In vivo inhibition of transcellular water channels (aquaporin-1) during acute peritoneal dialysis in rats

During peritoneal dialysis (PD), a major portion of the osmotically induced water transport to the peritoneum can be predicted to occur through endothelial water-selective channels. Aquaporin-1 (AQP-1) has recently been recognized as the molecular correlate to such channels. Aquaporins can be inhibited by mercurials. In the present study, HgCl2 was applied locally to the peritoneal cavity in rats after short-term tissue fixation, used to protect the tissues from HgCl2 damage. Dianeal (3.86%) was employed as dialysis fluid, 125I-albumin as an intraperitoneal volume marker, and 51Cr-EDTA (constantly infused intravenously) to assess peritoneal small-solute permeability characteristics. Immunocytochemistry and immunoelectron microscopy revealed abundant AQP-1 labeling in capillary endothelium in peritoneal tissues, representing sites for HgCl2 inhibition of water transport. HgCl2 treatment reduced water flow and inhibited the sieving of Na+ without causing any untoward changes in microvascular permeability, compared with that of fixed control rats, in which the peritoneal cavity was exposed to tissue fixation alone. In fixed control rats, the mean intraperitoneal volume (IPV) increased from 20.5 +/- 0.15 to 25.0 +/- 0.52 ml in 60 min, whereas in the HgCl2-treated rats, the increment was only from 20.7 +/- 0.23 to 23.5 +/- 0.4 ml. In fixed control rats, the dialysate Na+ fell from 135.3 +/- 0.97 to 131.3 +/- 1.72 mM, whereas in the HgCl2-treated rats the dialysate Na+ concentration remained unchanged between 0 and 40 min, further supporting that water channels had been blocked. Computer simulations of peritoneal transport were compatible with a 66% inhibition of water flow through aquaporins. The observed HgCl2 inhibition of transcellular water channels strongly indicates a critical role of aquaporins in PD and provides evidence that water channels are crucial in transendothelial water transport when driven by crystalloid osmosis.

Liver is not essential for solute transport during peritoneal dialysis

Based on theoretical calculations on solute exchange capacities of various peritoneal tissues, the liver has been predicted to account for up to 43% of the permeability-surface area product (PS) of the entire peritoneal "membrane" for a small solute (sucrose) during peritoneal dialysis (PD). In these calculations, the abdominal wall and the diaphragm were found to contribute only approximately 10 to 15% of the total PS. However, evisceration has in previous studies been shown to affect the PS characteristics during PD only marginally (10 to 30%). In such evisceration experiments the liver was usually not removed, and therefore it has been suggested that an intact liver might have significantly contributed to the solute exchange under these premises. We assessed the peritoneal PS of 51Cr-EDTA (constantly infused intravenously) and the plasma-to-peritoneal clearance (ClB-->P) of 125I-human serum albumin (RISA) (given as an i.v. bolus) in Wistar rats during acute PD. In one group of rats the liver surface was sealed off using Histoacrylate glue (N = 6) and in another group a 90% hepatectomy was performed, the remaining portion of the liver, usually the right lower lobe, being sealed off by glue (N = 6). A third group was sham operated to serve as control (N = 12). The PS for 51Cr-EDTA was 0.32 +/- 0.03(+/- SE) ml. min-1 (N = 12) during control, 0.32 +/- 0.04 ml.min-1 after "sealing" of the liver surface (N = 6, P > 0.1) and 0.40 +/- 0.03 after hepatectomy (N = 6, P > 0.1), that is, remained unchanged after experimental intervention. The CIB-->P of RISA during control was 5.88 +/- 1.0 microliter.min-1 (N = 10), and was not altered after hepatectomy, 6.17 +/- 0.48 microliters.min-1 (N = 5, P > 0.1), but slightly increased after liver surface sealing (9.69 +/- 1.09 microliters.min-1, N = 5, P < 0.05). In conclusion, the present experiments indicate that the liver does not play an essential role in the overall solute exchange between the plasma and the peritoneal cavity (PC) during PD.

