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

Therapeutic studies, level IV.

Fluid loss from the peritoneal cavity to surrounding tissue varies directly with intraperitoneal hydrostatic pressure (Pip). According to Darcy’s law [ Q = − KA(dPif/d x)], fluid flux ( Q) across a cross-sectional area ( A) of tissue will increase with an increase in either hydraulic conductivity ( K) or the interstitial fluid hydrostatic pressure gradient (dPif/d x, where x is distance). Previously, we demonstrated that in the anterior abdominal muscle (AAM) of rats, dPif/d xincreases by only 40%, whereas Krises fivefold between Pip of 1.5 and 8 mmHg. Because K is a function of interstitial volume (θif), we hypothesized that perturbations of Pip would change Pif and expand the interstitium, increasing θif. To test this hypothesis, we used dual-label quantitative autoradiography (QAR) to measure extracellular fluid volume (θec) and intravascular volume (θiv) in the AAM of rats within the Pip range from −2.8 to +8 mmHg. θif was obtained by subtraction (θec− θiv). dPif/d xwas measured with a micropipette and a servo-null system. Local θiv did not vary with Pip and averaged 0.010 ± 0.002 ml/g, and θif averaged 0.19 ± 0.01 ml/g at Pif ≤1.2 mmHg. However, θif doubled between Pif of 1.2 and 4.2 mmHg (from 0.20 ± 0.00 to 0.39 ± 0.01 ml/g, respectively) but did not increase with further increases in Pif. This nonlinear pressure-volume relationship does not explain the fivefold increase in K with Pip. Because the interstitial matrix contributes to the interstitial resistance to fluid flow, and because hyaluronan (HA) is the only component of the matrix that is not anchored to the tissue, we hypothesized that the loss of interstitial HA was responsible for the continued decrease in interstitial resistance to fluid flow. We determined HA concentration in the rat AAM and adjacent subcutaneous tissue (SC) at Pip = 0 mmHg and after 2 h of dialysis at constant Pip = 6 mmHg. The HA content (normalized to dry weight) in the AAM was reduced from 487 ± 16 to 360 ± 27 μg/g dry tissue ( n = 4, P < 0.05) and increased from 528 ± 72 to 1,050 ± 136 mg/g dry tissue ( n = 4, P > 0.001) in the SC. We conclude that the mechanisms responsible for the increase in K with Pip include expansion of the interstitium, dilution of interstitial macromolecules, and washout from the AAM to SC of interstitial macromolecules responsible for resistance to fluid flow.

Previously, we demonstrated that immunoglobulin G (IgG), dissolved in an isotonic solution in the peritoneal cavity, transported rapidly into the abdominal wall when the intraperitoneal (ip) pressure was >2 cmH2O. We hypothesized that this was chiefly caused by convection and that diffusion of IgG was negligible. To investigate the role of diffusion, we dialyzed rats with no pressure gradient across the abdominal wall muscle for 2 or 6 h with an ip isotonic solution containing125I-labeled IgG. At the end of the experiment, the animal was euthanized and frozen and abdominal wall tissue was processed to produce cross-sectional autoradiograms. Quantitative densitometric analysis resulted in IgG concentration profiles with far lower magnitude than profiles from experiments in which convection dominated. In other in vivo experiments, we determined the lymph flow rate to be 0.8 × 10−4ml ⋅ min−1 ⋅ g−1and the fraction of extravascular tissue (θs) available to the IgG to be 0.041 ± 0.001. An in vitro binding assay was used to determine the time-dependent, nonsaturable binding constant: 0.0065 min−1 × duration of exposure. A non-steady-state diffusion model that included effects of θs, time-dependent binding, and lymph flow was fitted to the diffusion profile data, and the IgG diffusivity within the tissue void was estimated to be 2 × 10−7cm2/s, a value much higher than that published by other groups. We also demonstrate from our previous data that convection of IgG through tissue dominates over diffusion at ip pressures >2 cmH2O, but diffusion may not be negligible. Furthermore, nonsaturable binding must be accounted for in the interpretation of tissue protein concentration profiles.

No abstract