Effects of Na(+)/Ca(2+)-exchanger overexpression on excitation-contraction coupling in adult rabbit ventricular myocytes.
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.
Ranu
Hardeep K
HK
Cardiac Medicine, National Heart and Lung Institute, Imperial College School of Medicine, London, UK.
Terracciano
Cesare M N
CM
Davia
Kerry
K
Bernobich
Elena
E
Chaudhri
Babar
B
Robinson
Susan E
SE
Bin Kang
Zhao
Z
Hajjar
Roger J
RJ
MacLeod
Kenneth T
KT
Harding
Sian E
SE
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D022062:Electrophysiologic Techniques, Cardiac; D005822:Genetic Vectors; D006352:Heart Ventricles; D008297:Male; D018485:Muscle Fibers, Skeletal; D009200:Myocardial Contraction / Q000502:physiology*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000096:biosynthesis / Q000502:physiology*
D019831:Sodium-Calcium Exchanger; D012964:Sodium