Transport of tracer albumin from peritoneum to plasma: role of diaphragmatic, visceral, and parietal lymphatics

Using a technique to acutely seal off various parts of the peritoneal membrane surface, with or without evisceration, we investigated the role of diaphragmatic, visceral, and parietal peritoneal lymphatic pathways in the drainage of 125I-labeled albumin (RISA) from the peritoneal cavity to the plasma during acute peritoneal dialysis in artificially ventilated rats. The total RISA clearance out of the peritoneal cavity (Cl) as well as the portion of this Cl reaching the plasma per unit time (Cl--> P) were assessed. Under non-steady-state conditions, the Cl was fivefold higher than the Cl--> P. Evisceration caused a 25-30% reduction in both Cl--> P and Cl. Sealing of the diaphragm, however, reduced the Cl--> P by 55% without affecting the Cl. A further reduction in the Cl--> P was obtained by combining sealing of the diaphragm with evisceration, which again markedly reduced the Cl. However, the greatest reduction in the Cl was obtained when the peritoneal surfaces of the anterior abdominal wall were sealed off in eviscerated rats. The discrepancy between the Cl and the Cl--> P can be explained by the local entrance of fluid and macromolecules into periabdominal tissues, where fluid is rapidly absorbed through the capillary walls via the Starling forces, while macromolecules are accumulating due to their very slow uptake by tissue lymphatics under non-steady-state conditions. Of the portion of the total Cl that rapidly entered the plasma, conceivably by lymphatic absorption, 55% could be ascribed to diaphragmatic lymphatics 30% to visceral lymphatics, and only some 10-15% to parietal lymphatics.

Are aldehydes in heat-sterilized peritoneal dialysis fluids toxic in vitro?

OBJECTIVE

Chemical analysis of several brands of peritoneal dialysis fluids (PD fluids) has revealed the presence of 2-furaldehyde, 5-HMF (5-hydroxymethylfuraldehyde), acetaldehyde, formaldehyde, glyoxal, and methylglyoxal. The aim of this study was to investigate if the in vitro side effects caused by glucose degradation products, mainly formed during heat sterilization, are due to any of these recently identified aldehydes.

DESIGN

Cell growth media or sterile filtered PD fluids were spiked with different concentrations of thealdehydes.

MEASUREMENTS

In vitro side effects were determined as the inhibition of cell growth of cultured mouse fibroblasts or stimulated superoxide radical release from human peritoneal cells.

RESULTS

Our results demonstrate that the occurrences of 2-furaldehyde, 5-HMF, acetaldehyde, formaldehyde, glyoxal, or methylglyoxal in heat-sterilized PD fluids are probably not the direct cause of in vitro side effects. In order to induce the same magnitude of cell growth inhibition as the heat-sterilized PD fluids, the concentrations of 2-furaldehyde, glyoxal, and 5-HMF had to be 50 to 350 times higher than those quantified in the PD fluids. The concentrations of acetaldehyde, formaldehyde, and methylglyoxal observed in the heat-sterilized PD fluids were closer to the cytotoxic concentrations although still 3 to 7 times lower.

CONCLUSION

Since none of these aldehydes caused in vitro toxicity at the tested concentrations, the toxicity found in PD fluids is likely to be due to another glucose degradation product, not yet identified. However, it is possible that these aldehydes may still have adverse effects for patients on peritoneal dialysis.

Elevated plasma levels of acute phase proteins in mesangioproliferative glomerulonephritis, membranous nephropathy and IgA nephropathy

In order to study the possible role of active inflammatory processes in clinical indolent primary chronic glomerulonephritides, plasma concentrations of the acute phase proteins: alpha 1-antitrypsin, haptoglobin, orosomucoid and C-reactive protein were measured in 166 glomerulonephritis patients. The patients had a diagnosis of either mesangioproliferative glomerulonephritis, membranous nephropathy or immunoglobulin A nephropathy and were divided in two groups, one with heavy urinary albumin losses and one with moderate to slight urinary albumin excretion. The median plasma concentration values for alpha 1-antitrypsin, haptoglobin and orosomucoid were increased in all three kinds of the investigated glomerulonephritides with exception for orosomucoid in patients with heavy urinary albumin losses and in the membranous nephropathy group. The plasma concentration values for C-reactive protein were not elevated at all in the material. The increase of plasma levels of acute phase proteins could be the result of persistent inflammatory stimuli that occur in primary chronic glomerulonephritides. The finding of unchanging plasma levels of C-reactive protein in contrast to increased concentrations of the other acute phase proteins could be of significance in diagnosing infections or other inflammatory diseases in patients with chronic glomerulonephritis.

