Importance of molecular charge for the passage of endogenous macromolecules across continuous capillary walls, studied by serum clearance of lactate dehydrogenase (LDH) isoenzymes

Fluid and Electrolyte Transport across the Peritoneal Membrane during CAPD According to the Three-pore Model

In the present review, we summarize the principles governing the transport of fluid and electrolytes across the peritoneum during continuous ambulatory peritoneal dialysis (CAPD) in “average” patients and during ultrafiltration failure (UFF), according to the three-pore model of peritoneal transport. The UF volume curves as a function of dwell time [V( t)] are determined in their early phase by the glucose osmotic conductance [product of the UF coefficient (LpS) and the glucose reflection coefficient (σg)] of the peritoneum; in their middle portion by intraperitoneal volume and glucose diffusivity; and in their late portion by the LpS, Starling forces, and lymph flow. The most common cause of UFF is increased transport of small solutes (glucose) across the peritoneum, whereas the LpS is only moderately affected. Concerning peritoneal ion transport, ions that are already more or less fully equilibrated across the membrane at the start of the dwell, such as Na+(Cl–), Ca2+, and Mg2+, have a convection-dominated transport. The removal of these ions is proportional to UF volume (approximately 10 mmol/L Na+and 0.12 mmol/L Ca2+removed per deciliter UF in 4 hours).

The present article examines the impact on fluid and solute transport of varying concentrations of Ca2+and Na+in peritoneal dialysis solutions. Particularly, the effect of “ultralow” sodium solutions on transport and UF is simulated and discussed. Ions with high initial concentration gradients across the peritoneum, such as K+, phosphate, and bicarbonate, display a diffusion-dominated transport. The transport of these ions can be adequately described by non-electrolyte equations. However, for ions that are in (or near) their diffusion equilibrium over the peritoneum (Na+, Ca2+, Mg2+), more complex ion transport equations need to be used. Due to the complexity of these equations, however, non-electrolyte transport formalism is commonly employed, which leads to a marked underestimation of mass transfer area coefficients (PS). This can be avoided by determining the PS when transperitoneal ion concentration gradients are steep.

Peritoneal Protein Loss, Leakage or Clearance in Peritoneal Dialysis, Where do we Stand?

Peritoneal protein loss (PPL) through peritoneal effluent has been a well-recognized detrimental result of peritoneal dialysis (PD) treatment since its inception. Investigation has focused mainly on PPL quantitative and qualitative determinations and evaluation of its prognostic value.

A comprehensive review of the pathophysiology of PPL (3-pore model revisited), methods of quantification, dialysate protein composition, and impact on clinical outcomes is presented herein. The author summarizes a brief analysis of associated cardiovascular disease and nutritional consequences, exploring the controversial cause-effect on mortality and technique failure.

Therapeutic modalities aiming to reduce PPL (angiotensin-converting enzyme inhibitors [ACEI]s and vitamin D therapies) were explored, although it is unclear whether PPL represents a valid therapeutic target or, on the other hand, is solely a manifestation of endothelial dysfunction.

Effects of pressure and electrical charge on macromolecular transport across bovine lens basement membrane

Molecular transport through the basement membrane is important for a number of physiological functions, and dysregulation of basement membrane architecture can have serious pathological consequences. The structure-function relationships that govern molecular transport in basement membranes are not fully understood. The basement membrane from the lens capsule of the eye is a collagen IV-rich matrix that can easily be extracted and manipulated in vitro. As such, it provides a convenient model for studying the functional relationships that govern molecular transport in basement membranes. Here we investigate the effects of increased transmembrane pressure and solute electrical charge on the transport properties of the lens basement membrane (LBM) from the bovine eye. Pressure-permeability relationships in LBM transport were governed primarily by changes in diffusive and convective contributions to solute flux and not by pressure-dependent changes in intrinsic membrane properties. The solute electrical charge had a minimal but statistically significant effect on solute transport through the LBM that was opposite of the expected electrokinetic behavior. The observed transport characteristics of the LBM are discussed in the context of established membrane transport modeling and previous work on the effects of pressure and electrical charge in other basement membrane systems.

Effect of charge on microvascular permeability in early experimental sepsis in the rat

A key feature of sepsis is hypovolemia due to increased microvascular permeability. It has been suggested that the negative charge of albumin and of the endothelial glycocalyx is important for maintenance of the normally low permeability for albumin. Here we tested the hypothesis that charge effects contribute to the increased permeability in sepsis. Transcapillary escape rate (TER) and initial distribution volume for (125)I-labeled bovine serum albumin (BSA, isoelectric point pH 4.6) and for (131)I-labeled charge modified BSA (cBSA, average isoelectric point, pH 7.1) was measured 3h after sepsis was induced by cecal ligation and incision (CLI) (n=11) and in control animals (n=12). The importance of charge for permeability in sepsis was estimated by comparing the ratio between TER for cBSA and TER for BSA during control conditions to that after CLI. Plasma concentration of the glycocalyx component glycosaminoglycans (GAGs) was measured in separate control and CLI animals. The initial distribution volume for BSA and cBSA in control animals was 38 ± 3 ml/kg and 47 ± 4 mL/kg and decreased by 17% and 19%, respectively, following CLI. TER for BSA increased from 16.7 ± 4.1% in the controls to 20.1 ± 1.9% following CLI. Corresponding values for cBSA were 26.7 ± 5.6% and 29.8 ± 3.5%, respectively. The ratio between TER for cBSA and TER for BSA was 1.62 ± 0.1 in the control group and 1.49 ± 0.1 following CLI (p<0.05). Plasma GAG concentrations were higher in CLI animals than in the control group. We conclude that CLI induce hypovolemia secondary to increased microvascular permeability. Negative charge contributes to the normally low permeability of albumin and the importance of charge is decreased in early experimental sepsis. The observed charge effects are associated with CLI-induced breakdown of the glycocalyx.

