Simulations of peritoneal solute transport during CAPD. Application of two-pore formalism

Blood peritoneal clearances of various endogenous solutes in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) were evaluated according to recent developments of the two-pore theory of membrane permeability, using a non-linear transport formalism for the analysis. Based on results obtained from these calculations and taking lymphatic drainage into account, transport from peritoneal cavity to the blood was also simulated. With respect to solute transport the data were compatible with a functional blood-peritoneal barrier consisting of a two-pore membrane containing a large number of paracellular "small pores" of radius 40 to 55 A and a small number of "large pores" of radius 200 to 300 A. Solutes smaller than 25 A in radius were found to be permeating across the peritoneal membrane mainly by means of diffusion across the small pores, whereas solutes larger than 40 A were calculated to reach the peritoneal cavity exclusively by unidirectional convection across the large pores. In addition, water was simulated to be transported through transcellular "ultrapores" (radius less than 8 A) not accessible to hydrophilic solute permeation. Small solute absorption from the peritoneal cavity was found to occur by diffusion across small pores. Molecules larger than 25 to 30 A in radius (molecular weight above 25,000) were simulated to be absorbed from the peritoneal cavity exclusively via non-size-selective lymphatic drainage.

Effects of oxygen free radical scavengers, xanthine oxidase inhibition and calcium entry-blockers on leakage of albumin after ischaemia. An experimental study in rabbit kidneys

The effect of pretreatment with various substances protecting against oxygen free radicals on the leakage of proteins across the vessel walls of rabbit kidneys induced by ischaemia has been studied. The leakage of proteins was estimated from the difference between the 120-min distribution space of [131I]albumin and the 5-min distribution space of [125I]albumin, the latter mainly measuring the intravascular volume. Neither SOD (superoxide dismutase), catalase, allopurinol or two different Ca2+ channel blockers (nifedipine, felodipine) could alone reduce the leakage induced by ischaemia. A combined pretreatment with SOD, catalase and nifedipine reduced the leakage in the cortex, and pretreatment with mannitol alone reduced the leakage in the cortex and outer stripe of the medulla. The results indicate that oxygen free radicals are involved in the leakage of proteins across the vessel walls induced by ischaemia, but that other mechanisms are involved as well.

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.

Fluid and protein fluxes across small and large pores in the microvasculature. Application of two-pore equations

Treating the blood-tissue barrier as a two-pore membrane the separate fluid and solute fluxes occurring across 'small pores' and 'large pores' were modelled in continuous capillaries employing two-pore equations for the calculations together with the non-linear flux equation and theories for restricted diffusion and for the reflection coefficient (sigma). The two-pore equations derived proved useful for analyses of transvascular protein flux data obtained at low as well as at high filtration rates. These equations were applied to lymphatic protein flux data from dog paw (Renkin et al. 1977a, b) and to tracer albumin uptake data from rat skeletal muscle (Rippe et al. 1979). For both sets of data the small- and large-pore radii became closely similar, 44 vs. 45 A and 240 vs. 225 A, which also holds for the large-pore fractions of hydraulic conductivity (0.097 vs. 0.056). The main result of this analysis is that the passage of macromolecules normally occurring across the microvascular walls is almost entirely convective, and hence, dependent on the transmural hydrostatic and oncotic pressure gradients and on the hydraulic conductivity. For example, 75-90% of the transvascular passage of albumin was found to be due to convection through large pores at normal lymph flows, the remaining portion being mainly due to diffusion across small pores. Solutes larger than albumin were almost exclusively transported by convection across large pores. Two-pore heterogeneity was found to explain the previously observed variations of the apparent overall large solute diffusion capacity (PSapp) and the overall reflection coefficient (sigma f) with filtration rate and also previous overestimations of PS. Furthermore, the present results were not compatible with protein transport across any 'non-hydraulically conductive capillary pathways' as previously postulated from the lymphatic protein flux data analysed here (Renkin 1985).

