Effects of Na⁺/Ca²⁺-exchanger overexpression on excitation-contraction coupling in adult rabbit ventricular myocytes

The Na⁺/Ca²⁺-exchanger (NCX) is the main mechanism by which Ca²⁺ 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 Ca²⁺ could reverse this depression. Resting membrane potential and action potential duration were largely unaffected by NCX overexpression, as was peak Ca²⁺ entry via the L-type Ca²⁺ channel. Systolic and diastolic Ca²⁺ levels were significantly reduced, with peak systolic Ca²⁺ in NCX-overexpressing myocytes lower than diastolic levels in control cells at 2 m M extracellular Ca²⁺. Both cell relengthening and the decay of the Ca²⁺ transient were significantly slowed. Sarcoplasmic reticulum (SR) Ca²⁺ 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 Ca²⁺ stores and low diastolic Ca²⁺ levels rather than reduced Ca²⁺ entry.