P20 Reduced t-tubule density and less synchronous Ca2+ release in cardiomyocytes from rats with congestive heart failure
Fredrik Swift1, William E. Louch1, Jon Arne Kro Birkeland1, Marion J. Thomas1, Mark B. Cannell2, Ivar Sjaastad1, 3, Ole M. Sejersted1
1 Institute for Experimental Medical Research and Center for Heart Failure Research, University of Oslo, Ullevaal University Hospital, Oslo, Norway
2 Department of Physiology, Faculty of Medicine and Health Sciences, University of Auckland, New Zealand
3 Department of Cardiology, Heart and Lung Center, Ullevaal University Hospital, Oslo, Norway
Release of Ca2+ from the sarcoplasmic reticulum (SR) in cardiomyocytes occurs at functional units where Ca2+ channels located in the t-tubules face SR Ca2+ release channels. The close proximity of these proteins ensure efficient excitation-contraction coupling. Experimentally reducing t-tubule density has been shown to increase the dyssynchrony of Ca2+ release. We investigated whether alterations in t-tubule density and Ca2+ release synchrony occur in cardiomyocytes from rats with post infarction congestive heart failure (CHF). Post infarction CHF was induced by ligation of the left coronary artery. Hearts were fixed with a solution containing 4% paraformaldehyde and mounted in Lowicryl HM20 resin. Thin sections were cut, mounted onto nickel grids, and randomized for blinded examination by electron microscopy. Mean density of the t-tubules was lower in micrographs from failing cardiomyocytes compared to sham (21±2 t-tubules/100 µm2 vs. 36±2 t-tubules/100 µm2, p<0.01). T-tubule density was also reduced in isolated cardiomyocytes labeled with Alexa 488 conjugated wheat germ agglutine (WGA). Line-scan images of Ca2+ transients (fluo-4 AM, 1Hz) in field stimulated isolated cardiomyocytes showed that SR Ca2+ release was significantly less synchronous in failing than in sham cells. The degree of dyssynchrony correlated with the severity of CHF. Thus, loss of t-tubules during CHF development may promote dyssynchrony of Ca2+ release, and contribute to decreased efficiency of excitation-contraction coupling in this disease.