Intraperitoneal fluid volume changes during peritoneal dialysis in the rat: indicator dilution vs. volumetric measurements

In order to validate the single injection RISA (125I human serum albumin) indicator diluation technique for assessing the alterations in intraperitoneal (i.p.) dialysate volume (IPV) which occur vs. time [V(t)] during peritoneal dialysis (PD), the RISA dilution technique was compared to V(t) determinations using a direct volume recovery method in Wistar rats. Sixteen milliliters of either 1.36 or 3.86% Dianeal or 0.9% NaCl were used as dialysis fluids in exchanges lasting between 1 and 360 min. Approximately 4% (4.41 +/- 0.59 (SE; n = 8) for 1.36% Dianeal and 4.07 +/- 0.72 (n = 4) for 3.86% Dianeal) of the RISA dose given intraperitoneally was lost from the dialysate during the first 1(-1.5) min after instillation, conceivably due to rapid tracer adsorption to peritoneal surfaces. Following the initial instant tracer loss and RISA dilution due to a residual volume (3.07 +/- 0.18 ml; n = 12), RISA disappeared at a fractional rate (FDR) of 2.10 +/- 0.14 x 10(-3) min-1 and 1.67 +/- 0.09 x 10(-3) min-1, during the first 30 min for 1.36 and 3.86% Dianeal, respectively. The overall FDR was 1.33 +/- 0.10 x 10(-3) and 0.707 +/- 0.082 x 10(-3) min-1 for 1.36% Dianeal (0-150 min) and 3.86% Dianeal (0-360 min), respectively, while the overall (0-150 min) FDR for the NaCl exchanges was 1.40 +/- 0.21 x 10(-3) min-1. These values correspond to RISA clearances out of the peritoneal cavity (KE) of 29.2 +/- 1.8, 22.1 +/- 1.6, and 25.7 +/- 2.4 microliter x min-1 for 1.36 and 3.86% Dianeal and 0.9% NaCl, respectively. The KE value for 3.86% Dianeal was significantly (p < 0.05) lower than for the two dialysates with lower osmolality. The slightly enhanced FDR of RISA during the first 30 min was partly due to the presence of nonprotein-bound free iodine in the RISA preparation used, and also to an enhanced disappearance of albumin during the first portion of the dwell. V(t) data from individual experiments using the RISA dilution technique (RISA curves) were analyzed by computer-aided nonlinear least-squares regression analysis.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced permselectivity in isolated perfused rat kidneys following small elevations of glomerular capillary pressure

A modified rat kidney preparation was used to explore how changes in hydrostatic pressure affect the permselective properties of the glomerular capillary bed. The maximally vasodilated kidneys of 18 rats were perfused with albumin solutions (16.7 g l-1) at different flow rates and hence arterial pressures (PA). One kidney in each rat was exposed to pressure elevations with the other kidney serving as a control perfused at constant PA of about 100 mmHg. Both the vascular resistance to flow and the glomerular filtration rate (GFR 34.6 +/- 2.9 ml min-1 100 g-1) were similar in the two kidneys at equal PA and remained constant throughout the experiment. The ratio of albumin clearance over GFR (theta) was initially around 0.4% at constant PA and gradually increased during 1.5 h to reach 0.7% at the end of the experiment. A direct increase of PA from 100 to 200 mmHg for 15 min resulted in a calculated increase of the effective glomerular filtration pressure gradient of 10-15 mmHg and in a two-fold increase of theta when measured at an identical PA of 100 mmHg. Albumin clearance was almost fully normalized within 20 min similar to that observed in e.g. skeletal muscle. However, the glomerular capillary barrier seemed to be far more sensitive to elevations of hydrostatic pressure than other capillary walls which require capillary pressure increments of 60 mmHg in order to induce similar reversible changes in permeability. Therefore, we conclude that an elevated PGC per se induces changes of glomerular permselectivity, which may have important pathophysiological implications during conditions of proteinuria.

Analysis of the pressure-flow characteristics of isolated perfused rat kidneys with inhibited tubular reabsorption

The renal hemodynamics were studied in an isolated perfused rat kidney model modified for investigations of the glomerular permeability characteristics. The tubular reabsorptive activity was inhibited by perfusion at low temperature (8 degrees C) in the presence of furosemide and nitroprusside resulting in a dramatic increase in the filtered load of fluid and solute reaching the tubules and hence in tubular pressure. The glomerular filtration rate (GFR), arterial pressure (PA) and needle pressure (intrarenal tissue pressure, PiR) were continuously recorded and the glomerular hydrostatic pressure was estimated by an arterial occlusion technique. The pre- to postglomerular resistance ratio was calculated from the pressure vs. GFR relationships for two perfusates having differing oncotic pressures (pi = 5.5 and pi = 20 mmHg), from which estimations of glomerular hydrostatic pressures (PGC) were concomitantly made. Thus, increases in delta pi could be exactly counterbalanced by equally large increases in PGC for any given GFR, the needle and Bowman's capsule pressures being dependent on GFR but not on plasma colloid oncotic pressure. The experimental interventions resulted in a pronounced elevation of PiR as compared with in vivo conditions, while the PGC values were in a normal range, resulting in reduced glomerular filtration pressures. Furthermore, the clearance of albumin varied with the oncotic pressure in agreement with the notion of heteroporosity.