Permeability of Peritoneal and Glomerular Capillaries: What are the Differences According to Pore Theory?

Pore and fiber-matrix theory can both be used to model the peritoneal and glomerular filtration barriers in an attempt to shed light on their differing structure–function relationships. The glomerular filtration barrier (GFB) is structurally more specialized, morphologically complex, and also highly dynamic; but paradoxically, because of its uniformity, it conforms more closely to the predictions of pore theory than does the peritoneum, and it in fact resembles a more simple synthetic membrane. Compared with the peritoneal capillary wall, the GFB has no transcellular “third” pores (aquaporins), and it is far less leaky and more size-selective to proteins, mainly as a result of having far fewer “large” pores. It does have charge-selective properties, although these are considered much less important in excluding albumin than was once thought, and it is also able to select polymers according to their shape and flexibility. Even this property might reflect the relative uniformity of the GFB, which has a high diffusion area and short diffusion distances, compared with the peritoneal barrier, which behaves more like a gel filtration column. Furthermore, the length of the diffusion path across the peritoneal membrane is much greater for small solutes, given the relatively high ultrafiltration coefficient for that membrane compared with the GFB—a situation that reflects both the tortuosity of the interendothelial clefts and the distribution of peritoneal capillaries within the interstitium. These comparisons reveal the peritoneal barrier as a relatively complex structure to model; and yet this model may be more representative of the general microcirculation, and thus shed light on systemic endothelial function in renal failure.

Interstitial exclusion of positively and negatively charged IgG in rat skin and muscle

Volume exclusion, i.e., the space not available for a specific probe, may be dependent on the probe charge. Therefore, interstitial exclusion was measured for positively and negatively charged immunoglobulin (IgG) in skin and muscle of rats by using a continuous infusion method (30). Steady-state concentration of (125)I-labeled IgG 1 (pI = 8.7) and (131)I- labeled IgG 4 (pI = 6.6) was maintained by infusion of tracer for 120-168 h with an implanted osmotic pump. At the end of the infusion period and before tissue sampling, the rat was anesthetized and nephrectomized, and (51)Cr-labeled EDTA was injected and allowed 4 h for equilibration to measure interstitial fluid volume (V(i)). Interstitial fluid was isolated from skin and muscle by using nylon wicks implanted post mortem. The relative IgG available space was measured as the ratio between labeled IgG and (51)Cr-labeled EDTA wick fluid equivalent spaces, and relative excluded volume fraction (V(e)/V(i)) was calculated as 1--V(a)/V(i). V(e)/V(i) in hindlimb skin averaged 0.37 +/- 0.05 (SE) and 0.65 +/- 0.06 (P < 0.01) for IgG 1 and 4, respectively, with corresponding figures of 0.24 +/- 0.05 and 0.51 +/- 0.04 (P < 0.01) in hindlimb muscle (n = 9 for both tissues). These experiments suggest that fixed negative charges, most likely glycosaminoglycans, influence distribution of macromolecules in the interstitium and therefore affect interstitial fluid balance.

A gel-membrane model of glomerular charge and size selectivity in series

We have analyzed glomerular sieving data from humans, rats in vivo, and from isolated perfused rat kidneys (IPK) and present a unifying hypothesis that seems to resolve most of the conflicting results that exist in the literature. Particularly important are the data obtained in the cooled IPK, because they allow a variety of experimental conditions for careful analysis of the glomerular barrier; conditions that never can be obtained in vivo. The data strongly support the classic concept of a negative charge barrier, but separate components seem to be responsible for charge and size selectivity. The new model is composed of a dynamic gel and a more static membrane layer. First, the charged gel structure close to the blood compartment has a charge density of 35-45 meq/l, reducing the concentration of albumin to 5-10% of that in plasma, due to ion-ion interactions. Second, the size-selective structure has numerous functional small pores (radius 45-50 A) and far less frequent large pores (radius 75-115 A), the latter accounting for 1% of the total hydraulic conductance. Both structures are required for the maintenance of an intact glomerular barrier.

Dextran, gelatin, and hydroxyethyl starch do not affect permeability for albumin in cat skeletal muscle

OBJECTIVE

To evaluate the effects of the three commercially available colloid solutions, 6% dextran 70, 6% hydroxyethyl starch (HES) 200/0.5, and 3.5% urea-linked gelatin on permeability for human albumin in a skeletal muscle in vivo model by evaluating their effects on the reflection coefficient for albumin.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

SUBJECTS

Eighteen adult cats.

INTERVENTIONS

The autoperfused and denervated calf muscles of the cat hindlimb were placed in a plethysmograph. The transvascular fluid absorption induced by an increase in the colloid osmotic pressure following a fixed intravenous bolus of human albumin was analyzed, first before start of, and then during an intra-arterial infusion to, the muscle preparation of the synthetic colloid to be analyzed. Capillary filtration coefficient as a measure of microvascular fluid permeability (conductance) was analyzed before and after start of the synthetic colloid.

