Differential secretion of proteins and glycoproteins by livers of immature and adult rats. Effect of antimicrotubule drugs.

This study was initiated to re-examine reported differences in the action of antimicrotubule agents on plasma protein secretion from livers of immature versus adult rats. The aim was (1) to determine the composition and to monitor the secretion of various plasma proteins and glycoproteins from liver slices labeled in vitro with specific amino acids and sugar residues, and (2) to correlate observed differences in secretion of these proteins with structural changes in the hepatocytes of the different aged animals. For the most part, slices of liver from fetal (term), neonatal (4- to 5 days old), and adult rats (70 days old) were incubated with radioactive amino acids or various tritiated sugars specific for N-linked core oligosaccharide and/or N-linked terminal oligosaccharide chains. Our findings indicate that liver slices of fetal and neonatal rats are efficient in synthesizing plasma proteins including fully glycosylated glycoproteins. The secretion of glycosylated and nonglycosylated proteins believed to be processed through Golgi complexes was inhibited to the same extent (approximately 70-80%) by antimicrotubule agents, regardless of the age of the host animal. However, other proteins and glycoproteins secreted by livers of immature rats were found to be relatively insensitive (i.e. inhibited to only 30-40%) to the action of various antimicrotubule drugs. The glycoproteins were found to lack N-linked terminal sugars (although the glycoproteins did contain N-linked core sugars), and it is likely that the drug-insensitive proteins bypassed critical glycosylating sites in the Golgi compartment prior to release. Overall, these findings support earlier data showing that antimicrotubule drugs have a special impact on Golgi-associated events in liver cells. To what extent these findings are related to the action of microtubules remains to be seen.