IN-VIVO HYDRAULIC CONDUCTIVITY OF MUSCLE - EFFECTS OF HYDROSTATIC-PRESSURE

We and others have shown that the loss of fluid and macromolecules fro m the peritoneal cavity is directly dependent on intraperitoneal hydro static pressure (P-ip). Measurements of the interstitial pressure grad ient in the abdominal wall demonstrated minimal change when P-ip was i ncreased from 0 to 8 mmHg. Because flow through tissue is governed by both interstitial pressure gradient and hydraulic conductivity (K), we hypothesized that K of these tissues varies with P-ip. To test this h ypothesis, we dialyzed rats with Krebs-Ringer solution at constant P-i p of 0.7, 1.5, 2.2, 3, 4.4, 6, or 8 mmHg. Tracer amounts of I-125-labe led immunoglobulin G were added to the dialysis fluid as a marker of f luid movement into the abdominal wall. Tracer deposition was corrected for adsorption to the tissue surface and for local loss into lymphati cs. The hydrostatic pressure gradient in the wall was measured using a micropipette and a servo-null system. The technique requires immobili zation of the tissue by a porous Plexiglas plate, and therefore a port ion of the tissue is supported. In agreement with previous results, fl uid flux into the unrestrained abdominal wall was directly related to the overall hydrostatic pressure difference across the abdominal wall (P-ip = 0), but the interstitial pressure gradient near the peritoneum increased only similar to 40% over the range of P-ip = 1.5-8 mmHg (20 -28 mmHg/cm). K of the abdominal wall varied from 0.90 +/- 0.1 x 10(-5 ) cm(2).min(-1).mmHg(-1) at P-ip = 1.5 mmHg to 4.7 +/- 0.43 x 10(-5) c m(2).min(-1).mmHg(-1) on elevation of P-ip to 8 mmHg. In contrast, for the same change in P-ip, abdominal muscle supported on the skin side had a significantly lower range of fluid flux (0.89-1.7 x 10(-4) vs. 1 .9-10.1 x 10(-4) ml.min(-1).cm(-2) in unsupported tissue). The differe nces between supported and unsupported tissues are likely explained in part by a reduced pressure gradient across the supported tissue. In c onclusion, the in vivo hydraulic conductivity of the unsupported abdom inal wall muscle in anesthetized rats varies with the superimposed hyd rostatic pressure within the peritoneal cavity.