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Aim: To investigate the ability of the microdialysis technique to measure capillary selectivity of different sized plasma proteins induced by local administration of platelet activating factor (PAF)., Methods: We used hollow plasmapheresis fibres with 3 cm membrane (cut off 3000 kDa) placed on the back of anaesthetized rats., Results: Platelet activating factor (50 microg mL(-1)) administered locally via the fibre, increased extravasation of radiolabelled 125I-HSA from plasma to the microdialysis fibre by approximately 900% compared both to baseline and the control fibre within 70 min (n = 6, P < 0.05). The extravasation in the control fibre did not change over time. HPLC measurement of plasma proteins in the microdialysis perfusate also demonstrated decreased capillary selectivity for proteins in the diameter range of 73 A, 56 A and 39 A after local administration of PAF (n = 6, P < 0.05). PAF also significantly lowered interstitial fluid (P(if)) pressure after subcutaneous administration (50 microg mL(-1)). Mean arterial pressure (MAP) after intravenous injection of PAF (0.4 microg kg(-1)) fell instantly by about 50 mmHg, and stabilized at 50 mmHg after 15 min (n = 6). MAP was unaltered when PAF was given through the microdialysis fibre (n = 4). Both total tissue water (TTW) and extravasation of albumin, measured as the plasma-to-tissue clearance (E-alb) showed a significant increase after PAF (n = 7, P < 0.05)., Conclusions: The present study demonstrates that PAF induces plasma protein extravasation and decrease capillary selectivity of different sized plasma proteins. It also increases transcapillary fluid flux, and lowers P(if), indicating a role for PAF in the interstitium for generation of transcapillary transport of water and large molecules followed by formation of oedema.

Interstitial fluid pressure (P(if)) is a key determinant in increasing the transcapillary driving pressure, pulling fluid from the microcirculation into the interstitial space at the onset of acute inflammatory reactions and the oedema formation associated with these. Prostaglandin E1 (PGE1) induces lowering of P(if) in rat skin which increases transcapillary transport of 51Cr-EDTA into the center of a tumor as measured by microdialysis. The aim of this study was twofold: First, to evaluate and develop the microdialysis technique thoroughly with regard to its suitability for investigating transcapillary water transport in rat skin using 51Cr-EDTA as a tracer. Secondly, to evaluate the effect of PGE1 on transcapillary transport of 51Cr-EDTA. This study demonstrates that PGE1 increases transcapillary transport of 51Cr-EDTA into skin interstitium. There were no significant differences between the experimental probe and the control probe when calculations from the entire experiment (90 min) were compared. On the other hand, significant differences were observed by examining the experiment in smaller time intervals. PGE1 increased transcapillary transport of 51Cr-EDTA during the first 15 min when administered through the microdialysis probe. This observation suggests that increased blood flow and/or permeability-surface area product are responsible for raising the transcapillary transport of 51Cr-EDTA, i.e. the transport is diffusion limited. Administration of PGE1 through the probe rather than around the probe resulted in less scatter between experiments than when PGE1 was injected subcutaneously around the probe.