R U Naqvi1, D Tweedie, K T MacLeod. 1. Cardiac Medicine, National Heart and Lung Institute, Imperial College School of Medicine, Dovehouse Street, SW3 6LY, London, UK.
Abstract
BACKGROUND: We investigated the effects of cardiac hypertrophy on intracellular calcium (Ca(2+)) homeostasis, the amounts of proteins involved in calcium regulation and the influence of the action potential on such changes. METHODS: Cardiac hypertrophy was induced in rabbits by constriction of the ascending aorta. They were kept for 6 weeks then the heart was removed and left ventricular myocytes isolated. A portion of these myocytes was immediately frozen and stored for subsequent protein analyses using Western blotting. RESULTS: After aortic banding, cardiac myocyte two-dimensional area and membrane capacitance were increased by 53% and 23% respectively. Hypertrophy prolonged cell contraction and relaxation and the corresponding Indo-1 Ca(2+) transients. Hypertrophied cells displayed longer action potentials but Ca(2+) current densities were unchanged compared with myocytes from sham hearts. If Ca(2+) was released from the sarcoplasmic reticulum using rapid cooling, so bypassing the normal mechanisms involved in excitation-contraction coupling, then no functional differences between hypertrophied and control cells could be observed. Western blot analysis showed that the amounts of sarcoplasmic reticulum Ca(2+) ATPase, its regulatory protein phospholamban and the sodium/calcium exchanger were unchanged whereas the amount of calsequestrin was increased by 65% and the alpha(1) subunit of the sodium/potassium ATPase was reduced by 72%. These changes do not appear to evoke functional consequences under these conditions. CONCLUSION: In this model of cardiac hypertrophy, the increase in action potential duration is responsible for changes in contraction and relaxation.
BACKGROUND: We investigated the effects of cardiac hypertrophy on intracellular calcium (Ca(2+)) homeostasis, the amounts of proteins involved in calcium regulation and the influence of the action potential on such changes. METHODS:Cardiac hypertrophy was induced in rabbits by constriction of the ascending aorta. They were kept for 6 weeks then the heart was removed and left ventricular myocytes isolated. A portion of these myocytes was immediately frozen and stored for subsequent protein analyses using Western blotting. RESULTS: After aortic banding, cardiac myocyte two-dimensional area and membrane capacitance were increased by 53% and 23% respectively. Hypertrophy prolonged cell contraction and relaxation and the corresponding Indo-1 Ca(2+) transients. Hypertrophied cells displayed longer action potentials but Ca(2+) current densities were unchanged compared with myocytes from sham hearts. If Ca(2+) was released from the sarcoplasmic reticulum using rapid cooling, so bypassing the normal mechanisms involved in excitation-contraction coupling, then no functional differences between hypertrophied and control cells could be observed. Western blot analysis showed that the amounts of sarcoplasmic reticulum Ca(2+) ATPase, its regulatory protein phospholamban and the sodium/calcium exchanger were unchanged whereas the amount of calsequestrin was increased by 65% and the alpha(1) subunit of the sodium/potassium ATPase was reduced by 72%. These changes do not appear to evoke functional consequences under these conditions. CONCLUSION: In this model of cardiac hypertrophy, the increase in action potential duration is responsible for changes in contraction and relaxation.