Hydrostatic and Osmotic Pressures Modulate Partitioning of Tissue Water in Abdominal Muscle during Dialysis

Objectives To investigate the effect of simultaneous exposure of anterior abdominal muscle (AAM) to changes in intraperitoneal hydrostatic pressure (Pip) and to osmolality of peritoneal fluid on total tissue water (TTW) and on the pattern of distribution of TTW in the AAM.Design A pilot study of single 60-min dwells in anesthetized Sprague-Oawley (SO) rats, dialyzed with either isotonic (290 mOsm/kg) or hypertonic (510 mOsm/kg) dialysis solutions at nominal Pi of 0 mmHg or 6 mmHg.Measurements TTW (from dry-weight-to-wet -weight ratios) can be divided into the extracellular volume [0ec’ from quantitative autoradiography (QAR) with 14C-mannitol] and intracellular volume (0ic = TTW -0ec). 0ec = 0if + 0iv’ where 0if = interstitial volume and 0iv = vascular volume [from QAR with 1311-immunoglobulin G (lgG)]. All measured parameters are standardized to tissue dry weight and expressed as mean ± standard error.Results Regardless of the osmolality of the dialysis solution, elevation of PiP to 6 mmHg results in tissue expansion, primarily in 0if’ which is doubled to 1.71 ± 0.11 mUg dry weight and 1.60 ± 0.17 mUg dry weight with isotonic and hypertonic dialysis, respectively, as compared to controls (0.64 ± 0.04 mUg dry weight). The local 0iv was not affected by Pip or osmolality of the bathing solution. The overall 0iv is 0.046 ± 0.006 mUg dry weight. A two-way analysis of variance (ANOVA) to access the effect of osmolality and Pip on 0ic demonstrated no significant change in 0ic (F = 1.2, p > 0.1) as calculated for controls (3.13 ± 0.19 mUg dry weight), after isotonic dialysis (3.13 ± 0.20 mUg dry weight), or after hypertonic dialysis (2.77 ± 0.30 mUg dry weight).Conclusion Elevation of Pip to 6 mmHg significantly increased TTW and expanded the tissue. Tissue expansion is primarily in interstitium (0if, which is doubled from control value regardless of dialysis fluid osmolality.