Daniele Venturoli

Transendothelial transport: the vesicle controversy.

The relative contribution of transcytosis vs. large pore transport to the passage of macromolecules across microvascular endothelia has been a controversial issue for nearly half a century. To separate transcytosis from ‘porous’ transport, the transcytosis inhibitors N-ethylmaleimide (NEM) and filipin have been tested in in situ or ex vivo perfused organs with highly conflicting results. In continually weighed isolated perfused organs, where measurements of pre- and post-capillary resistances, capillary pressure and capillary filtration coefficients can be repeatedly performed, high doses of NEM and filipin increased the bulk transport of macromolecules from blood to tissue, despite producing vasoconstriction. By contrast, in in situ perfused organs, marked reductions in the tissue uptake of albumin tracer have been observed after NEM and filipin. When tissue cooling has been employed as a means of inhibiting (active) transcytosis, results have invariably shown a low cooling sensitivity of albumin transport, compatible with passive transendothelial passage of albumin. This observation is further strengthened by the commonly observed dependence of albumin transport upon the capillary pressure and the rate of transcapillary convection. For low-density lipoprotein (LDL), a cooling-sensitive, non-selective transport component has been discovered, which may be represented by filtration through paracellular gaps, lateral diffusion through transendothelial channels formed by fused vesicles, or by transcytosis. From a physiological standpoint there is little evidence supporting active transendothelial transport of most plasma macromolecules. This seems to be supported by studies on caveolin-1-deficient mice lacking plasmalemmal vesicles (caveolae), in which there are no obvious abnormalities in the transendothelial transport of albumin, immunoglobulins or lipoproteins. Nevertheless, specific transport in peripheral capillaries of several hormones and other specific substances, similar to that existing across the blood-brain barrier, still remains as a possibility.

The effects of low-sodium peritoneal dialysis fluids on blood pressure, thirst and volume status

Background. Poor ultrafiltration is associated with worse outcomes in peritoneal dialysis (PD) patients. This might in part reflect problems associated with salt and water excess. Increasing the diffusive component of peritoneal sodium removal using low-sodium PD fluids might have beneficial effects on blood pressure (BP), thirst and fluid status that could translate into clinical benefits. Methods. Using a multicentre, prospective, baseline controlled (1 month), non-randomized intervention (2 months) design, two novel solutions designed from predictions using the three-pore model were investigated. In group A ([Na+] = 115 mmol/l), the glucose (G) was increased to 2.0% to compensate for reduced osmolality whereas in group B ([Na+] = 102 mmol/l), it was unchanged (2.5%). Both solutions were substituted for one 3- to 5-h exchange per day and no change was made to the rest of the dialysis regime. Results. Ten patients in group A and 15 in group B completed the study. Both solutions resulted in significant increases (30–50 mmol/dwell) in diffusive sodium removal during the test exchanges, P < 0.001. Ultrafiltration was maintained in group A but reduced in group B. Ambulatory nocturnal mean BP fell in group A [93.1 ± 10.6 mmHg (±SD) versus 85.1 ± 10.2 mmHg, P < 0.05], but was stable in group B (95.4 ± 9.4 versus 95.1.1 ± 10.7 mmHg, NS). Thirst reduced independent of appetite and mood in both groups by 2 months, more markedly in group A. Indices of fluid status, including TBW by bioimpedance and D dilution also improved in group A, P < 0.05, whereas weight increased in group B. Conclusions. Increasing the diffusive component of sodium removal whilst maintaining ultrafiltration is associated with improvements in BP, thirst and fluid status. The lack of effect seen with uncompensated low-sodium dialysate suggests that these benefits cannot be achieved by manipulation of dialysate sodium removal alone. These observations provide valuable information of the design of future randomized studies to establish the clinical role for low-sodium dialysis fluids.

Glomerular sieving of three neutral polysaccharides, polyethylene oxide and bikunin in rat. Effects of molecular size and conformation.