Pathophysiological description of the ultrastructural changes of the peritoneal membrane during long-term continuous ambulatory peritoneal dialysis

Some of the patients on continuous ambulatory peritoneal dialysis (CAPD) develop with time on treatment an increased transperitoneal transport of small solutes, implying that glucose is more rapidly absorbed from the dialysate. Hence, the dialysate/serum crystalloid osmotic gradient dissipates at a faster rate, so that ultrafiltration failure may result. The pathophysiological correlates to these changes are not well understood. However, it seems that with time on CAPD, there are changes in the submesothelial interstitium, affecting both the ground substance and spacing and orientation of collagen fibers. There may also be mesothelial alterations with patchy shedding of the cells. The present article discusses these changes in terms of a modified three-pore model of peritoneal permeability. In this model, the capillary walls act as a major barrier for solutes ranging in size from inulin (molecular radius 14 A) to macromolecules (molecular radius > 30 A). However, for solutes smaller than inulin both capillary wall and insterstitium contribute to the blood-peritoneum transport impedance. The increased small-solute exchange sometimes occurring in long-term CAPD can be explained either by recruitment of vascular surface area, due, e.g., to an increased capillarization of the peritoneal membrane with time, or, more likely, a drop in the interstitial transport resistance to small solutes. The latter possibility is supported by the often more pronounced increase in the transperitoneal transfer of small solutes than that of macromolecules over time in CAPD.

Transport of macromolecules across microvascular walls: the two-pore theory

In this review we summarized the evidence favoring the concept that the major plasma proteins are passively transported across vascular walls through water-filled pathways by means of convection and diffusion. With regard to solute transport, a majority of microvascular walls seems to show a bimodal size selectivity. This implies the presence of a high frequency of functional small pores, restricting proteins, and an extremely low number of non-size-selective pathways, permitting the passage of macromolecules from blood to tissue, here denoted large pores. We discussed the general behavior of such a heteroporous system. A major consequence of two-pore heteroporosity is that large-solute transport must mainly occur due to convection through large pores at low filtration rates, that is, at normal or even zero lymph flows. Indeed, convection must be the predominating transport mode for most solutes across large pores when the net filtration rate is zero. Under these (transient) conditions, the convective leak of macromolecules across large pores will be counterbalanced by absorption of essentially protein-free fluid through protein-restrictive pores. In a heteroporous membrane, proteins can thus be transported by solvent drag across vascular walls in the absence of a net convection. Normally the steady-state transcapillary fluid flow (lymph flow) is about equally partitioned among small and large pores, which makes lymph essentially a "half and half" mixture of protein-free ultrafiltrate and plasma. With increasing fluid flows, however, the plasma filtrate will be progressively diluted, eventually reaching a protein concentration largely in proportion to the fractional hydraulic conductance accounted for by the large pores (alpha L). Under these high lymph flow conditions, not only the large-pore transport but also the small-pore transport (of smaller macromolecules) will become convective. At low lymph flows, however, the small-pore transport of smaller macromolecules is usually mostly diffusive. An important implication of capillary heteroporosity is that single-pore formalism is inadequate for correctly evaluating the capillary sieving characteristics. With the use of homoporous transport formalism, the "lumped" macromolecular PS and sigma will therefore vary as a function of transcapillary fluid flow (Jv). However, it is approximately correct to use single-pore formalism for conditions when Jv is very high during steady state. Thus, if minimal sieving coefficients can be measured for macromolecules, then these values will accurately reflect (1 - sigma).(ABSTRACT TRUNCATED AT 400 WORDS)

Osmotic barrier properties of the rat peritoneal membrane

In this study the osmotic barrier characteristics of the rat peritoneal membrane were investigated. Fluid movements between the peritoneal cavity and the blood were measured following instillation of isotonic saline (control) and hypertonic solutions of NaCl, glucose, sucrose, raffinose and myoglobin (test solutions). Moreover, 5 and 8% albumin in NaCl were investigated. Osmotic transients were assessed using a simple volume recovery technique. Peritoneal osmotic conductances (i.e. products of peritoneal hydraulic conductances [LpS] and solute reflection coefficients [sigma]) were calculated from the differences in the rates of peritoneal fluid loss and in osmotic pressures between test solutions and the isotonic saline control solution. The osmotic conductance to glucose was estimated to be 1.63 microliters min-1 mmHg-1 m-2 and that for albumin to be 59.6 microliters min-1 mmHg-1 m-2. Assuming an albumin sigma of 0.9, the sigma of glucose was estimated to be 0.025, in accordance with previous measurements for the cat peritoneal membrane. The osmotic conductances assessed here were compatible with an 'overall' peritoneal equivalent small pore radius of 47-48 A, but could also be fitted to a three-pore model of peritoneal permselectivity, including a transcellular (ultra-small pore) pathway and a large pore pathway. The great discrepancy between peritoneal sigma for small solutes and that for albumin obtained in this study indicates that small solute reflection coefficients are close to zero while that for albumin is not far from unity. Furthermore, the peritoneal hydraulic conductance (ultrafiltration coefficient) is large enough to allow for a substantial absorption of fluid directly into the plasma when the crystalloid osmotic pressures in blood and peritoneal dialysate are in equilibrium.

