Ontogeny of the receptor for polymeric immunoglobulins in rat hepatocytes

Based on in vitro experiments measuring daily secretion rates in the culture media of rat hepatocytes and in vivo experiments using pulse labeling of intracellular precursors, the present study examines the ontogenic expression of the polymeric immunoglobulin receptor and secretory component by hepatocytes during growth. Our data indicate that hepatocytes from infant and suckling rats (day 5, 15) cultured in serum-free and hormone-free conditions only secreted trace amounts of secretory component. Beginning on day 20, basal secretion rate showed a marked upsurge with a 10-fold increase by day 35. The addition of dexamethasone (10−7 mol/L) to the culture media enhanced by 2.5-fold the basal secretion of secretory component by hepatocytes from 20-, 25-, and 35-day-old rats, while addition of insulin to the media had no effect. The response to dexamethasone was dose-dependent (10−5, 10−6, 10−7 mol/L) and specific. In vivo pulse labeling of receptor precursors in hepatocytes from 40-day-old rats allowed the identification of three intracellular forms: a 105-kilodalton peptide and a 116–120-kilodalton mature doublet. In 13-day-old rats, three immature precursors were detected: a 105-kilodalton peptide and a high molecular weight doublet of 185–190 kilodaltons. Sucklings (13 days) treated with corticosterone showed a pattern of precursors similar to controls. These findings support the following conclusions: (a) hepatocytes from infant and suckling rats synthesize and process immature receptor precursors whose expression is unaffected by corticosterone treatment, and (b) active secretion of secretory component is initiated at weaning independently from humoral and hormonal factors while the magnitude of its production by the liver is under the control of glucocorticoids.