Aim:  Polysaccharides and many other non‐protein polymers generally have a more open, flexible and asymmetrical structure compared with globular proteins. For a given molecular weight (MW), the Stokes–Einstein radius (ae) of the following polymers increases in the order: Ficoll < dextran ≤ pullulan < polyethylene oxide (PEO). We have tested the hypothesis that such an increase in ‘molecular extension’ will increase the molecule’s glomerular permeability. Thus, we investigated the glomerular sieving coefficients (θ) of the mentioned polymers and of the negatively charged and extended protein bikunin.

The Peritoneal Microcirculation in Peritoneal Dialysis

This paper deals with the peritoneal microcirculation and with peritoneal exchange occurring in peritoneal dialysis (PD). The capillary wall is a major barrier to solute and water exchange across the peritoneal membrane. There is a bimodal size‐selectivity of solute transport between blood and the peritoneal cavity, through pores of radius ∼40–50 Å as well as through a very low number of large pores of radius ∼250 Å. Furthermore, during glucose‐induced osmosis during PD, nearly 40% of the total osmotic water flow occurs through molecular water channels, termed “aquaporin‐1.” This causes an inequality between 1−σ and the sieving coefficient for small solutes, which is a key feature of the “threepore model” of peritoneal transport. The peritoneal interstitium, coupled in series with the capillary walls, markedly modifies small‐solute transport and makes large‐solute transport asymmetric. Thus, although severely restricted in the blood‐to‐peritoneal direction, the absorption of large solutes from the peritoneal cavity occurs at a high clearance rate (∼1 mL/min), largely independent of molecular radius. True absorption of macromolecules to the blood via lymphatics, however, seems to be occurring at a rate of ∼0.2 mL/min. Several controversial issues regarding transcapillary and transperitoneal exchange mechanisms are discussed in this paper.

Low-Dose Prostacyclin Improves Cortical Perfusion following Experimental Brain Injury in the Rat

It was recently shown that prostacyclin at a low dose reduces cortical cell death following brain trauma in the rat. Conceivably, prostacyclin with its vasodilatory, anti-aggregatory, anti-adhesive and permeability-reducing properties improved a compromised perfusion caused by post-traumatic vasoconstriction, microthrombosis and increased microvascular permeability. The objective of the present study was therefore to investigate the hemodynamic effects of low-dose prostacyclin in the traumatized rat cortex. Following a fluid percussion brain injury or a sham procedure, animals were treated with a continuous intravenous infusion of prostacyclin of 1 or 2 ng x kg(-1) x min(-1), or vehicle. Blood flow ([(14)C]-iodoantipyrine), the permeability-surface area product (PS) for [(51)Cr]-EDTA, and brain water content were measured after 3 or 48 h of treatment. Blood flow values in the injured cortex were transiently reduced to 0.42 +/- 0.2 mL x min(-1) in the vehicle group 3 h following trauma from a corresponding value of about 1.6 mL x min(-1) in the sham group, with recovery of blood flow after 48 h. Prostacyclin treatment caused a dose-dependent increase in blood flow which reached statistical significance 48 h following trauma. Brain water content and PS increased in the injured cortex post trauma and the higher dose of prostacyclin increased these parameters further at 48 h compared to the vehicle group (p < 0.05). The latter effects of prostacyclin cannot be attributed to an increase in permeability, as prostacyclin did not influence PS or brain water content following sham trauma. In fact prostacyclin has been shown to have permeability-decreasing properties. We conclude that prostacyclin improves cortical perfusion following brain trauma. The simultaneous aggravation of brain edema can be explained by an increased surface area, perhaps in combination with increased capillary hydrostatic pressure.

