CHARACTERIZATION OF PUTRESCINE TRANSPORT ACROSS THE INTESTINAL EPITHELIUM - STUDY USING ISOLATED BRUSH-BORDER AND BASOLATERAL MEMBRANE-VESICLES OF THE ENTEROCYTE

Putrescine transport was investigated in isolated brush border and basolateral membrane vesicles prepared from the rabbit enterocyte. Brush border vesicles were oriented right-side-out and basolateral vesicles inside-out, forming a model representing uptake and extrusion across the intestinal epithelium. Putrescine transport across both membranes was initially rapid, and 66% of the equilibrium uptake was achieved within the first minute. According to osmo-plots and measurements at 4 degrees C, 20% of total incorporation presented binding to the membrane. In order to estimate actual uptake into the vesicles, K-m was calculated from the differences in putrescine incorporation at 37 degrees C and 4 degrees C, and was 12.7 mu mol L(-1) for brush border uptake and 38.2 mu mol L(-1) for basolateral extrusion. Putrescine uptake into brush border and basolateral membrane vesicles was not enhanced in the presence of an Na+ gradient. When Na+ was substituted with an uncharged solute, mannitol, putrescine incorporation was increased, indicating that putrescine uptake is not Na+-dependent and that cations might interfere with the carrier. Paraquat and methylglyoxal-bis(guanylhydrazone), known to share the polyamine transport system, inhibited putrescine incorporation in both membrane vesicle preparations. Basolateral carrier showed significantly higher sensitivity to cations. We conclude that putrescine uptake across the apical. membrane and extrusion across the basolateral membrane of the enterocyte are mediated by two different and independent carriers which differ in their electrical properties.