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-Dawley (SD) rats, dialyzed with either isotonic (290 mOsm/kg) or hypertonic (510 mOsm/kg) dialysis solutions at nominal Pip of 0 mmHg or 6 mmHg. MEASUREMENTS: TTW (from dry-weight-to-wet-weight ratios) can be divided into the extracellular volume [theta(ec), from quantitative autoradiography (QAR) with 14C-mannitol] and intracellular volume (theta(ic) = TTW - theta(ec)). Theta(ec) = theta(if) + theta(iv), where theta(if) = interstitial volume and theta(iv) = vascular volume [from QAR with 131I-immunoglobulin G (IgG)]. 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 theta(if), which is doubled to 1.71+/-0.11 mL/g dry weight and 1.60+/-0.17 mL/g dry weight with isotonic and hypertonic dialysis, respectively, as compared to controls (0.64+/-0.04 mL/g dry weight). The local theta(iv) was not affected by Pip or osmolality of the bathing solution. The overall theta(iv) is 0.046+/-0.006 mL/g dry weight. A two-way analysis of variance (ANOVA) to access the effect of osmolality and Pip on theta(ic) demonstrated no significant change in theta(ic) (F = 1.2, p > 0.1) as calculated for controls (3.13+/-0.19 mL/g dry weight), after isotonic dialysis (3.13+/-0.20 mL/g dry weight), or after hypertonic dialysis (2.77+/-0.30 mL/g dry weight). CONCLUSION: Elevation of Pip to 6 mmHg significantly increased TTW and expanded the tissue. Tissue expansion is primarily in interstitium (theta(if)), which is doubled from control value regardless of dialysis fluid osmolality.

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