Effects of early endotoxemia and dextran-induced anaphylaxis on the size selectivity of the glomerular filtration barrier in rats

This study was performed to investigate the glomerular permeability alterations responsible for the microalbuminuria occurring in endotoxemia and during anaphylactic shock. In anesthetized Wistar rats, the left ureter was catheterized for urine collection while, simultaneously, blood access was achieved. Endotoxemia was induced by lipopolysaccharide (LPS) from Escherichia coli, and glomerular permeability was assessed at 60 and 90 (n = 7) and 120 (n = 7) min. Anaphylaxis was induced by a bolus dose of Dextran-70, and glomerular permeability assessed at 5 min (n = 8) and 40 min (n = 9). Sham animals were followed for either 5 or 120 min. The glomerular sieving coefficients (theta) to fluorescein isothiocyanate-Ficoll (70/400) were determined from plasma and urine samples and assessed using size-exclusion chromatography (HPLC). After start of the LPS infusion (2 h), but not at 60 or 90 min, theta for Ficoll(70A) had increased markedly [from 2.91 x 10(-5) +/- 6.33 x 10(-6) to 7.78 x 10(-5) +/- 6.21 x 10(-6) (P < 0.001)]. In anaphylaxis, there was a large increase in theta for Ficolls >60 A in molecular radius already at 5 min, but the glomerular permeability was completely restored at 40 min. In conclusion, there was a transient, immediate increment of glomerular permeability in dextran-induced anaphylaxis, which was completely reversible within 40 min. By contrast, endotoxemia caused an increase in glomerular permeability that was manifest first after 2 h. In both cases, theta to large Ficoll molecules were markedly increased, reflecting an increase in the number of large pores in the glomerular filter.

Transvascular Passage of Macromolecules into the Peritoneal Cavity of Normo- and Hypothermic Rats in vivo: Active or Passive Transport?

During the last decades there has been a debate regarding whether transvascular protein transport is an active (transcytosis) or a passive (porous) process. To separate cooling-sensitive transcytosis from passive transport processes between blood and peritoneal fluid, we induced hypothermia in rats in vivo, reducing their body temperature to 19°C. Control rats were kept at 37°C. Either human albumin, or IgG, or IgM, or LDL, radiolabeled with 125I, was given intra-arterially together with 51Cr-EDTA. During tracer administration, a 2-hour peritoneal dialysis dwell was performed. Clearance of the tracers to dialysate, and the permeability-surface area coefficient (PS) for 51Cr-EDTA and glucose were assessed. During cooling, mean arterial blood pressure (MAP) was reduced to 40% of control and plasma viscosity increased by 48.5%, while peritoneal blood flow was reduced to 10%. At 19°C, clearance of albumin to dialysate fell from 9.30 ± 1.62 (SEM) to 3.13 ± 0.28 µl/min (p < 0.05), clearance of IgG from 6.33 ± 0.42 to 2.54 ± 0.12 µl/min (p < 0.05), clearance of IgM from 3.65 ± 0.33 to 1.10 ± 0.12 µl/min (p < 0.05), and clearance of LDL from 3.54 ± 0.20 to 0.73 ± 0.06 µl/min (p < 0.05). The fall in PS for 51Cr-EDTA was from 0.320 ± 0.01 to 0.075 ± 0.003 ml/min (p < 0.05), and that for glucose from 0.438 ± 0.02 to 0.105 ± 0.01 ml/min (p < 0.05). Tissue cooling reduced large solute transport largely in proportion to the cooling-induced reductions of MAP (to 40%), and the concomitant increase in viscosity (to 67%), i.e. to ≈20–30% (0.40 × 0.67) of control, though LDL clearance was reduced further. The fall in small solute PS, in excess of the viscosity effect, mirrored the fall in peritoneal blood flow occurring during hypothermia. In conclusion, the good correlation of predicted to calculated changes suggests that the overall transendothelial macromolecular passage in vivo occurs passively, and not due to active processes.

Fluid loss from the peritoneal cavity by back-filtration through the small pores of the three-pore model

The partitioning of fluid flows among small and ultrasmall pores of the three-pore model in peritoneal dialysis has been traditionally assessed using 4-hour dwells with 3.86% glucose solutions. Under these conditions, however, back-filtration through small pores has been hard to demonstrate. As nicely shown by Asghar and Davies, however, the use of low-concentration (1.36%) glucose-based solutions allows accurate studies of the partitioning of fluid flows from the peritoneal cavity under conditions of fluid loss.