The functional effect of adenoviral Na⁺/Ca²⁺ exchanger overexpression in rabbit myocytes depends on the activity of the Na⁺/K⁺-ATPase

Objectives: The functional consequences of Na⁺/Ca²⁺ exchanger (NCX) overexpression in heart failure have been controversially discussed. NCX function strongly depends on intracellular sodium which has been shown to be increased in heart failure. Methods and results: We investigated the Na⁺/K⁺-ATPase (NKA) inhibitor ouabain (0.5–16 μmol/l) in electrically stimulated, isotonically contracting adult rabbit cardiocytes overexpressing NCX after adenoviral gene transfer (Ad-NCX-GFP, 48 h culture time). Myocytes transfected with adenovirus encoding for green fluorescent protein (Ad-GFP) served as a control. Contractions were analyzed by video-edge detection. In the Ad-NCX-GFP group, the maximum inotropic response was significantly reduced by 50.7% (P<0.05). This was a result of an enhanced susceptibility to contracture after exposure to the drug (median concentration (25–75%): 4 (4–8) vs. 8 (6–16) μmol/l, P<0.05). When analyzing relaxation before contracture, the maximum relaxation velocity was reduced (0.15±0.04 vs. 0.27±0.04 μm/s, P<0.05) and the time from peak shortening to 90% of relaxation was increased (298±39 vs. 185±15 ms, P<0.05). No differences in systolic and diastolic parameters were observed with the Na⁺ channel modulator BDF9198 (1 μmol/l). Conclusions: Inhibition of NKA by ouabain induces a combined diastolic and systolic dysfunction in NCX overexpressing rabbit myocytes. This may be the consequence of cytoplasmic Ca²⁺ overload due to inhibition of forward mode or induction of reverse mode Na⁺/Ca²⁺ exchange. In end-stage failing human myocardium and during digitalis treatment this mechanism may be of major importance.