The bloodless rat: a new model for macromolecular transport studies across lung endothelium

To examine the effect of circulating proteins on the passage of intravenously injected native ferritin across pulmonary capillary endothelium, rats were exchange transfused with FC-43 fluorocarbon emulsion (FCE) under ether anesthesia. Protein concentration was reduced to less than 1.1 mg/ml by exchange transfusion, followed by FCE containing 15, 30, or 60 mg/ml of lyophilized rat serum protein (LRSP). Two minutes after ferritin injection lungs were prepared for ultrastructural morphometry. The diameter and numerical density of vesicles remained unchanged under all experimental conditions; however, at 0.6 mg/ml of circulating protein there was a 5- and 10-fold increase, respectively in percent vesicle (%VL) and basement membrane labeling (BML) by ferritin. This was reversible; at 60 mg/ml of circulating protein %VL and BML was indistinguishable from controls. Following a reduction of circulating protein to less than 1.1 mg/ml, the addition of 15 mg/ml LRSP reduced %VL but had no effect on BML. This suggests that in addition to shuttling vesicles there may be a second mechanism for the transport of ferritin, possibly involving transendothelial chains of vesicles.