Mechanisms of transendothelial and tissue transport in the peritoneum. Experimental studies on rat

In the present thesis, the quantitative role of active transport (transcytosis) in bulk transvascular protein movement in vivo was evaluated. Using chemical transcytosis inhibition (N-ethylmaleimide (NEM) and filipin), we were able to show that NEM actually increases endothelial permeability to macromolecules, probably as a result of unspecific toxic action on endothelial cells. Filipin, on the other hand had no effects on transperitoneal protein permeability in vivo. Our permeability effects of NEM have been confirmed in perfused rat lungs, in isolated perfused rat hindquarters and, on a morphological level, in rat aorta. Thus, the array of papers providing morphological evidence for a decreased protein transport after NEM and filipin administration in situ and in vitro should be handled with great caution. We therefore turned to a more physiologic way of inhibiting active transport, namely by in vivo cooling. We found that protein transport in hypothermic rats did not cease, as could be expected if transcytosis were the main route of transport, but was reduced in proportion to the reduced blood pressure and to the cooling-induced increases in viscosity. The largest molecule tested, low-density lipoprotein (LDL), was slightly more cooling-sensitive than the other macromolecules, however. This result is probably reflecting the presence of inflammatory gaps at 37°C, that were absent in the cold animals (19°C). The opening of such gaps has previously been shown to be an active, calcium-dependent process that can be inhibited by cooling. Another transport issue treated in this thesis was the impact of the interstitium on transperitoneal transport. Addition of hyaluronan (HA), a major component of the interstitial ground substance, to the dialysis fluid resulted in an improved net ultrafiltration. This finding was shown to be due to the formation of a HA-filter cake, in the late phase of the dwell, when net fluid movement is directed towards tissue capillaries, thus reducing the fluid back-filtration. The present thesis further evaluates a highly controversial transport issue, i.e. whether or to what extent small solute transport is blood flow limited in the peritoneum. We thus induced blood flow reductions by way of bleeding rats. We found a significantly lower small solute transport when blood flow was reduced, but the reduction was much lower than what has been found in other organs. The reason for this is probably that the peritoneal interstitium imposes a transport resistance, which conceals the high level of blood flow limitation occurring at the capillary level.