MEASUREMENTS AND MAIN RESULTS

Arterial blood flow, arterial and venous blood pressures, total vascular resistance, tissue volume changes, capillary filtration coefficient, and plasma volume were measured before and during the colloid infusion. According to the Starling fluid equilibrium, the ratio between the reflection coefficients for albumin on two occasions (before and after infusion of the synthetic colloid) can be calculated from the maximum osmotic absorption rates induced by a fixed intravenous bolus infusion of albumin and from the capillary filtration coefficients. Obtained data were adjusted for different plasma volume at the two occasions. We found that none of the three synthetic colloids analyzed had any significant effect on the reflection coefficient for albumin.

CONCLUSION

An effect on albumin microvascular permeability of the synthetic colloids dextran 70, HES 200/0.5, and urea-linked gelatin could not be shown by a method analyzing their effect on the reflection coefficient for albumin.

Glomerular size and charge selectivity in the rat as revealed by FITC-ficoll and albumin

The fractional clearances (theta) for FITC-Ficoll and albumin were estimated in isolated perfused rat kidneys in which the tubular activity was inhibited by low temperature (8 degrees C) and/or 10 mM NH(4)Cl. The Ficoll data were analyzed according to a two-pore model giving small and large pore radii of 46 A and 80-87 A, respectively. The estimated negative charge density was 35-45 meq/l at 8 degrees C. Perfusion with erythrocyte-free solutions of kidneys at 37 degrees C reduced glomerular size and charge permselectivity. Thus the large pore fraction of the glomerular filtrate (f(L)) was 1.64% at 37 degrees C compared with 0.94% at 8 degrees C. The theta for albumin was four times higher at 37 degrees C than at 8 degrees C (0.86% vs. 0.19%, respectively). NH(4)Cl caused further irreversible damage to the glomerular barrier. We conclude that there are no deleterious effects on the glomerular barrier of a reduction in temperature from 37 degrees C to 8 degrees C. Therefore our data seem to disprove the hypothesis of low glomerular permselectivity and transtubular uptake of intact albumin and support the classic concept of a highly selective glomerular barrier.

Vascular permeability in a human tumour xenograft: molecular charge dependence

Molecular charge is one of the main determinants of transvascular transport. There are, however, no data available on the effect of molecular charge on microvascular permeability of macromolecules in solid tumours. To this end, we measured tumour microvascular permeability to different proteins having similar size but different charge. Measurements were performed in the human colon adenocarcinoma LS174T transplanted in transparent dorsal skinfold chambers in severe combined immunodeficient (SCID) mice. Bovine serum albumin (BSA) and IgG were fluorescently labelled and were either cationized by conjugation with hexamethylenediamine or anionized by succinylation. The molecules were injected i.v. and the fluorescence in tumour tissue was quantified by intravital fluorescence microscopy. The fluorescence intensity and pharmacokinetic data were used to calculate the microvascular permeability. We found that tumour vascular permeability of cationized BSA (pI-range: 8.6-9.1) and IgG (pI: 8.6-9.3) was more than two-fold higher (4.25 and 4.65x10(-7) cm s(-1)) than that of the anionized BSA (pI approximately 2.0) and IgG (pI: 3.0-3.9; 1.11 and 1.93x10(-7) cm s(-1), respectively). Our results indicate that positively charged molecules extravasate faster in solid tumours compared to the similar-sized compounds with neutral or negative charges. However, the plasma clearance of cationic molecules was approximately 2x faster than that of anionic ones, indicating that the modification of proteins enhances drug delivery to normal organs as well. Therefore, caution should be exercised when such a strategy is used to improve drug and gene delivery to solid tumours.

Effects on capillary fluid permeability and fluid exchange of albumin, dextran, gelatin, and hydroxyethyl starch in cat skeletal muscle

OBJECTIVE

To evaluate the peripheral hemodynamic effects in a skeletal muscle in vivo model of the four commercially available colloid solutions, 20% human albumin, 6% dextran-70, 6% hydroxyethyl starch 200/0.5 (HES), and 3.5% urea-linked gelatin.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

SUBJECTS

Fifteen adult cats.

INTERVENTIONS

The isolated, autoperfused, and denervated calf muscles of the cat hindlimb were placed in a plethysmograph. The colloids tested were given intra-arterially to the muscles in increasing rates of clinically relevant doses.

MEASUREMENTS AND MAIN RESULTS

Arterial blood flow, arterial and venous blood pressures, total vascular resistance, tissue volume changes, and capillary filtration coefficient were measured before, during, and after the colloid infusions. The altered capillary filtration coefficient reflects a change in capillary fluid permeability. The capillary filtration coefficient was decreased by albumin and dextran, not affected by HES, and increased by urea-linked gelatin. Albumin induced transcapillary fluid absorption, gelatin induced transcapillary filtration, and no transcapillary fluid exchange was observed with dextran and HES. After discontinuation of the infusions, HES and gelatin induced a rebound transcapillary filtration. No such effect was seen after dextran and albumin. All colloids increased muscle blood flow.

CONCLUSION

We conclude that capillary fluid permeability is decreased by albumin and dextran, unchanged by HES, and increased by gelatin. This and the differences in the rebound effect may contribute to the differences in the plasma volume expanding properties of the respective colloid. The increased blood flow induced by the colloids was more an effect of reduced vascular tone than of lowered blood viscosity.

