Hepatic mitochondrial proliferation in rats with secondary biliary cirrhosis: Time course and mechanisms

It is well known that the hepatic mitochondrial protein content is increased in rats 4 weeks after bile duct ligation. In the present study, we measured the time course of this increase and assessed the levels of selected mitochondrial messenger RNA (mRNA) species and the race of mitochondrial protein synthesis by isolated mitochondria. Three days after surgery, the mitochondrial protein content was not significantly different between bile duct-ligated (BDL) and control rats, averaging 1,140 +/- 220 mg/liver in BDL and 1,260 +/- 50 mg/liver in sham-operated control rats. However, in comparison with control rats, it was increased in BDL rats by 35% at 7 days, by 81% at 14 days, and by 27% at 28 days after surgery. In vitro mitochondrial protein synthesis, which was assessed as the fractional incorporation of [S-35]-methionine into mitochondrial protein, was not different between BDL and control rats at 3 days after surgery, but was decreased in BDL rats by 63% at 7 days, by 55% at 14 days, and by 36% at 28 days after surgery, Northern blot analysis revealed an increase in the mRNA levels of adenosine triphosphate (ATP)ase subunit 6 and apocytochrome b in BDL rats at day 7, but no significant differences between BDL and control rats in mitochondrial mRNA and ribosomal RNA species 14 and 28 days after surgery, These results show that the hepatic mitochondrial protein content rises early after surgery in BDL rats, but this rise cannot be ascribed to elevated rates of mitochondrial protein synthesis. Thus. increased synthesis of nuclearly encoded mitochondrial proteins and/or decreased degradation of mitochondrial proteins appear likely mechanisms that lead to the observed increase in the hepatic mitochondrial protein content in BDL rats.