Effects of ischaemia on leakage of albumin in rabbit kidneys

The effect of ischaemia on the leakage of proteins across the vessel walls in rabbit kidneys was studied. The leakage of proteins was estimated from the distribution space of labelled albumin. Various methods to calculate the vascular volume yielding the true extravascular leakage were tested. The most reliable method appeared to be to use the same tracer (131I-albumin, 125I-albumin) for both estimating the leakage and the intravascular volume. One hour of ischaemia induced a drastic increase in the extravasation of proteins in the cortex and outer zone of the medulla but not in the inner zone.

How Are Macromolecules Transported Across the Capillary Wall?

It is generally believed that macromolecules penetrate the capillary wall via mobile plasmalemmal vesicles (transcytosis). However, recent evidence suggests that filtration across rare capillary leaks is the main exchange process for large solutes. Furthermore, diffusion of albumin and other small negatively charged proteins via interendothelial clefts seems to be severely restricted by negative charges of the cleft wall, thus retaining albumin in the circulatory system.

Estimation of equivalent pore radii in pulmonary microvasculature after lung lymph fistula preparation

The effect of lung lymph fistula preparation on pulmonary microvascular permeability was investigated in sheep. Acutely prepared animals (n = 9) were compared with animals with a chronic lung lymph fistula (n = 5). The osmotic reflection coefficients (sigma) for total protein, albumin, immunoglobins (Ig) G and M, and the equivalent pore dimensions were calculated. Data were achieved at maximal possible lymph flows (QL) following elevation of left atrial pressure. In sheep with a chronic lung lymph fistula sigma's for total protein, albumin, IgG, and IgM at maximal lymph flows were 0.76 +/- 0.01, 0.65 +/- 0.09, 0.79 +/- 0.03, and 0.91 +/- 0.01, respectively. In the acutely prepared group the minimum lymph-to-plasma protein concentration for total protein was 0.39 +/- 0.06, corresponding to a sigma of 0.61 +/- 0.01. The sigma for albumin, IgG, and IgM were 0.48 +/- 0.04, 0.64 +/- 0.02, and 0.87 +/- 0.01, respectively. The equivalent pore radii in the chronic group were determined to be 54 and 190 A with 29% of the filtration accounted for by large pores. In the acute group the small pores were 56 A and the large pores 175 A with 53% of total volume flow at maximum lymph flows occurring through the large pores. Assuming a constant small-pore population the large pore number increased 4.5 times after surgery. For total protein, IgG, and IgM, sigma's in the acutely prepared group were significantly lower than in the control group. These results thus indicate that surgical preparation of a lung lymph fistula in sheep may cause acute increases in pulmonary microvascular permeability.

Influence of perfusate oncotic pressure on the transcapillary clearance of albumin in maximally vasodilated rat skeletal muscle

An external detection technique was developed for repetitive and reliable measurements of clearance (Cl) of 99mTc-albumin during alterations of serum colloid osmotic pressure (pi p). Isolated, maximally vasodilated rat hindquarters were perfused with serum of two pi ps (20 mmHg and 45 mmHg) at four or five different filtration rates (Fv) for each pi p in each animal. The recorded accumulation rates of 99mTc-albumin (AR) were converted into dimensions of albumin clearance, setting the isogravimetric Cl at normal vascular pressures (pi p = 20 mmHg) at 0.0246 +/- 0.0012 ml min-1 per 100 g, which was obtained in defined muscle samples in 11 separate experiments. Serum perfusion with higher colloid osmotic pressure (45 mmHg) shifted the albumin clearance values upwards, without affecting the slope of the Cl vs. Fv relationship. Thus, the reflection coefficient (sigma) for albumin did not seem to be affected by the changes in pi p, while the isogravimetric albumin clearance was increased to roughly 0.058 ml min-1 per 100 g. Explicit two-pore equations were found to describe the experimental data fairly well, yielding an average sigma for albumin of 0.92 and a minor contribution of diffusion to overall transport even at low Fvs. Moreover, a coupling of macromolecular clearance to pi p may serve to minimize alterations in plasma protein concentration.