Glomerular charge selectivity for proteins larger than serum albumin as revealed by lactate dehydrogenase isoforms

It is well known that macromolecules like albumin are markedly restricted in their passage across the glomerular capillary wall. However, the relative importance of solute size, charge and shape is currently debated since much of the previous work is based on dextran in neutral or charge-modified forms. These polymers have certain drawbacks that make them less suitable for analysis of capillary permeability and the notion of a glomerular charge barrier has therefore been questioned. Moreover, macromolecules larger than albumin (mol. wt. 69,000) have been suggested to pass through nonselective 'shunt' pathways. In order to study glomerular permeability, isolated rat kidneys were perfused with albumin solutions containing trace amounts of two differently radiolabelled isoenzymes of lactate dehydrogenase (LDH) at low temperature to inhibit tubular function. The isoenzymes have similar size (mol. wt. 140,000) and shape but differ in charge, one carrying a negative net surface charge (LDH1, -19) and the other being slightly cationic (LDH5, +2). The urine and perfusate samples were subjected to high pressure liquid chromatography (HPLC) gel-filtration to allow for measurements of intact LDH. The fractional clearance was 0.11% +/- 0.04% for the anionic LDH1 and 0.56% +/- 0.07% for LDH5, whereas that for albumin was 0.21% +/- 0.03% at a glomerular filtration rate of 0.11 +/- 0.01 mL min-1 g-1 kidney wet weight. The results were analysed using a homogenously charged membrane model and are compatible with a charge density of 35 mEq L-1, with 95% confidence interval of 26-41 mEq L-1. These findings suggest a significant glomerular charge selectivity for proteins substantially larger than albumin. The charge density is, however, far less than estimated from dextran studies.

Increased peritoneal permeability to albumin in streptozotocin diabetic rats

The mechanism behind the increased peritoneal permeability to albumin in diabetics is still unclear. In this study, streptozotocin diabetic rats developed albuminuria and significantly increased D/P of albumin after the fourth week of disease, reaching peak levels at the end of the 24 week period of follow-up. Coincidentally, extravasation of albumin to the interstitial tissue was evaluated with the Evans-blue method. Age-matched control rats showed Evans-blue concentrations of 0.023 +/- 0.013 micrograms/100 mg of dry tissue, whereas in diabetics the numbers were 1.22 +/- 0.719 micrograms (P < 0.001). Perfusion with Ruthenium-Red (RR) done in control at zero time, and in age-matched intact as well as in diabetic rats after 24 weeks of disease showed that the density distribution of capillary subendothelial anionic sites was significantly lower for diabetics (13 +/- 3/microns basement membrane vs. 31 +/- 3 and 34 +/- 4 in control groups; P < 0.001). Similar findings were made on the mesenteric submesothelial basement membrane. Mean density of RR decorated anionic sites was 12 +/- 2/microns basement membrane in diabetics, whereas those observed in both control groups were 31 +/- 2 and 31 +/- 3/microns (P < 0.001). Therefore, this reduced density of microvascular and submesothelial negative charges, equivalent to that induced by diabetes in other capillary beds, appears to be at the origin of the decreased permselectivity of the diabetic peritoneum for anionic serum albumin.

Altering the blood-brain barrier in the rat by intracarotid infusion of polycations: a comparison between protamine, poly-L-lysine and poly-L-arginine

To evaluate the role of surface charge for the blood-brain barrier permeability, the albumin content was determined in the cerebrospinal fluid and in the brain 1 h after intracarotid infusion of protamine sulphate, a natural polycationic protein with a high content of arginine (mol. wt 4000-4400), poly-L-arginine (mol. wt 11,600) or poly-L-lysine (mol. wt 10,200). Five milligrams (4 x 10(-4) mmol) poly-L-arginine increased the albumin content in the brain 15 times more than 5 mg (5 x 10(-4) mmol) poly-L-lysine (P < 0.001) and 3.5 times more than 5 mg (1 x 10(-3) mmol) protamine (P < 0.001); the difference between protamine and poly-L-lysine was also significant (P < 0.05). After 0.5 mg (4 x 10(-4) mmol) poly-L-arginine the albumin extravasation was still higher than after 5 mg protamine (P < 0.01) and 5 mg poly-L-lysine (P < 0.001). Cisternal albumin increased from control values 0.08 mg ml-1 to 0.30, 0.46 and 1.21 mg ml-1 in rats given 5 mg poly-L-lysine, protamine and poly-L-arginine, respectively (P < 0.01 for difference between arginine and the other two substances). The higher mol. wt and positive charge of poly-L-arginine may at least in part explain the more pronounced albumin leakage after arginine than after protamine. However, the difference between poly-L-arginine and poly-L-lysine suggests that other factors, possibly related to the guanidino groups, contribute to the blood-brain barrier opening by poly-L-arginine.