Tissue uptake of insulin and inulin in red and white skeletal muscle in vivo

By employing a tissue uptake technique, the kinetics of the plasma-interstitial equilibration of radiolabeled insulin, inulin, and albumin were followed in four muscles of differing capillarity in anesthetized rats. The soleus muscle (SOL), and the red portion of the gastrocnemius muscle (RG), as well as the extensor digitorum longus muscle (EDL) and the white portion of the gastrocnemius muscle (WG) were investigated. After constant intravenous tracer infusions and repeated plasma sampling under euglycemic clamp conditions, animals were killed at varying time intervals and the muscles mentioned above were dissected out. The radioactivity of tracer per gram of tissue in each muscle divided by the plasma activity of tracer per milliliter of plasma, i.e., "the plasma equivalent space" of tracer, thus could be followed as a function of time. From this function the permeability-surface area (PS) of inulin as well as the distribution volumes at time 0 (V0) of inulin and insulin and their equilibrium distribution volumes (VE) were assessed. The PS for inulin (in ml.min-1.100 g muscle-1) was 0.52 +/- 0.10 (mean +/- SE) in WG, increasing with more red fibers to 1.37 +/- 0.18 in SOL. Also the inulin interstitial distribution volume (at blood-tissue tracer equilibrium; VE) increased in this order (in ml/100 g) from 7.30 +/- 0.91 in WG to 12.93 +/- 0.89 in SOL. The V0 for insulin was found to be approximately fivefold larger than the plasma volume in each muscle sample, indicating a high degree of binding of insulin to structures within the vascular compartment, conceivably to the vascular endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular permselectivity is dependent on adequate serum concentrations of orosomucoid

Orosomucoid, or alpha 1-acid glycoprotein, a serum protein known to be an "acute phase reactant" has recently been shown to be needed for the maintenance of normal capillary permeability in skeletal muscle and mesentery. Therefore, we were interested in studying whether the glomerular capillary wall is affected by orosomucoid as well. For this purpose, left and right kidneys from nine rats (group A) were isolated and perfused in situ and in parallel using separate solutions of human albumin (1.8% in Tyrode), differing in their content of orosomucoid, one containing 0.21 g/liter, the other less than 0.005 g/liter. The temperature was kept at 8 degrees C in order to minimize tubular reabsorption of fluid and albumin. The two kidneys showed identical and stable vascular resistances during the experiments. Also the glomerular filtration rates (GFR) were stable between 30 and 33 ml/min/100 g kidney. Initially, the two kidneys showed similar fractional albumin clearance (theta) values of approximately 0.003. However, in the "absence" of orosomucoid theta increased to become four- to fivefold higher in the test kidney than in the control kidney at the end of the 1 1/2 hour experiment. This difference was observed in all rats, suggesting that orosomucoid is needed also for the maintenance of the glomerular permselectivity. In a separate group of eight animals (group B), orosomucoid-containing albumin solutions were used in parallel with horse serum solutions to perfuse the two kidneys of each rat, at 8 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Lymphatic versus nonlymphatic fluid absorption from the peritoneal cavity as related to the peritoneal ultrafiltration capacity and sieving properties

