Absorption of plasma proteins from peritoneal cavity of normal rats

The present study was undertaken to examine whether the uptake of plasma proteins from the peritoneal cavity is quantitative so that tracers could be introduced that way for measuring their turnover. To this end, the metabolic behavior of seven homologous plasma proteins, labeled with 125I, was compared in rats after intravenous or intraperitoneal administration. The animals were maintained under physiological conditions. Total body radiation measurements showed that the degradation rates of albumin, immunoglobulins A and G, alpha 1-macroglobulin, and transferrin were the same regardless of the route of injection. This implies that these proteins are quantitatively absorbed from the peritoneum without undergoing modifications. The half-life of intraperitoneally injected alpha 1-acid glycoprotein was consistently shorter by an average 9%, thus suggesting that this protein becomes slightly altered if introduced that way. Only one-half of intraperitoneally injected fibrinogen survived normally, whereas the other underwent rapid degradation. The surviving molecules had the same half-life as fibrinogen injected intravenously. The fraction of surviving fibrinogen could be augmented by mixing the dose with serum. Within a wide range of concentrations and quantities injected, the degradation rate of transferrin remained the same. Analysis by deconvolution of the plasma curves of albumin and alpha 1-macroglobulin absorbed from the peritoneum showed that the transport process was independent of protein size and, at least up to 35 mg, of the amount injected. According to the same technique, intraperitoneally administered diferric transferrin retained its iron during passage into the circulation.