Molecular charge influences transperitoneal macromolecule transport

The influence of molecular charge on macromolecule transport during peritoneal dialysis was assessed by determining transperitoneal transport rates for fluorescent-labeled macromolecules (molecular radii from 15 to 40 A) that differed only in molecular charge: neutral dextran, anionic dextran sulfate and cationic diethylaminoethyl (DEAE) dextran. The test macromolecules were infused into the bloodstream of unanesthetized New Zealand White rabbits at a constant rate, and isotonic dialysis solution (40 ml/kg) was instilled into the peritoneal cavity. Blood and dialysate samples were taken hourly over a four hour dwell. Transperitoneal transport rates were assessed by calculating both the dialysate to plasma concentration ratio at four hours and the permeability-area product for the peritoneum, the latter parameter determined from the increase in the dialysate concentration with time. Transport rates for DEAE dextran were less (P < 0.05) than those for both neutral dextran and dextran sulfate; transport rates for neutral dextran and dextran sulfate were not different. Moreover, transperitoneal transport rates for fluorescent-labeled DEAE dextran were not affected by adding unlabeled DEAE dextran to the intravenous infusion solution, an observation suggesting that low transport rates for DEAE dextran were not due to its binding to plasma protein. We conclude that molecular charge is a determinant of transperitoneal macromolecule transport.

Addition of purified orosomucoid preserves the glomerular permeability for albumin in isolated perfused rat kidneys

The serum protein, orosomucoid has been shown to be essential for the maintenance of normal capillary permeability in several different organs, including the kidney. Thus, the clearance of albumin was found to be almost fivefold higher in the absence of orosomucoid in a previous study on isolated rat kidneys, perfused with either of two commercially available human albumin solutions of similar composition, but differing in their content of orosomucoid (0.21 g l-1 vs. < 0.005 g l-1). The following experiments were performed in order to verify the hypothesis that this effect on glomerular permselectivity was due to orosomucoid per se and not to other ingredients in the two solutions. Both kidneys of 12 rats were isolated and perfused with identical albumin solutions without orosomucoid, but with the addition of purified orosomucoid (0.25 g l-1) to one of the kidneys. No significant differences in vascular resistance, urine flow or glomerular filtration rate (GFR), which was found to be 27 +/- 2 ml min-1 100 g-1, were observed between the two groups of kidneys. The fractional clearance of albumin (theta) was initially similar for both kidneys (0.0022 +/- 0.0002). In the absence of orosomucoid, theta gradually increased to 0.0076 +/- 0.0013 after 1 h of perfusion compared to 0.0040 +/- 0.0006 for the kidneys with orosomucoid added to the perfusate (P < 0.001, n = 12). We conclude that the plasma glycoprotein orosomucoid indeed plays an important role in regulating the dynamic properties of the glomerular capillary wall by reducing the permeability towards macromolecules such as albumin.

Transcapillary filtration of plasma protein in long-term type 1 (insulin dependent) diabetic patients

To study the pathophysiologic mechanism leading to increased transcapillar sieving of albumin in patients with diabetic nephropathy, subcutaneous interstitial concentrations of albumin, transferrin, total IgG and IgG-4 was measured in 30 long-term type 1 diabetic patients using the skin suction blister method. Eight normal subjects served as controls. The patients were divided in groups according to their urinary albumin excretion: normal (n = 11), micro-albuminuria (n = 9), and clinical nephropathy (n = 10). Results were expressed as the blister to serum concentration ratio (Cb:Cs) of each macromolecule. Normoalbuminuric patients had lower Cb:CsIgG ratio than healthy controls (0.28 +/- 0.04 vs. 0.35 +/- 0.09, p = 0.03). The lowest Cb:CsIgG ratio (0.23 +/- 0.07) was found in patients with clinical nephropathy. The same trend could be demonstrated in the other Cb:Cs ratios, but differences there were not significant. No differences related to capillary charge- or size-selectivity could be demonstrated between the groups. The results might reflect an increase of intracapillary hydrostatic pressure or increased capillary hydraulic permeability in the diabetic state per se, augmented during the development of microvascular complications.

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)

Passage of low density lipoproteins through monolayers of human arterial endothelial cells. Effects of vasoactive substances in an in vitro model

The endothelium controls the influx of lipoproteins into the arterial wall, a process that may be disturbed in arteriosclerotic blood vessels. We have used an in vitro model to investigate the characteristics of the passage of low density lipoproteins (LDL) through monolayers of human arterial endothelial cells. Umbilical artery, aorta, or carotid artery endothelial cells were cultured on polycarbonate filters and formed a tight monolayer in which the cells were connected by tight junctions. Passage of 125I-LDL through these monolayers proceeded linearly over a 24-hour period. It was threefold lower through monolayers of aorta or carotid artery cells than through monolayers of umbilical artery cells. The LDL passage process did not show saturation with LDL concentrations up to 800 micrograms/ml LDL-protein (i.e., 1.6 nmol/ml apolipoprotein B) between 2 and 4 hours after addition. However, during the first 30 to 60 minutes after addition of high concentrations of LDL, a reduction of the passage rate of both LDL and peroxidase, resulting in an apparent saturation of the passage process, was observed. The passage rate of the negatively charged acetylated LDL was twofold lower than that of native LDL. Addition of histamine to the endothelial monolayer resulted in a large, but transient, increase in permeability paralleled by a decrease in electrical resistance. The effects of histamine were mediated via an H1 receptor. Thrombin and Ca++ ionophore also induced an increase in permeability of the monolayer, while bradykinin did not. The effects of histamine and thrombin were paralleled by a rapid and marked increase in cytoplasmatic Ca++ concentration of the endothelial cells, while bradykinin induced only a small increase. Although the cyclic adenosine 5'-monophosphate-elevating agent, forskolin, markedly decreased the basal rate of LDL passage through the endothelial cell monolayers, it did not change the relative increase in permeability induced by histamine. Thus, histamine induces small, but significant, increases in the permeability of tight endothelial cell monolayers.

