Effects of Na(+)/Ca(2+)-exchanger overexpression on excitation-contraction coupling in adult rabbit ventricular myocytes

• Published in: Journal of molecular and cellular cardiology 2002-04, Vol.34 (4), p.389-400
• Main Authors: Ranu, Hardeep K, Terracciano, Cesare M N, Davia, Kerry, Bernobich, Elena, Chaudhri, Babar, Robinson, Susan E, Bin Kang, Zhao, Hajjar, Roger J, MacLeod, Kenneth T, Harding, Sian E
• Format: Article
• Language: English
• Subjects: Adenoviridae - genetics; Animals; Electrophysiologic Techniques, Cardiac; Genetic Vectors; Heart Ventricles; Male; Muscle Fibers, Skeletal; Myocardial Contraction - physiology; Rabbits; Sarcoplasmic Reticulum - metabolism; Sodium - metabolism; Sodium-Calcium Exchanger - biosynthesis; Sodium-Calcium Exchanger - physiology
• ISSN: 0022-2828
• DOI: 10.1006/jmcc.2001.1521

The Na(+)/Ca(2+)-exchanger (NCX) is the main mechanism by which Ca(2+) is transported out of the ventricular myocyte. NCX levels are raised in failing human heart, and the consequences of this for excitation-contraction coupling are still debated. We have increased NCX levels in adult rabbit myocytes by adenovirally-mediated gene transfer and examined the effects on excitation-contraction coupling after 24 and 48 h. Infected myocytes were identified through expression of green fluorescent protein (GFP), transfected under a separate promoter on the same viral construct. Control experiments were done with both non-infected myocytes and those infected with adenovirus expressing GFP only. Contraction amplitude was markedly reduced in NCX-overexpressing myocytes at either time point, and neither increasing frequency nor raising extracellular Ca(2+) could reverse this depression. Resting membrane potential and action potential duration were largely unaffected by NCX overexpression, as was peak Ca(2+) entry via the L-type Ca(2+) channel. Systolic and diastolic Ca(2+) levels were significantly reduced, with peak systolic Ca(2+) in NCX-overexpressing myocytes lower than diastolic levels in control cells at 2 m m extracellular Ca(2+). Both cell relengthening and the decay of the Ca(2+) transient were significantly slowed. Sarcoplasmic reticulum (SR) Ca(2+) stores were completely depleted in a majority of myocytes, and remained so despite increasingly vigorous loading protocols. Depressed contractility following NCX overexpression is therefore related to decreased SR Ca(2+) stores and low diastolic Ca(2+) levels rather than reduced Ca(2+) entry.