Expression of inositol 1,4,5-trisphosphate receptor isoforms in rat cirrhosis

Ca2+ signals mediate the hepatic effects of numerous hormones and growth fa ctors. Hepatic Ca2+ signals are elicited by the inositol trisphosphate rece ptor, an intracellular Ca2+ channel. Three isoforms of this receptor have b een identified; they are expressed and regulated differently. We investigat ed the effect of liver fibrosis and cirrhosis on the hepatic expression of the inositol trisphosphate receptor isoforms. Two different rat models were used: bile duct ligation (fibrosis) and chronic exposure to CCl4/phenobarb ital (cirrhosis). Messenger RNA levels were determined by ribonuclease prot ection assay (RPA), competitive polymerase chain reaction (PCR) followed by Southern blotting, and real-time quantitative PCR. Protein expression was assessed by Western blotting; tissue distribution was assessed by immunohis tology. In control animals, isoform 2 was the predominant isoform, isoform 1 represented less than one third, and isoform 3 less than 1%. After bile d uct ligation, expression of types 1 and 3 increased 1.9- and 5.7-fold, and expression of type 2 decreased 2.5-fold at the protein level. After exposur e to CCl4/phenobarbital, expression of types 1, 2, and 3 were 2.4-, 0.9-, a nd 4.2-fold their expression in control animals. Type 2 was localized to th e apical domain of hepatocytes, consistent with a role for Ca2+ signals in canalicular function. Type 3 was detectable in intrahepatic bile duct epith elial cells and not in hepatocytes, suggesting that Ca2+ signals may be reg ulated differently in these cells. Signaling through inositol trisphosphate receptor participates in the pathogenesis of cirrhosis, because this proce ss affects the expression of its isoforms.