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ROLE OF TITIN AND COLLAGEN IN DIASTOLIC DYSFUNCTION OF CIRRHOTIC CARDIOMYOPATHY
TK Glenn, H Liu, SS Lee
Liver Unit, GI Research Group, University of Calgary
BACKGROUND: Significance of diastolic dysfunction in cirrhotic cardiomyopathy (CCM) has been brought to the forefront with several reports of unexpected heart failure following liver transplantation and transjugular intrahepatic portosystemic stent-shunt. Titin- a sarcomeric protein- is the prime source of passive tension and thus an important determinant of diastolic stiffness in cardiomyocytes. Moreover, Protein Kinase A (PKA) phosphorylation of titin has been shown to reduce passive tension and allow for more rapid diastolic filling. Collagen (COL), which is believed to contribute to passive tension and alter diastolic function, may also be a major contributor to diastolic dysfunction observed in cirrhosis.
AIM: To determine the possible role of titin, PKA and collagen in the pathophysiology of diastolic dysfunction in CCM.
HYPOTHESIS: Changes in titin expression and phosphorylation, as well as an increase in COL ratio will result in slower diastolic filling marked by an increase in the relaxation time of the cirrhotic heart.
METHODS: Cirrhosis was induced in Sprague-Dawley (175-250g) rats by bile duct ligation (BDL); controls underwent a sham operation without ligation. Four weeks after operation, diastolic function was examined in vitro in isolated functioning cardiomyocytes for any change in force, muscle length and sarcomere length. Gel electrophoresis and RT-PCR were examined for N2B and N2BA expression. RT-PCR and Western Blots were also used to examine PKA and COL levels in ventricular tissue.
RESULTS: We have shown previously that in the BDL isolated cardiomyocytes, diastolic return velocity was significantly decreased and the time taken to complete relaxation was increased in BDL when compared to sham groups (p<0.05). However, there was no significant difference in mRNA (p=0.43) and protein (p=0.12) levels of N2B isoform, and no visible protein levels of N2BA isoform. RT-PCR showed that PKA levels are decreased in the cirrhotic group (Sham=0.2720±0.026, BDL=0.1780±0.026). COLI was increased in BDL, while COLIII was decreased.
CONCLUSIONS: Diastolic dysfunction manifests in cirrhotic rat but titin isoforms do not appear to be the only determining factor of this condition. The reduction of PKA in the cirrhotic model corresponds to a decrease in titin phosphorylation and thus an increase in passive tension, which subsequently can result in slower diastolic filling. The increase in COLI, which forms a well-structured matrix that resists extension, and the decrease in the more compliant COLIII could be a compensatory mechanism to counteract the decreased phosphorylation of titin and the increased relaxation time.