Na super(+)/Ca super(2+) exchanger overexpression impairs frequency- and ouabain-dependent cell shortening in adult rat cardiomyocytes

• Published in: American journal of physiology. Heart and circulatory physiology 2004-10, Vol.287 (4), p.H1435-H1445
• Main Authors: Boelck, Birgit, Muench, Goetz, Mackenstein, Peter, Hellmich, Martin, Hirsch, Ingo, Reuter, Hannes, Hattebuhr, Nadja, Weig, Hans-Joerg, Ungerer, Martin, Brixius, Klara, Schwinger, Robert HG
• Format: Article
• Language: English
• ISSN: 0363-6135; 1522-1539

The Na super(+)/Ca super(2+) exchanger (NCX) may influence cardiac function depending on its predominant mode of action, forward mode or reverse mode, during the contraction-relaxation cycle. The intracellular Na super(+) concentration ([Na super(+)] sub(i)) and the duration of the action potential as well as the level of NCX protein expression regulate the mode of action of NCX. [Na super(+)] sub(i) and NCX expression have been reported to be increased in human heart failure. Nevertheless, the consequences of altered NCX expression in heart failure are still a matter of discussion. We aimed to characterize the influence of NCX expression on intracellular Ca super(2+) transport in rat cardiomyocytes by adenoviral-mediated gene transfer. A five- to ninefold (dose dependent) overexpression of NCX protein was achieved after 48 h by somatic gene transfer (Ad.NCX.GFP) versus control (Ad.GFP). NCX activity, determined by Na super(+) gradient-dependent super(45)Ca super(2+)-uptake, was significantly increased. The protein expressions of sarco(endo)plasmic reticulum Ca super(2+)-ATPase, phospholamban, and calsequestrin were unaffected by NCX overexpression. Fractional shortening (FS) of isolated cardiomyocytes was significantly increased at low stimulation rates in Ad.NCX.GFP. After a step-wise enhancing frequency of stimulation to 3.0 Hz, FS remained unaffected in Ad.GFP cells but declined in Ad.NCX.GFP cells. The positive inotropic effect of the cardiac glycoside ouabain was less effective in Ad.NCX.GFP cells, whereas the positive inotropic effect of beta-adrenergic stimulation remained unchanged. In conclusion, NCX overexpression results in a reduced cell shortening at higher stimulation frequencies as well as after inhibition of sarcolemmal Na super(+)-K super(+)-ATPase, i.e., in conditions with enhanced [Na super(+)] sub(i). At low stimulation rates, increased NCX expression enhances both intracellular systolic Ca super(2+) and contraction amplitude.