The complex molecular mechanisms underlying spontaneous cardiac pacemaking are not fully understood. Recent findings point to a co-ordinated interplay between intracellular Ca(2+) cycling and plasma membrane-localized cation transport determining the origin and periodicity of pacemaker potentials. The sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Here, we investigated the contribution of NCX1 to cardiac pacemaking. We used an inducible and sinoatrial node-specific Cre transgene to create micelacking NCX1 selectively in cells of the cardiac pacemaking and conduction system (cpNCX1KO). RT-PCR and immunolabeling experiments confirmed the precise tissue-specific and temporally controlled deletion. Ablation of NCX1 resulted in a progressive slowing of heart rate accompanied by severe arrhythmias. Isolated sinoatrial tissue strips displayed a significantly decreased and irregular contraction rate underpinning a disturbed intrinsic pacemaker activity. Mutant animals displayed a gradual increase in the heart-to-body weight ratio and developed ventricular dilatation; however, their ventricular contractile performance was not significantly affected. Pacemaker cells from cpNCX1KO showed no NCX1 activity in response to caffeine-induced Ca(2+) release, determined by Ca(2+) imaging. Regular spontaneous Ca(2+) discharges were frequently seen in control, but only sporadically in knockout (KO) cells. The majority of NCX1KO cells displayed an irregular and a significantly reduced frequency of spontaneous Ca(2+) signals. Furthermore, Ca(2+) transients measured during electrical field stimulation were of smaller magnitude and decelerated kinetics in KO cells. Our results establish NCX1 as a critical target for the proper function of cardiac pacemaking.