Sepsis in sheep reduces pulmonary microvascular sieving capacity

The changes in pulmonary microvascular permeability in sheep, after infusion of live Escherichia coli, were studied using estimations of the osmotic reflection coefficients (sigma) for total protein, albumin, immunoglobins (Ig) G and M and based on these estimations equivalent pore dimensions were calculated. A chronic lung lymph fistula was prepared in seven sheep. After a base-line period, left atrial pressure (Pla) was increased. E. coli (10(9) X kg body wt) were given after attaining filtration independent L/P values. The sigma's for the normal lung were calculated to 0.73 for total protein and to 0.65, 0.76, and 0.91 for albumin, IgG, and IgM, respectively. The equivalent pore radii were determined to 50 and 175 A with 35% of the filtration accounted for by the large pores. After bacterial infusion, the sigma's for total protein, albumin, IgG, and IgM decreased significantly from preseptic values to 0.58, 0.50, 0.64, and 0.83, respectively. After sepsis the small pores were 50 A and the large pores 200 A with 49% of total volume flow at maximum lymph flows occurring through the large pores. Assuming a constant small-pore population the large-pore number increased 32% after bacterial infusion. These results indicate that pulmonary microvascular permeability may have increased due to the sepsis.

Segmental vascular resistances and compliances in dog lung

The segmental distribution of vascular resistances and compliances were evaluated in isolated blood perfused lung lobes using arterial, venous, and double-occlusion pressures and were compared with filtration midpoint capillary pressures (Pc,f). We separated total vascular resistance (RT) and compliance (CT) into large artery (Ra, Ca), large vein (Rv, Cv), and microvascular compartments (Rmc, Cmc) at base-line and increased vascular pressures and during infusions of histamine, serotonin, and norepinephrine. In control lobes, double-occlusion pressure (Pdo) closely approximated Pc,f at all vascular pressures. Pre- and postcapillary resistance were approximately equal when referenced to either Pc,f or Pdo. Although Rmc comprised 42% of RT and Cmc constituted 76% of CT, a twofold increase in base-line Pc,f caused RT to decrease to 67% and Rmc/RT to 29% of control values, whereas CT decreased to 87% and Cmc/CT decreased to 88% of control values over the same Pc,f range. Mean static CT was 2.25 +/- 0.09 ml X cmH2O-1. 100 g-1, whereas dynamic CT was 1.54 +/- 0.08 ml X cmH2O-1. 100 g-1, or only 68% of static vascular compliance. Drug infusions increased mean RT from 4.2- to 5.3-fold and significantly decreased both static and dynamic CT. Although all vascular segments were constricted, histamine affected primarily large veins, serotonin increased Ra greater than Rv, and norepinephrine constricted upstream and downstream vessels about equally. Increased Pc,f in the presence of these drugs decreased RT significantly in every case primarily through attenuation of the drug vasoconstrictor effect on Rmc and decreased CT primarily due to a decrease in Cmc, but increased Cmc/(Ca + Cv). Thus the microvascular compartment appears to be the major site of both fluid filtration and vascular compliance and contributes significantly to total vascular resistance. Drug infusions constricted large and small vessel compartments as defined here, but increased Pc,f attenuated microvascular vasoconstriction and to a lesser extent large vessel vasoconstriction resulting in a reduced microvascular resistance in both drug-treated and control lobes. This effect can be attributed to recruitment and/or distension of microvessels and distension of larger vessels.

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'.

Calcium dependence of histamine-induced increases in capillary permeability in isolated perfused rat hindquarters

Experiments were performed on isolated maximally vasodilated perfused rat hindquarters to evaluate the role of calcium and magnesium for the capillary permeability increase(s) elicited by histamine. Changes in capillary permeability were quantified by determinations of capillary filtration coefficient (CFC) with gravimetric technique, and capillary diffusion capacity (PS) for vitamin B12 (MW = 1,355) with a single injection indicator dilution technique. During control, vascular resistance was 2.2 PRU100 at a flow of 9.4 ml min-1 per 100 g, and PS for B12 was 3.7 +/- 0.1 ml min-1 per 100 g, while CFC was 0.0377 +/- 0.0004 ml min-1 mmHg-1 per 100 g. Perfusion with 'Mg-free' solution for 1 h caused a 24% increase in CFC, while neither 'Ca-free' perfusion nor perfusion with verapamil (5 X 10(-5) M) nor felodipine (1 X 10(-6) M) induced any changes in CFC. Histamine (100-200 microM) caused in all preparations a 150-200% increase in CFC with only small changes in PS for B12. This histamine effect was absent after 1 h of 'Ca-free' perfusion and was partially blocked after 1 h of perfusion with 0.1 mM calcium, while the calcium antagonists verapamil and felodipine had no effects on the histamine-induced changes. The results imply that histamine exerts its action on the endothelial cells through a calcium-dependent process, probably involving low affinity calcium sites but this process could not be inhibited by the calcium antagonists used. Thus, endothelial cell contractility, which probably is responsible for the histamine-induced increase in capillary permeability, exhibits unique characteristics, differing from those of vascular smooth muscle.