Cyanide metabolism in the isolated, perfused, bloodless hindlimbs or liver of the rat

Female CD1 rats weighing 250-300 g were anesthetized with ip pentobarbital, 50 mg/kg, and either the liver or the hindlimbs were surgically isolated and perfused in situ with a Krebs-Henseleit buffer, pH 7.4, at 38 degrees C, containing 40 g/liter dextran and 30 mg/liter papaverine. Perfusion pressure was continuously monitored, and in most experiments, flow was maintained at the physiological rate of 8.5 ml/min. In-line Clark-type electrodes allowed the continuous measurement of oxygen extraction. Potassium cyanide to 0.15 mM was usually added to the perfusate just prior to the start of a run. After a period of equilibration, samples of the perfusate were taken periodically for cyanide (CN) and thiocyanate (SCN) analyses. The results were used to determine CN extraction ratios or clearance and rates of SCN formation. When it was apparent that a steady state had been reached with respect to the above, sodium thiosulfate (TS) was added to the perfusate (to 0.1, 1.0, or 2.0 mM), and periodic samples were again collected after an equilibration period. In the absence of albumin, TS rapidly and significantly increased the rate of conversion of CN to SCN in both the liver and the hindlimbs. The rate of CN clearance in milliliters per minute per kilogram perfused tissue was 20-fold greater in the liver than in the hindlimbs. However, when the results from hindlimbs were extrapolated to the total body skeletal muscle mass, the rate of CN clearance by the total liver mass was only 1.5-fold greater than in total muscle mass. In the absence of TS, total muscle mass cleared CN at a rate that was 2.6-fold greater than the total liver mass, but the rates in both tissues were very much less than in the presence of TS. The extraction ratio for CN in the liver was 0.8 and the clearance was dependent on the flow rate. The extraction ratio for CN in the hindlimbs was 0.2, and the clearance was independent of the flow rate. Thus, CN clearance by the liver probably increases (within limits) with increasing portal blood flow. Evidence was obtained for the existence of a significant CN "sink," particularly in the liver, which presumably represents reversible binding to unknown tissue constituents.

Transcapillary clearance of albumin in rat skeletal muscle monitored by external detection. Effects of alterations in capillary surface area

The effects of noradrenaline (NA)-induced vasoconstriction on the transcapillary passage of albumin was evaluated by an external detection technique, allowing repetitive measurements of albumin clearance (Cl) during various conditions (in the same animal). Six isolated rat hindquarters were perfused with serum-albumin solutions during maximal vasodilation (papaverine 90 microM) and Cl was determined at different net filtration rates (Fv) induced by elevations of venous pressure. Then, the perfusate was changed to one of similar composition but containing noradrenaline (2-4 microM), and the procedure of determining Cl vs. Fv was repeated. Tissue accumulation of [99mTc]albumin was expressed in terms of clearance, using the isogravimetric Cl of defined muscle samples during maximum vasodilation in separate experiments as reference, the latter being 0.0246 +/- 0.0012 ml min-1 per 100 g. Noradrenaline caused an increase in vascular resistance from 2 to 14 mmHg min 100 g ml-1, while the Cl vs. Fv relationship was shifted downwards in a fashion parallel with the control Cl vs. Fv curve. For Fv = 0, Cl was 0.0101 +/- 0.0014 ml min-1 per 100 g during NA challenge. The average reflection coefficient for albumin (sigma tot) was 0.92 +/- 0.01 irrespective of vascular tone. Thus, both albumin clearance and the capillary filtration coefficient (CFC) seem to vary in direct proportion to the capillary surface area available for exchange.

Molecular charge effects on the protein permeability of the guinea-pig placenta

The effect of molecular charge on protein permeability of the dually perfused guinea-pig placenta has been investigated by measurement of the permeability surface area products (PS) and observation of the ultrastructural localization of cationic horseradish peroxidase (cHRP) and anionic horseradish peroxidase (aHRP) molecules. Steady-state PS calculated from the experimental data was 0.032 +/- 0.0045 and 0.0045 +/- 0.0008 ml min-1 (mean +/- s.e.m.) for cHRP and aHRP respectively (P less than 0.01). The PS for a diffusional marker, 51Cr-ethylenediaminetetraacetic acid, showed no significant difference between the two groups. Ultrastructurally, placentae perfused with cHRP showed fewer microvilli and a dilated interstitial space compared with placentae perfused with aHRP; large vacuoles were also found in the syncytiotrophoblast in the former but not in the latter tissue. Reaction product in placentae perfused with cHRP was localized to the maternal-facing plasma membrane of the syncytiotrophoblast, in vacuoles and vesicles of the syncytiotrophoblast, and also in the basement membrane of the interstitial space, whereas placentae perfused with aHRP only had reaction product in vesicles in the syncytiotrophoblast. These results suggest that anionic sites in the guinea-pig placenta affect its permeability to charged proteins, cationic molecules inducing structural changes associated with increased permeability.