In this article we discuss the role of capillary fluid absorption via Starling mechanisms (the transcapillary hydrostatic pressure gradient opposed by the colloid osmotic pressure gradient as multiplied by the capillary UF coefficient) vs. lymphatic fluid absorption as determinants of the total fluid loss from the peritoneal cavity during continuous ambulatory peritoneal dialysis (CAPD). We also mention that, under nonsteady state conditions, there is in addition some net absorption of fluid into the interstitium of tissues surrounding the peritoneal cavity. Support for the contention that nonlymphatic fluid absorption directly into the capillaries is the major mode of fluid transport from the peritoneal cavity to the blood is given by measurements of the peritoneal-to-blood clearance of tracer albumin (or other proteins). Such measurements yield clearance values of the order of 0.2-0.3 ml/min in CAPD. This represents only about 20% of the total peritoneal fluid loss rate (1.2-1.3 ml/min) in ordinary CAPD dwells. Indirect support for a relatively low lymph flow is also derived from capillary physiology. Like continuous capillary walls, the peritoneal membrane shows a bimodal selectivity towards molecules of graded molecular size. Thus, small solute transport can be described as occurring by diffusion through numerous 'small' (approximately 50 A radius) pores, whereas large solute transport is consistent with blood-peritoneal convection through smaller numbers of 'large' (radius approximately 250 A) pores. Furthermore, peritoneal sieving data are compatible with the notion that large crystalloid osmotic pressure gradients cause fluid flow through a water-exclusive ('ultra-small' pore) pathway. A three-pore model of peritoneal selectivity can explain why small solute sieving coefficients are only 0.5-0.6, even though small solute reflection coefficients are close to zero. Another important implication of the three-pore concept is that the peritoneal UF-coefficient is much higher than previously thought, emphasizing the role of capillary absorption in the fluid loss from the peritoneal cavity in CAPD. It is concluded that fluid loss from the peritoneal cavity is dominated by capillary fluid absorption. Hence, lymphatic absorption accounts for just a small fraction of the peritoneal-to-blood absorption of fluid in peritoneal dialysis.

Net fluid absorption under membrane transport models of peritoneal dialysis

The effect of oncotic pressure and lymphatic flow on intraperitoneal dialysate volumes in peritoneal dialysis is investigated under each of two membrane transport models: one assuming a homogeneous single-pore membrane and the other a heteroporous three-pore membrane. In both cases, solute and fluid removal are assumed to occur via a mass transport model in which the peritoneum acts like a synthetic membrane separating two well-mixed compartments (body and dialysate). The homoporous mass transport model of Pyle and Popovich and the three-pore model of Rippe et al., although conceptually different, are shown to be equivalent mathematically. This feature allows one to apply the analytical solutions of Vonesh et al. to either model. It also enables one to apply parameter estimates from one model to another; for example, one can apply the lumped sum reflection coefficients of the three-pore model to a homoporous membrane model. A comparison is made between the use of empirically estimated rejection coefficients computed under the homoporous membrane model of Pyle and Popovich versus lumped-sum reflection coefficients calculated in accordance with the three-pore model of Rippe et al. The two models predict similar drain volumes provided the exchange is conducted using glucose as the osmotic agent. However, one does see a significantly different contribution of protein oncotic pressure and lymphatic drainage to fluid absorption under the two sets of osmotic reflection coefficients. Moreover, for a simulated exchange employing an osmotic agent with a molecular weight of 20,000 daltons, the use of reflection coefficients calculated under the three-pore model yields net ultrafiltration values which are more consistent and physiological than results obtained using the empirically estimated rejection coefficients. Since estimates of 'lymphatic flow' will vary according to the quantity and quality of input parameter values (i.e., hydrostatic pressure, protein concentrations, osmotic reflection coefficients), it would be better to label these estimates as the sum of lymphatic and unmodeled net fluid absorption.

A note on the errors of using venous congestion in intact rats for determinations of microvascular permeability

The established ideas of transcapillary exchange have recently been challenged based on studies in intact rats. In vivo measurements of net fluid flux and albumin clearance in muscle (and skin) have given estimates of the reflection coefficient (sigma) for albumin of 0.98-0.99 compared to the sigma value of 0.90 obtained by most other techniques. This discrepancy has vast consequences for the understanding of the transcapillary passage of macromolecules. A sigma for albumin near unity implies that there is virtually no coupling between protein and fluid transfer as induced by, for example, increases in vascular hydrostatic pressures. However, there are several assumptions inherent in the seemingly straight-forward experiments on intact rats, and in the present study we tested the hypothesis that occlusion of the femoral vein by ligation induces only moderate and transient increments of venous pressure (PV). During control conditions PV was 6.3 mmHg and pressure increased to 12.8 mmHg immediately following venous occlusion. However, PV declined with time and after 30 minutes of occlusion the capillary hydrostatic pressure was only increased by 3.0 mmHg. Calculations of the capillary filtration coefficient gave completely unrealistic values, close to those of maximally vasodilated skeletal muscle. The findings suggest that data obtained from intact rats, albeit important and interesting, should be evaluated with great care due to possible experimental errors in the in vivo approaches. In particular, the technique of estimating sigma for albumin in intact rats must be subjected to modifications before allowing any reliable conclusions.(ABSTRACT TRUNCATED AT 250 WORDS)

Upper and lower bounds on capillary permeability ratios of Cr-EDTA to cyanocobalamin in rat hindquarters