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).

Validation of double vascular occlusion method for Pc,i in lung and skeletal muscle

Capillary pressures in isogravimetric lung and skeletal muscle measured with the double vascular occlusion technique (Pdo) were compared to those measured using the traditional gravimetric technique (Pc,i). Pressures were measured using both techniques in isolated blood-perfused canine lungs (n = 18), blood-perfused rat hindquarters before (n = 8) and after (n = 6) maximal dilatation with papaverine and in rat hindquarters perfused with an artificial plasma (n = 6). In both organs, regardless of vascular tone, the double vascular occlusion isogravimetric pressure was the same as the gravimetric Pc,i, and the two measurements were highly correlated. Lung: Pdo = -0.22 + 1.06 Pc,i (r = 0.85, P less than 0.01); hindquarter: Pdo = -1.03 + 0.99 Pc,i (r = 0.91, P less than 0.01). In addition, Pdo was the same at every combination of isogravimetric arterial and venous pressures tested. The results indicate that the more rapidly applied double vascular occlusion pressure yields an accurate measure of isogravimetric capillary pressure in isolated organs over a wide range of isogravimetric pressures.

Fluid filtration and protein clearances through large and small pore populations in dog lung capillaries

Solvent drag reflection coefficients (sigma f) for six protein fractions with hydrodynamic radii ranging from 37 to 120 A were determined using dog lung lymph CL/CP data over a range of lung lymph flows (QL). Two equivalent pore populations with effective radii of 75-85 A and 200-325 A were determined for the pulmonary capillaries over a range of mean lymph flows for 1.8 to 6.3 times control. Fractional fluid clearance through the large pore population was observed to decrease from 0.28 to 0.16 as QL increased. These data indicate that lymph flow (filtration) increased primarily by an increased filtration through small pores with relatively little increase in filtration through the large pore population. The shift to small pore filtration at high filtration rates accounts for the dependence of the homoporous estimate of total protein sigma f on lymph flow previously observed using lung lymph (Parker et al., 1981). If capillary membrane heteroporosity is present then there should be a net convective clearance of protein across the capillary caused by a volume circulation between large and small pore populations. Minimal estimates of 6-10% of protein clearance are attributed to this mechanism even in the absence of net fluid movement across the capillary. The optimal solute radius for clearance due to volume circulation is about 60 A, based on the membrane pore sizes determined for lung capillaries. Although convective transport of macromolecules is generally acknowledged to account for most transcapillary protein clearance at high filtration rates, the inclusion of a volume circulation component suggests that convection may also be the dominant mode of macromolecular transport at low filtration rates.

Evaluation of the 'stretched pore phenomenon' in isolated rat hindquarters

In order to study the changes in capillary permeability occurring upon marked elevations in microvascular pressure, capillary filtration coefficient (CFC) and diffusion capacity (PS) for Cr-EDTA were repeatedly measured 'on-line' before and after brief periods (3 min) of large venous pressure (PV) elevations in maximally vasodilated perfused rat hindquarters. First at PV's greater than or equal to 55 mmHg, increases in CFC and Ps-Cr-EDTA were observed immediately after the pressure elevations. While the CFC increases were then always pronounced (up to 4-to 5-fold), the concomitant increases in Ps-Cr-EDTA were small (at most 30-40%). For PV greater than or equal to 55 mmHg there was a rough proportionality between PV and CFC. While PS for Cr-EDTA showed little reversibility with time upon PV normalization, CFC was usually almost completely reversed after 10-20 min. The mentioned CFC and PS increases at PV's greater than or equal to 55 mmHg were quantitatively similar to those induced by histamine-type mediators in the same preparation. It is concluded that capillary hydraulic conductivity can increase markedly but reversibly upon large PV elevations, and that this is mainly due to forceful opening of ordinarily closed 'large pores' in the microvascular membrane, rather than being caused by lesional rifts in the endothelium.