Interstitial distribution of charged macromolecules in the dog lung: a kinetic model

A mathematic model was constructed to investigate conflicting physiologic data concerning the charge effect of continuous capillaries to macromolecules in the lung. We simulated the equilibration kinetics of lactate dehydrogenase (MR 4.2 nM) isozymes LDH 1 (pI = 5.0) and LDH 5 (pI = 7.9) between plasma and lymph using previously measured permeability coefficients, lung tissue distribution volumes (VA) and plasma concentrations (CP) in lung tissue. Our hypothesis is that the fixed anionic charges in interstitium, basement membrane, and cell surfaces determine equilibration rather than charged membrane effects at the capillary barrier, so the same capillary permeability coefficients were used for both isozymes. Capillary filtration rates and protein fluxes were calculated using conventional flux equations. Initial conditions at baseline and increased left atrial pressures (PLA) were those measured in animal studies. Simulated equilibration of isozymes over 30 h in the model at baseline capillary pressures accurately predicted the observed differences in lymph/plasma concentration ratios (CL/CP) between isotopes at 4 h and equilibration of these ratios at 24 h. Quantitative prediction of isozyme CL/CP ratios was also obtained at increased PLA. However, an additional cation selective compartment representing the surface glycocalyx was required to accurately simulate the initial higher transcapillary clearances of cationic LDH 5. Thus experimental data supporting the negative barrier, positive barrier, and no charge barrier hypotheses were accurately reproduced by the model using only the observed differences in interstitial partitioning of isozymes without differences in capillary selectivity.

Permeability of renal capillaries. II. Transport of neutral and charged protein molecular probes

The permselectivity of the renal capillaries was investigated from the transport of a series of molecular probes: inulin, positive (+2, net charge at pH 7.4), neutral (0), and negative (-6) myoglobin, neutral (0) and negative (-14) horseradish peroxidase (HRP) and two isomers of lactate dehydrogenase (LDH), namely the positively charged (+2) LDH-M4 and the negatively charged (-19) LDH-H4. The determination of the concentration of tracer proteins necessitated gel separation of both plasma and renal hilar lymph. The reason for this is that the proteins, after filtration, will be reabsorbed and degraded by the proximal tubular cells into small molecular compounds (amino acids), which will return to both the renal interstitium and systemic plasma. Even if this degradation is of low degree, as for high-molecular-weight proteins, separation is still required, since the relative lymph concentration (plasma concentration put at 1) is also low, that is, even small amounts of low molecular compounds will distort the relative lymph concentration obtained. The transport from plasma to renal hilar lymph of the tracer molecules fell with increasing Stokes-Einstein radius. The relative lymph concentration of the 11 A inulin was 1.06 +/- 0.03 (mean +/- SEM), of the neutral 17.5 A myoglobin 0.76 +/- 0.05, of the neutral 32 A HRP 0.32 +/- 0.02 and of the neutral 46 A LDH 0.12 +/- 0.01. The data are compatible with a two-pore system. The negative tracer molecules were in general proportionally more restricted than the neutral (or positive) moieties (P less than 0.001) thus suggesting a negatively charged peritubular capillary membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Orosomucoid as one of the serum components contributing to normal capillary permselectivity in rat skeletal muscle

The effects of human serum orosomucoid (normal serum concentration 0.7-1.0 g l-1) on capillary permeability were investigated in 12 isolated maximally vasodilated rat hindquarters perfused with bovine serum albumin (50 g l-1) in modified Tyrode. Measurements were made of capillary filtration coefficient (CFC), permeability surface area product (PS) for vitamin B12 and isogravimetric clearance of radiolabelled albumin (Cl alb). The results were compared with those obtained using perfusates without addition of orosomucoid ('albumin group') or perfusates containing horse serum ('serum group'). Clearance of albumin was almost four-fold higher in the albumin than in the serum group, 0.0895 +/- 0.0066 (n = 12) and 0.0252 +/- 0.016 ml min-1 per 100 g (n = 18), respectively, while intermediate Cl alb values were obtained with human orosomucoid in the perfusate (greater than 0.1 g l-1), 0.0436 +/- 0.0034 ml min-1 per 100 g) (n = 8). These changes in Cl alb were not accompanied by any differences in CFC or PS. We conclude that orosomucoid is one of the components in serum (besides albumin) needed for the maintenance of normal permselectivity of the capillary walls of rat skeletal muscle. Alternatively, human orosomucoid is structurally related to other substances exerting this 'serum effect'.

Capillary permeability in rat hindquarters as determined by estimations of capillary reflection coefficients

Osmotic reflection coefficients (sigma) for a variety of solutes ranging from NaCl to albumin were determined in perfused maximally vasodilated rat hindquarters employing the osmotic transient method (Vargas & Johnson 1964). Measurements were performed at high flows and using short tubings with small volumes. Intracapillary solute concentrations of the osmotic transients were measured or estimated for solutes of the size of inulin or smaller. The PS for Cr-EDTA and cyanocobalamine were determined repeatedly in half of the experiments using an on-line modification of the single injection (indicator diffusion) method (Rippe & Stage 1978) and capillary filtration coefficients (CFC or LpS) were followed in all experiments. The capillary osmotic reflection coefficient was determined to 0.05 for NaCl, to 0.08 for sucrose, to 0.39 for inulin, to 0.57 for myoglobin and to 0.87 for albumin. These reflection coefficients were compatible with a 'small pore radius' of approximately 40 A (slit width (w) of approximately 50 A) according to modern hydrodynamic theories for the reflection coefficient and the parallel transcapillary pathway hypothesis. The best fit of the osmotic transient data to current theories for the reflection coefficient occurred if the major portion (86-87%) of the hydraulic conductivity (Lp) was accounted for by this paracellular 'small pore' (slit) pathway and if 3.0-4.1% of Lp could be ascribed to a transcellular pathway (sigma approximately I) while the remaining fraction (10%) of Lp was accounted for by a non-selective paracellular pathway (sigma approximately o); that is, by 'large pores'.

