Differential toxicity of RCAII (ricin) on rabbit intestinal epithelium in relation to postnatal maturation.

The purpose of this work was to assess the toxic lectin ricin (RCAII) as a probe for the study of intestinal permeability in the developing small bowel. Jejunal explants from suckling and adult rabbits were exposed to varying dosages of RCAII for 30 min at 25 degrees C and then cultured in toxin-free medium. The RCAII dose required to inhibit protein synthesis during 6 h of culture increased from 0.1 microgram/ml in 4-day-old rabbits to 25 micrograms/ml in weanling rabbits. RCAII cytotoxicity was almost completely blocked by 0.1 M lactulose in all age groups. The kinetics of 125I-RCAII binding to purified microvillus membranes were determined by incubating a fixed concentration of membrane protein (30 micrograms) with increasing concentrations of labeled lectin (2-18 micrograms/ml). Binding attained saturation with adult but not with suckling animal membranes. The latter yielded a curvilinear relationship in Scatchard plots, suggesting either several classes of binding sites or negative cooperativity. RCAII binding was confined to the delipidated fraction of the membranes and decreased by 42% from 6 days old to adult age. The extreme sensitivity of colostral epithelium to RCAII is probably related to the high level of endocytosis exhibited by the immature membrane of suckling rabbits. The development of increasing resistance to the toxin, and associated decrease in binding, might be related to disappearance of saccharide sites in productive surface receptors occurring in the developmental course of intestinal glycosylation.