Hydraulic conductivity, albumin reflection and diffusion coefficients of pig mediastinal pleura

Physiology of pericardial fluid production and drainage

The pericardium is one of the serosal cavities of the mammals. It consists of two anatomical structures closely connected, an external sac of fibrous connective tissue, that is called fibrous pericardium and an internal that is called serous pericardium coating the internal surface of the fibrous pericardium (parietal layer) and the heart (visceral layer) forming the pericardial space. Between these two layers a small amount of fluid exists that is called pericardial fluid. The pericardial fluid is a product of ultrafiltration and is considered to be drained by lymphatic capillary bed mainly. Under normal conditions it provides lubrication during heart beating while the mesothelial cells that line the membrane may also have a role in the absorption of the pericardial fluid along with the pericardial lymphatics. Here, we provide a review of the the current literature regarding the physiology of the pericardial space and the regulation of pericardial fluid turnover and highlight the areas that need to be further investigated.

Transport properties of the mesothelium and interstitium measured in rabbit pericardium

The contribution of the pleural mesothelium to pleural liquid and protein transport is still vigorously debated. Recent in vitro studies of stripped pleural membrane and free-standing pericardium have demonstrated active ion solute coupled transport of liquid and transcytosis of protein. However, the relative contribution of the passive transport properties of the pleural mesothelium compared to the pleural interstitium has not been extensively studied. In in vitro studies, we measured the albumin diffusion coefficient, reflection coefficient, hydraulic conductivity and electrical resistance of rabbit pericardium. We used two techniques, treatment with 40 muM nocodazole and a 1-min hypotonic cell lysis with distilled water, to eliminate the effect of the two mesothelial layers on diffusional and hydraulic resistances. Each technique increased the albumin diffusion coefficient and hydraulic conductivity 3- to 4-fold. In hydraulic conductivity experiments using tracer 125I-albumin, nocodazole reduced the reflection coefficient to zero, rendering the pericardium completely permeable to albumin. We applied the cell-lysis technique to the pleural and pericardial mesothelium in sequence to evaluate the separate contribution of each mesothelium. Both diffusional and hydraulic resistances, but not electrical resistance, of the mesothelium were overestimated by the cell-lysis technique. The pleural mesothelium contributed at most 30% of diffusional resistance, 10% of hydraulic resistance and 14% of electrical resistance of the total pericardial resistances. We conclude that the pleural mesothelium is not the primary barrier to protein diffusion or bulk flow of liquid from the pericardial microcirculation to the pleural liquid.

Pleural mechanics and fluid exchange

The pleural space separating the lung and chest wall of mammals contains a small amount of liquid that lubricates the pleural surfaces during breathing. Recent studies have pointed to a conceptual understanding of the pleural space that is different from the one advocated some 30 years ago in this journal. The fundamental concept is that pleural surface pressure, the result of the opposing recoils of the lung and chest wall, is the major determinant of the pressure in the pleural liquid. Pleural liquid is not in hydrostatic equilibrium because the vertical gradient in pleural liquid pressure, determined by the vertical gradient in pleural surface pressure, does not equal the hydrostatic gradient. As a result, a viscous flow of pleural liquid occurs in the pleural space. Ventilatory and cardiogenic motions serve to redistribute pleural liquid and minimize contact between the pleural surfaces. Pleural liquid is a microvascular filtrate from parietal pleural capillaries in the chest wall. Homeostasis in pleural liquid volume is achieved by an adjustment of the pleural liquid thickness to the filtration rate that is matched by an outflow via lymphatic stomata.

