Abstract 581: Rapid Down-Regulation of Connexin43 after Stretch-Induced Early Myocardial Hypertrophy
Gap junctions are important for cardiac conduction and consequent synchronized contraction. Heterogeneous gap junction distribution or reduced numbers of gap junctions can cause arrythmogenic components in hypertrophic cardiomyopathy. It is known that during end stage failure expression of critical gap junction proteins are down-regulated. It is unknown if in early stages of hypertrophy, mechanical load plays a role in reduction of gap junction protein levels. To assess the hypothesis that regulation of connexin43 expression occurs during early stage of hypertrophic induction, expression was measured in cultured multicellular myocardium subjected to high mechanical loading. Right ventricular trabeculae from rabbit hearts were subjected to high mechanical load or no load (slack) conditions in a multicellular muscle culture system. These small ventricular trabeculae were continuously electrically stimulated at 1 Hz at 37 Celsius, and isometric contractions were recorded for periods 24 – 48 hours. Muscles exposed to high load gradually increased developed force with time. Average developed force measured at 24 h was 178 +/− 22 % (n=12, P<0.05) from starting values. We observed a significant increase in muscle diameter, by 4.3 +/− 1.4 (n=8, P<0.05) and 17.4 +/− 3.7 % (n=14, P<0.05) after 24 and 48 hours of high load contractions. Edema was absent, indicating myocyte hypertrophy. Protein quantification showed a down-regulation of connexin43 in stretched muscles to 49 +/− 14% (n=5, P<0.05) and 31 +/− 10% (n=5, P<0.05) of control fresh muscles after 24 and 48 h of high load, respectively. There was no significant different in connexin43 expression between fresh control myocardium and muscles cultured and stimulated in absence of load for 48 hours. Reduced connexin43 levels resulted in a functional impairment of gap junctions. Only in non-stretched muscles iontophoretically loaded bis-fura 2 into a single central myocyte could spread form cell to cell with a good success rate (~80 –90%). Out of 8 high load muscles we were able to spread dye via gap junctions in only a single muscle, and this only at levels well below normal. We conclude that mechanical loading alone can decrease expression and function of gap junctions during early cardiac hypertrophy.