The single injection indicator dilution technique, often used for assessing capillary permeability, was employed for estimations of the equivalent pore radius of skeletal muscle microvessels according to the theory of restricted diffusion. There are, however, certain important sources of error that must be considered in order to allow conclusions regarding the degree of restricted diffusion. In this study, we have recalculated previously published data in order to minimize the effect of heterogeneity of the second kind, i.e. a transit-time dependence of the fractional extraction values. The method used makes it possible to calculate reliable intervals of confidence for the permeability surface area products, and hence for the permeability surface area product-ratios (and equivalent pore radius), taking into account the maximal theoretical impact of back diffusion on measured extraction data. After correction for transit-time dependent effects of heterogeneity, the permeability surface area product-ratio of Cr-EDTA to cyanocobalamin (vit. B12) from 48 measurements in eight rats was found to have a theoretical 'upper bound' of 2.63 +/- 0.06 and a lower bound of 2.10 +/- 0.07, corresponding to an equivalent pore radius of 60 to 109 A. This minimum pore radius estimate was even further reduced by corrections for plasma flow dependent reductions in overall extraction fraction (heterogeneity of the first kind) to 45 A, whereas the upper bound on pore radius was reduced to 60 A. These data strongly support the presence of marked restricted diffusion of small solutes in the maximally vasodilated rat hindquarter microvasculature.

Computer simulations of peritoneal fluid transport in CAPD

To model the changes in intraperitoneal dialysate volume (IPV) occurring over dwell time under various conditions in continuous ambulatory peritoneal dialysis (CAPD), we have, using a personal computer (PC), numerically integrated the phenomenological equations that describe the net ultrafiltration (UF) flow existing across the peritoneal membrane in every moment of a dwell. Computer modelling was performed according to a three-pore model of membrane selectivity as based on current concepts in capillary physiology. This model comprises small "paracellular" pores (radius approximately 47 A) and "large" pores (radius approximately 250 A), together accounting for approximately 98% of the total UF-coefficient (LpS), and also "transcellular" pores (pore radius approximately 4 to 5 A) accounting for 1.5% of LpS. Simulated curves made a good fit to IPV versus time data obtained experimentally in adult patients, using either 1.36 or 3.86% glucose dialysis solutions, under control conditions; when the peritoneal UF-coefficient was set to 0.082 ml/min/mm Hg, the glucose reflection coefficient was 0.043 and the peritoneal lymph flow was set to 0.3 ml/min. Also, theoretical predictions regarding the IPV versus time curves agreed well with the computer simulated results for perturbed values of effective peritoneal surface area, LpS, glucose permeability-surface area product (PS or "MTAC"), intraperitoneal dialysate volume and dialysate glucose concentration. Thus, increasing the peritoneal surface area caused the IPV versus time curves to peak earlier than during control, while the maximal volume ultrafiltered was not markedly affected. However, increasing the glucose PS caused both a reduction in the IPV versus time curve "peak time" and in the "peak height" of the curves. The latter pattern was also seen when the dialysate volume was reduced. It is suggested that computer modelling based on a three-pore model of membrane selectivity may be a useful tool for describing the IPV versus time relationships under various conditions in CAPD.

Vascular clearance by the reticuloendothelial system--measurements using two different-sized albumin colloids

Normal and reticuloendothelial system (RES) stimulated rats were examined with dynamic liver RES scintigraphy using a computerized gamma camera. 99Tcm-labelled albumin colloid, albures (radius 250 nm) or nanocoll (radius 25 nm), or both were used as test substances to study the kinetics of vascular clearance after RES stimulation. Registrations were made of 30 s per frame for 5 min and 300 s per frame for 15 min or 25 min and a region of interest (ROI) was indicated over the liver. Whole body and liver RES clearance rate constants (k) were calculated from the liver uptake vs time curve. Liver parenchyma blood flow was estimated with 133Xe washout technique. The blood clearance rate constant of albures in non-activated rats was twice that for nanocoll (1.08 +/- 0.05 vs 0.49 +/- 0.02 10(-2)s-1). There was no mutual interaction between the two colloids, implying that they may be eliminated from the blood-stream by slightly different processes. In zymosan-stimulated animals, nanocoll given in a single injection showed a significantly increased k-value. Neither the albures clearance rate constant nor the nanocoll/albures k-value ratio revealed RES macrophage activation. By contrast the nanocoll/albures ratio, calculated for the liver, rose significantly. The final colloid uptake in the liver revealed RES macrophage activation. No changes in liver parenchyma blood flow per g tissue could be registered after administration of zymosan. The nanocoll and albures colloid particles did not impair the normal liver parenchyma blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

A phenomenological interpretation of the variation in dialysate volume with dwell time in CAPD