Effect of concentration and hyaluronidase on restriction of hetastarch flux through lung interstitial segments

The transport properties of lung interstitium were studied by measuring the flow of hetastarch solution (2 and 6%) through 1-cm perivascular interstitial segments of rabbit lungs. Hetastarch (10(4)-10(7) Da) solution has a colloid osmotic pressure similar to that of albumin solution. Driving pressure was 5 cm H(2)O and mean interstitial pressure was 0 cm H(2)O. The flows of 2 and 6% hetastarch solutions were measured before (Q(1)) and after (Q(2)) the addition of 0.02% hyaluronidase. Hetastarch molecular distributions in effluent samples were measured by high-performance size-exclusion chromatography (HPSEC) to determine sieving ratio (C(out)/C(in), downstream-to-upstream concentration ratio). Hyaluronidase significantly (P < 0.0004) increased flow sixfold, but the increase in flow (Q(2)/Q(1)) was reduced through the interstitium around smaller vessels. A similar behavior was observed with the flow of albumin solution without and with hyaluronidase. C(out)/C(in) decreased monotonically with molecular weight, was greater with 6% than with 2% (low colloid osmotic pressure) hetastarch, and increased with hyaluronidase. Modeling the transport through uniform pores, equivalent pore radius was 10 and 15 nm with 2 and 6% hetastarch, respectively, and doubled with hyaluronidase. In conclusion, interstitial pores expand in response to an increase in colloid osmotic pressure both before and after tissue degradation by hyaluronidase.

Effect of concentration on restriction and diffusion of albumin in the excised rat diaphragm

In tissue samples of rat diaphragm mounted between two chambers, we measured the flow of albumin solution (0-5 g/dl) containing radioactive tracer (125)I-albumin in response to a driving pressure of 20 cmH(2)O. The ratio of the albumin concentration of the output solution to that of the input (sieving ratio, C(out)/C(in)) was measured from solution radioactivity. C(out)/C(in) increased monotonically from 0.5 with the flow of approximately 0 g/dl albumin solution (tracer) to 0.9 with the flow of 5 g/dl albumin solution. We modeled the tissue as a membrane subjected to flows of high Peclet No. with a reflection coefficient sigma = 1 - C(out)/C(in). Values of sigma decreased from 0.5 with Ringer solution to 0.1 with 5 g/dl albumin solution. Hydraulic conductivity measured with the flow of Ringer solution increased with the flow of 5 g/dl albumin solution. Wet-to-dry weight ratio and radioactivity of tissue samples immersed in 0.01-5 g/dl albumin solutions indicated a 40% increase in tissue water, associated with an albumin volume fraction of 0.3 measured at 0.5-2 h. The slower rate of albumin uptake occurring up to 20-30 h indicated intracellular diffusion that was equal with 1 and 5 g/dl albumin solution but reduced with a 0.01 g/dl albumin solution. The results suggest that interstitial pores increase in size in response to an increase in albumin concentration. We postulate a two-pore model made of intracellular pores that coalesce into a set of larger pores by osmotic flow.

Effect of concentration and hydration on restriction of albumin by lung interstitium

In previous studies, the flow of albumin solution through hydrated lung interstitial segments was higher than a prior flow of Ringer solution (A. Tajaddinni et al., 1994, J. Appl. Physiol. 76, 578-583). We wondered whether this effect was caused by an increased pore size. We measured the flow of albumin solutions through interstitial segments subjected to a driving pressure of 5 cm H(2)O and various mean interstitial pressures (P(if)). The ratio of albumin concentration (C(alb)) of the output solution to that of the input solution (C(out)/C(in), sieving ratio) was measured using tracer (125)I-albumin. At normal hydration (0 cm H(2)O P(if)), C(out)/C(in) was minimal (0.6) with the flow of Ringer solution, increased to 0.8 with the flow of 5 g/dl albumin solution, and increased to 1 with increased hydration at 15 cm H(2)O P(if). We modeled the interstitium as a membrane subjected to flows of high Peclet numbers. Accordingly, the albumin reflection coefficient [sigma = 1 - (C(out)/C(in))] at 0 cm H(2)O P(if) was 0.4 with the flow of Ringer solution and decreased to 0 at 5 g/dl C(alb) and 15 cm H(2)O P(if). This behavior suggests that the flow of albumin occurred through interstitial pores that increased in size as either C(alb) or hydration increased. We conceive of an interstitium that consists of pores with permeable moveable walls across which osmotic interaction occurs between the pore liquid and the surrounding tissue.

