OBJECTIVE: To assess the clearance of radiolabeled tracer albumin (RISA) from peritoneal cavity to plasma (Cl-->P) in rats under essentially "normal" conditions, that is, when intraperitoneal hydrostatic pressure (IPP) is subatmospheric and the intraperitoneal (IP) "free" fluid volume (IPV) is low. METHODS: A volume of 0.3 mL of RISA was injected IP into anesthetized Wistar rats (wt = 300 g) when the IPV was approximately 2 mL (normal) or the IPV was approximately 10 mL, and IPP was either -1.8 mmHg (normal) or +1.5 mmHg (produced by an external cuff). Plasma samples (25 microL) were obtained repeatedly during the dwell, which lasted 30-300 min, after which the peritoneal cavity was opened to recover the IPV and residual IP RISA activity.The Cl-->P was assessed as the mass transfer of RISA into plasma, occurring per unit time, divided by the calculated mean IP RISA concentration (CD). The interstitial RISA space was measured as the mass of RISA accumulated, per unit tissue weight, in peritoneal tissue samples divided by the CD. RESULTS: A markedly lower Cl-->P (2.47+/-0.67 microL/min), as well as total RISA clearance out of the peritoneal cavity (Cl), was found under "normal" conditions (an IPV of approximately 2 mL and an IPP of approximately -1.8 mmHg) compared to the situation during peritoneal dialysis (an IPV of approximately 20 mL and an IPP of +1 mmHg). Furthermore, the interstitial RISA space increased linearly over time even at negative IPPs and at an unchanging peritoneal interstitial fluid volume. At a low (normal) IPV the Cl-->P did not increase significantly with elevating IPP, and increased only marginally when tracer distribution was improved by artificial vibration of the rats. However the Cl-->P increased when larger volumes were infused to increase the total IPV. CONCLUSIONS: It is concluded that the Cl-->P and Cl at low IPPs and IPVs are not as high as during peritoneal dialysis. Increases in Cl-->P were, however, coupled to increases in IPV.This highlights the importance of the IPV per se and of a sufficient IP tracer distribution for direct lymphatic absorption to be efficient.

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