Degradation and resynthesis of gap junction protein in plasma membranes of regenerating liver after partial hepatectomy or cholestasis.

Changes in the total amount of the gap junction protein (M(r) 26,000) after partial hepatectomy or bile duct ligation and recanalization were investigated in rat liver membranes by quantitative immunoblot with rabbit antiserum to the M(r) 26,000 protein. The loss and reappearance of the M(r) 26,000 protein roughly paralleled loss and reappearance of gap junction plaques analyzed previously under similar physiological conditions by freeze-fracture of hepatocyte surfaces. The total amount of the hepatic M(r) 26,000 protein in liver plasma membranes and the total area of the hepatocyte surface occupied by gap junction plaques appeared to be proportional under these conditions. However, at the minimum, 28-35 hr after partial hepatectomy we still find about 15% of the M(r) 26,000 protein, in contrast to <1% of gap junction plaques, determined by morphometric analysis. This discrepancy is probably due to the fact that very small gap junction plaques, single connexons, and free M(r) 26,000 gap junction subunits are missed by the morphometric analysis. At the times of the minimal amount of the M(r) 26,000 protein in hepatic plasma membranes after partial hepatectomy or bile duct ligation we found that crude hepatic lysosomal membranes of these rats contained less M(r) 26,000 protein than lysosomal membranes of nonoperated control animals. Thus, we conclude that the decrease and increase of the total amount of the M(r) 26,000 protein cannot be explained only by dispersal and reuse of gap junction subunits but are largely due to degradation and resynthesis of the M(r) 26,000 protein. No significant change in the amount of the M(r) 21,000 protein that had been isolated with gap junction plaques was observed in liver plasma membranes after partial hepatectomy. This confirms our previous conclusion that the M(r) 26,000 and M(r) 21,000 proteins are independent of each other.