Restricted diffusion of CrEDTA and cyanocobalamine across the exchange vessels in rat hindquarters

The degree of diffusional restriction of skeletal muscle capillary walls to small solutes was estimated from the permeability surface area products (PS) of CrEDTA (MW = 341) and cyanocobalamine (MW = 1355), using computerized 'on-line' recordings of venous single injection indicator dilution curves. Experiments were performed on isolated perfused maximally vasodilated rat hindquarters during largely isogravimetric conditions and the arrangements allowed for measurements of capillary filtration coefficients (CFC). Extraction of tracer varied markedly as a function of transit time and, furthermore, PS increased with increasing flows, both these phenomena indicating tissue and flow heterogeneity. At maximal flows the disturbing influence of heterogeneity will be minimal and hence the diffusion capacities obtained by extrapolating PS area to infinite flows, so called PS tot values, were considered to give the best estimation of the 'true' capillary diffusion capacities. The value of PS tot was 12.9 +/- 0.5 for CrEDTA and 5.1 +/- 0.3 ml min-1 per 100 g for vitamin B12. The calculated PS tot ratio of 2.59 +/- 0.11 indicates restricted diffusion through equivalent pores of radius 53 A, whereas the ratio of the free diffusion coefficients for these solutes is 1.79. Using PS peak for the calculations (totally neglecting heterogeneity) the pore radius was, however, markedly overestimated. Thus, for a PS-ratio of 1.89 +/- 0.04 for CrEDTA vs. B12 an equivalent pore radius of 300 A was calculated. Also, using PS area (only partly correcting for heterogeneity) overestimated the pore radius (70 A) from a mean PS-ratio of 2.33 +/- 0.05. It was concluded that the equivalent pore radius in rat hindquarter microvascular walls is 53 A or even smaller in essential agreement with data from osmotic transient experiments in the same preparation (r approximately 40 A).

Serum factors other than albumin are needed for the maintenance of normal capillary permselectivity in rat hindlimb muscle

To investigate the effects of different perfusates on capillary permeability, we determined the capillary filtration coefficient (CFC), the capillary diffusion capacity (PS) for Cr-EDTA and clearance of albumin during isogravimetric conditions and maximal vasodilatation in the isolated, perfused rat hindquarter preparation. Experiments were conducted in 30 rats with different perfusates. We were able to confirm the classical 'protein effect'. Absence of proteins, using pure dextran solution as perfusate, induced a 45% increase in CFC and a three-fold increase in albumin clearance. However, we also found evidence for a 'serum effect'. Hence, the clearance of albumin was normal when the serum content exceeded 5% (v/v) in perfusates otherwise composed of albumin in Tyrode, but increased three-fold from 0.0305 to 0.0912 ml (min X 100 g)-1 when the rats were perfused with albumin in Tyrode with no serum present, without any change in CFC, PS for Cr-EDTA or vascular resistance to flow. Thus, certain non-dialysable serum factors, other than albumin, seem to be needed for the maintenance of normal capillary permselectivity in rat hindquarters. These factors are probably needed for the capillary membrane to maintain its character of a negatively charged barrier.

Macromolecule transport across the pulmonary microvessel walls

In summary, it has been difficult to assess the permeability properties of the pulmonary capillary membrane. However, new mathematical and experimental techniques have recently been developed which are of sufficient sensitivity and specificity to begin to evaluate the complex mechanisms responsible for many forms of lung pathology. While future work will undoubtedly need to address problems associated with heterogeneity of pulmonary blood flow, and with an equally heterogeneous population of vascular permeability and fluid formation sites, we currently need to focus on using the correct experimental approaches for assessing vascular permeability. The appropriate techniques are described in the text and indicate that the measurements of reflection coefficients using lymph obtained at high vascular pressures, filtration coefficients obtained from both isolated and intact lungs, and two-pore models are useful in assessing vascular permeability.

Distribution of anionic binding sites in extravascular space of skeletal muscle demonstrated with polycationized ferritin

Polycationized ferritin (PCF) and native ferritin (NF) were microinjected into rat muscle interstitium. The injection site was fixed in situ and processed for electron microscopy. PCF decorated the external aspect of the lamina densa of pericapillary and muscular basal lamina as quasi-regular aggregates, up to approximately 200 nm wide, separated with spaces greater than 20 nm wide. The internal aspect of the lamina densa possessed negative binding sites with a similar distribution, whereas PCF was never retained within the lamina densa. PCF bound very closely to the sarcolemma but did not approximate intimately the endothelial plasmalemma including vesicles. PCF also interacted with strict regularity with collagen fibrils (at an axial repeat of greater than 60 nm) forming transverse bands of PCF across collagen bundles. No ordered interaction between NF and extracellular structures was observed. The ordered distribution of negative binding sites in interstitium, as revealed with PCF, should affect the blood-lymph movement of macromolecules.