Reticuloendothelial system uptake of infused 125I-trypsin in newborn and adult rats

Previous studies have demonstrated enhanced intestinal trypsin uptake and decreased liver clearance of trypsin in newborn rats compared to adults. In order to examine the effectiveness of the reticuloendothelial system (RES) in clearing trypsin, bovine trypsin (1.25 mg/100 g body weight) plus trace 125I-trypsin were injected into the portal vein of 2-week-old (n = 57) and adult (n = 44) control rats or following RES stimulation using intraperitoneally injected lipopolysaccharide or RES suppression with intraperitoneally injected oleic acid emulsion. Plasma, liver and spleen 125I activities were assessed at 1, 5 or 15 min following infusion in control, stimulated and suppressed animals. Newborn control rats had significantly increased 125I plasma levels with decreased liver and spleen 125I activity compared to control adults. RES stimulation in the newborns did not lead to any change in liver or plasma levels although splenic values increased while adults had a decrease in liver 125I activity. RES suppression in the newborns led to increased plasma and decreased spleen 125I-trypsin values while adult rat levels were unchanged. The immature reticuloendothelial system in newborns is poorly responsive to RES stimulation although it can be made even further inefficient by RES suppression. The combination of RES immaturity and lack of response to stimulation may make newborns susceptible to proteolytic damage, especially during times of increased systemic levels of proteolytic enzymes.