Effects of plasma- and cell-free perfusates on filtration coefficient of perfused canine lungs

The filtration coefficient (Kf,c) of the microvessels in isolated dog lungs were studied for whole and diluted blood, whole and diluted plasma, Tyrode's solution, and Tyrode's plus dextran (4%, 63,000 mol wt) perfusates. When whole blood and plasma were diluted, Kf,c increased abruptly at a plasma protein concentration between 4 and 5 g/l, an effect which was not dependent on the erythrocyte mass. Both Tyrode's and Tyrode's plus dextran produced increases in Kf,c (60 and 30%, respectively). The difference in Kf,c measured between these latter perfusates was completely abolished when Kf,c were corrected for viscosity differences. Thus the pulmonary microvasculature responds similarly to the systemic circulation in that complete removal of plasma proteins from the perfusate increases Kf,c by 50%. This effect is independent of erythrocyte mass or colloid osmotic pressure of the perfusate, since perfusion with dextran solutions alone also increased Kf,c.

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.

Osmotic reflection coefficient for total plasma protein in lung microvessels

The osmotic reflection coefficient (sigma) for total plasma proteins was estimated in 11 isolated blood-perfused canine lungs. Sigma's were determined by first measuring the capillary filtration coefficient (Kf,C in ml X min-1 X 100g-1 X cmH2O-1) using increased hydrostatic pressures and time 0 extrapolation of the slope of the weight gain curve. Kf,C averaged 0.19 +/- 0.05 (mean +/- SD) for 14 separate determinations in the 11 lungs. Following a Kf,C determination, the isogravimetric capillary pressure (Pc,i) was determined and averaged 9.9 +/- 0.5 cmH2O for all controls reported in this study. Then the blood colloids in the perfusate were either diluted or concentrated. The lung either gained or lost weight, respectively, and an initial slope of the weight gain curve (delta W/delta t)0 was estimated. The change in plasma protein colloid osmotic pressure (delta IIP) was measured using a membrane osmometer. The measured delta IIP was related to the effective colloid osmotic pressure (delta IIM) by delta IIM = (delta W/delta t)0/Kf,C = sigma delta IIP. Using this relationship, sigma averaged 0.65 +/- 0.06, and the least-squares linear regression equation relating Pc,i and the measured IIP was Pc,i = -3.1 + 0.67 IIP. The mean estimate of sigma (0.65) for total plasma proteins is similar to that reported for dog lung using lymphatic protein flux analyses, although lower than estimates made in skeletal muscle using the present methods (approximately 0.95).

Permselectivity of the peritoneal membrane

To investigate the osmotic barrier characteristics of the peritoneal membrane during conditions similar to peritoneal dialysis in man, yet transperitoneal fluid movement was measured in 20 cats following intraabdominal placement of isotonic saline and hypertonic solutions of NaCl, glucose, raffinose, and inulin. Also, isooncotic solutions of hemoglobin and albumin and two sulfated high-molecular-weight dextrans were investigated. Transperitoneal fluid movement was measured by a volume recovery method. Oncotic pressures of test solutions and plasma were measured by osmometry. Peritoneal osmotic conductances were calculated from the rate of transperitoneal water movement and the difference in osmotic pressures between the test solution and isotonic saline. The average glucose osmotic conductance per unit body surface are was found to be 2.3 +/- 0.18 x 10(-3) ml . min-1 . mm Hg-1 . m-2, in good agreement with previous reports, and the glucose osmotic reflection coefficient (sigma) was estimated to be 0.02. All the osmotic conductances measured could be fitted to a peritoneal equivalent pore radius of approximately 6 nm according to current hydrodynamic theories. The peritoneal membrane filtration coefficient was estimated to be 0.12 ml . min-1 . mm Hg-1 . m-2, of which 0.5-1% was found to be due to transcellular water flow. In conclusion the results of this study indicate that the peritoneum is a highly selective membrane with restrictive properties comparable to those reported for continuous capillary beds.