Myocardial function with reduced expression of the sodium-calcium exchanger

The complete removal of the cardiac sodium-calcium exchanger (NCX1) is associated with embryonic lethality, whereas its overexpression is linked to heart failure. To determine whether or not a reduced expression of NCX1 is compatible with normal heart structure and function, we studied 2 knockout (KO) mouse models with reduced levels of NCX1: a heterozygous global KO (HG-KO) with a 50% level of NCX1 expression in all myocytes, and a ventricular-specific KO (V-KO) with NCX1 expression in only 10% to 20% of the myocytes.

Both groups of mice were evaluated at baseline, after transaortic constriction (TAC), and after acute or chronic beta-adrenergic stimulation. At baseline, the HG-KO mice had smaller hearts and the V-KO mice had larger hearts than their wild-type (WT) controls (P < .05). The HG-KO and their control WT mice had normal responses to TAC and beta-adrenergic stimulation. However, the V-KO group was intolerant to TAC and had a significantly (P < .05) blunted response to beta-adrenergic stimulation as compared with the HG-KO mice and WT controls. Unlike the HG-KO mice, the V-KO mice did not tolerate chronic isoproterenol infusion. Telemetric analysis of the electrocardiogram, body temperature, and activity revealed a normal diurnal rhythm in all groups of mice, but confirmed shorter QT intervals along with increased arrhythmias and reduced R wave to P wave amplitude ratios in the V-KO mice.

Though NCX1 can be reduced by half in all myocytes without significant functional alterations, it must be expressed in more than 20% of the myocytes to prevent severe remodeling and heart failure in mouse heart.