junction. 4. Measurement of the tissue isotope specific activity ratio together with bidirectional transcellular Na+ fluxes allows calculation of the four unidirectional fluxes across the mucosal and serosal boundaries. Values obtained for Na+ entry (J12) and exit (J21) across the mucosal boundary are 7.97 alnd 7.13 mumol-cm(-2)-h(-1) respectively. Entry flux (J12) is a saturable function of Ringer [Na+]. The calculated Km is 295 mM and the V is 17.6 mumul-cm(-2)-h(-1). Na+ entry flux is insensitive to ouabain (0.1 mM). Ouabain results in elevation of exit (J21) flux of Na+ across the brush border. D-Galactose causes a saturable increase in Na+ flux (J12) across the mucosal boundary; the Km for this relationship is 1.2 mM and the V 2.17 mumol-cm(-2)-h(-1). The stoichiometry between sugar and Na+ entry is applixmately 1:1. In contrast to the effect of galactose on entry flux, no change in Na+ efflux across the mucosal boundary is observed when Ringer [galactose] is raised. This finding is dissonant with the prediction of the Na+ -gradient hypothesis. The calculated values of exit (J23) and entry (J32) Na+ fluxes across the serosal border are 16.74 and 15.90 mumol-cm(-2)-h(-1). 0.1 mM ouabain markedly reduces both these unidirectional fluxes. This result is consistent with a serosal location of the Na+-pump. Serosal Na+ exit flux J23 increases as a hyperbolic function of Ringer [galactose]. A small galactose-dependent decrease in entry (J32) is also observed. 0.1 mM ouabain abolishes these galactose-dependent changes. 5. The present findings together with those in the previous paper are discussed in relation to the convective-diffusion model for sugar transport.

Zonulae occludentes of the rat nephron under conditions of experimental expansion of blood and/or fluid volume

The zonula occludens (Z.O.) has been implicated as a major pathway for passive fluid movement from lateral intercellular spaces to lumen in response to an expansion of blood or fluid volume. The present study examines to Z.O. of adult rat nephrons in control and Ringer-Locke or while blood infused animals to determine whether varying the conditions of passive fluid movement would influence the structure of the Z.O. Rats were infused over 20 minutes with Ringer-Locke (7% body weight) or while blood (2.3% body weight). In the controls, 92%-94% of all Z.O. observed in proximal tubules and 100% of Z.O. from distal tubules and collecting ducts had fused outer leaflets. The approximate linear extent of the Z.O. was 140 A for proximal tubules, 2,500 A for distal tubules and 3,000 A for collecting ducts. There was no significant difference in any of these values following the infusion with either Ringer-Locke or whole blood. We conclude that expansion of blood or fluid volume causes no detectable alteration in the fine structure of Z.O. under the conditions used in the present study.

Specialized junction in the distal convoluted tubule of rat kidney

The ultrastructure and permeability of the apical junctions between epithelial cells of the distal nephron have been studied in rat kidney using a collidal lanthanum tracer and uranyl acetate staining en bloc. The apical intercellular junctions of the macula densa and juxtaglomerular segment of the early distal convoluted tubule measure up to 0.5 mu in length and about 50 A in width. Lanthanum permeates the occluding portion of these junctions in a discontinuous manner, defining a series of closely spaced and parallel lines of fusion that run in a direction perpendicular to the apical-basal axis of the tubular cells. The apical junctions of the remainder of the distal convoluted and cortical collecting tubules are impermeable bolanthanum. This distinctive apical tight junction can account for the greater permeability to ions of the early distal convoluted vs. late distal convoluted and cortical collecting tubules.