Quantitative analysis of intratumoral infusion of color molecules

Intratumoral infusion has a potential for improving distribution of drugs. To optimize the infusion, we developed a novel technique to quantify the distribution volume of color molecules (Vd) in solid tumors. Evans blue-labeled albumin was infused locally with the use of a needle into a rat fibrosarcoma ex vivo under different pressures. After the infusion, tumor tissues were sectioned serially into thin slices. The blue area in each slice was quantified with the use of the newly developed technique. The Vd was calculated based on the blue area and the slice thickness. Our data showed that infusion pressure and volume (V(i)) had significant effects on Vd. The median of Vd/V(i) decreased from 2.99 to 1.79 when infusion pressure was increased from 50 to 163 cmH2O, presumably due to retardation of convective transport. In addition, the coefficient of variation in Vd/V(i) was increased from 0.13 at 50 cmH2O to 0.64 at 163 cmH2O. The dependence of Vd/V(i) and its variation on infusion pressure suggests that 1) infusion-induced tissue deformation is unpredictable and 2) both the unpredictability and the interstitial retardation of convective transport increase with infusion pressure.

Role of tumor-host interactions in interstitial diffusion of macromolecules: cranial vs. subcutaneous tumors

The large size of many novel therapeutics impairs their transport through the tumor extracellular matrix and thus limits their therapeutic effectiveness. We propose that extracellular matrix composition, structure, and distribution determine the transport properties in tumors. Furthermore, because the characteristics of the extracellular matrix largely depend on the tumor-host interactions, we postulate that diffusion of macromolecules will vary with tumor type as well as anatomical location. Diffusion coefficients of macromolecules and liposomes in tumors growing in cranial windows (CWs) and dorsal chambers (DCs) were measured by fluorescence recovery after photobleaching. For the same tumor types, diffusion of large molecules was significantly faster in CW than in DC tumors. The greater diffusional hindrance in DC tumors was correlated with higher levels of collagen type I and its organization into fibrils. For molecules with diameters comparable to the interfibrillar space the diffusion was 5- to 10-fold slower in DC than in CW tumors. The slower diffusion in DC tumors was associated with a higher density of host stromal cells that synthesize and organize collagen type I. Our results point to the necessity of developing site-specific drug carriers to improve the delivery of molecular medicine to solid tumors.

Macromolecule transfer through mesothelium and connective tissue

Diffusional permeability (P) to inulin (P(in)), albumin (P(alb)), and dextrans [70 (P(dx 70)), 150 (P(dx 150)), 550 (P(dx 550)), and 2, 000 (P(dx 2,000))] was determined in specimens of parietal pericardium of rabbits, which may be obtained with less damage than pleura. P(in), P(alb), P(dx 70), P(dx 150), P(dx 550), and P(dx 2, 000) were 0.51 +/- 0.06 (SE), 0.18 +/- 0.03, 0.097 +/- 0.021, 0. 047 +/- 0.011, 0.025 +/- 0.004, and 0.021 +/- 0.005 x 10(-5) cm/s, respectively. P(in), P(alb), and P(dx 70) of connective tissue, obtained after removal of mesothelium from specimens, were 10.3 +/- 1.42, 2.97 +/- 0.38, and 2.31 +/- 0.16 x 10(-5) cm/s, respectively. Hence, P(in), P(alb), and P(dx 70) of mesothelium were 0.54, 0.20, and 0.10 x 10(-5) cm/s, respectively. Inulin (like small solutes) fitted the relationship P-solute radius for restricted diffusion with a 6-nm "pore" radius, whereas macromolecules were much above it. Hence, macromolecule transfer mainly occurs through "large pores" and/or transcytosis. In line with this, the addition of phospholipids on the luminal side (which decreases pore radius to approximately 1.5 nm) halved P(in) but did not change P(alb) and P(dx 70). P(in) is roughly similar in mesothelium and capillary endothelium, whereas P to macromolecules is greater in mesothelium. The albumin diffusion coefficient through connective tissue was 17% of that in water. Mesothelium provides 92% of resistance to albumin diffusion through the pericardium.