Intraperitoneal fluid volume (IPV) changes versus time were followed in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) using a simple volume recovery method. In each patient dialysates containing 1.36 and 3.86 percent glucose as an osmotic agent were investigated. The patients' IPV versus time data were fitted to a function determined by four "arbitrary" coefficients, from which both the initial ultrafiltration (UF) rate immediately following intraperitoneal (i.p.) fluid instillation and the "final" peritoneal-to-blood fluid absorption rate could be assessed. The peritoneal osmotic conductance to glucose, that is, the peritoneal ultrafiltration coefficient (Kf), times the peritoneal osmotic reflection coefficient to glucose (sigma g), Kf sigma g, was determined using two related approaches. Kf sigma g is a major determinant of the transperitoneal volume exchange, and it was calculated to be 3.54 +/- 0.85 (+/- SE) and 3.81 +/- 0.52 microliters/min/mm Hg, respectively, depending on the assumption employed. Kf sigma g was further analysed according to a three-pore model of membrane permeability to determine the possible range of Kf and sigma g compatible with a peritoneal small solute sieving coefficient (phi) ranging from 0.3 to 0.61. According to these calculations both Kf and sigma g ranged from 0.043 to 0.081 (ml/min/mm Hg and dimensionless, respectively). The maximal peritoneal lymph flow (L) realistic according to this analysis, and compatible with a measured total peritoneal-to-blood fluid absorption rate of 1.25 +/- 0.14 ml/min, was 0.75 ml/min, the most plausible values, however, falling between 0.3 to 0.5 ml/min.

Pulmonary vascular permeability and resistance measurements in control and ANTU-injured dog lungs

Because questions have arisen regarding pulmonary vascular permeability and resistance measurements in isolated, perfused lungs, we sought to determine the 1) stability of repeated measurements of permeability and resistance in control lungs; and 2) magnitude of change in these measurements when permeability was greatly increased. Using blood-perfused dog lungs, we measured filtration coefficient (Kf) and isogravimetric capillary pressure (Pci) as indexes of vascular permeability, and we also determined total vascular resistance (Rt) as well as the segmental resistances using the double-occlusion pressure (Pdo). In a control group (n = 8), the base-line measurement of Kf (0.21 +/- 0.02 ml.min-1.cmH2O-1.100 g-1) and Pci (10.2 +/- 0.9 cmH2O) did not change over 4 h, indicating no changes in endothelial barrier function. Base-line Rt (13.9 +/- 2.6 cmH2O.l-1.min.100 g) also did not significantly increase. In a second group (n = 5), alpha-naphthylthiourea (ANTU) increased the initial Kf more than eight times (from 0.17 +/- 0.03 to 1.40 +/- 0.32 ml.min-1.cmH2O-1.100 g-1) and decreased Pci by 56% (from 9.4 +/- 0.6 to 4.1 +/- 0.4 cmH2O) at 1 h, indicating severely damaged endothelium. In addition, the Pdo determined during isogravimetric conditions correlated very well with Pci not only in control lungs (observed previously) but also in very permeable lungs (not previously reported). We conclude that this experimental model provides an excellent means of assessing changes in pulmonary microvascular permeability, with a spectrum ranging from no changes in hourly measurements for 4 h to obvious changes in permeability by 1 h.

Oleic acid reduces pulmonary microvascular sieving capacity in sheep

Changes in pulmonary microvascular permeability in sheep, after oleic acid injection, were studied using estimations of the osmotic reflection coefficient (sigma d) for total protein, albumin, immunoglobulins (Ig) G and M and calculation of the equivalent small and large pores of the microvessels. A chronic lung fistula was prepared in eight sheep. After a base-line period, left atrial pressure (Pla) was increased. Oleic acid (0.05 mg/kg body wt) was injected after a filtration-independent state had been obtained, and the spontaneously ventilating animals were then followed for 2 h. The sigma d for the normal lung was 0.65 +/- 0.03, 0.59 +/- 0.02, 0.72 +/- 0.04, and 0.84 +/- 0.02 for total protein, albumin, IgG, and IgM, respectively. The equivalent pore radii were 54 and 225 A. After oleic acid infusion, arterial pressure and arterial O2 tension decreased and leukocytes and platelets were consumed. At the end of the experiment, sigma d's were 0.27 +/- 0.04, 0.24 +/- 0.07, 0.33 +/- 0.06, and 0.55 +/- 0.04 for total protein, albumin, IgG, and IgM, respectively. The equivalent pore radii were 54 and 275 A, and the number of large pores was increased by 195%. The results indicate that oleic acid produces an increased vascular permeability by increasing the size and the numbers of large pores of the pulmonary microvascular walls.