Excitation-contraction coupling in Na+-Ca2+ exchanger knockout mice -: Reduced transsarcolemmal Ca2+ flux

Cardiac-specific Na+-Ca2+ exchanger (NCX) knockout ( KO) mice surprisingly survive into adulthood without compensatory changes in protein expression levels. To determine how cardiac function is maintained in the absence of NCX, we investigated membrane currents, intracellular Ca2+, and action potentials (APs) in whole cell patch-clamped myocytes from wild-type (WT) and NCX knockout mice. There was no difference in resting Ca2+ or sarcoplasmic reticular Ca2+ load between KO and WT. During prolonged caffeine exposure, the decrease of the Ca2+ transient was drastically slowed in KO versus WT myocytes, indicating that no alternative Ca2+-extrusion mechanism is upregulated to compensate for the absence of NCX. Peak L-type Ca2+ current (I-Ca) was reduced by 62% in KO myocytes compared with WT. Nevertheless, the corresponding Ca2+ transients were similar, implying an increase in the gain of excitation-contraction coupling in KO cells. APs recorded from KO cells repolarized more rapidly than in WT. In WT myocytes, applying a KO AP waveform voltage clamp reduced Ca2+ influx via I-Ca by 59% compared with WT AP waveform clamps. Again, the corresponding Ca2+ transients remained similar. Our findings indicate that NCX KO myocytes limit Ca2+ influx to approximate to 20% of that in WT by reducing ICa and by abbreviating the AP. Contractility is maintained by an increase in the gain of excitation contraction coupling resulting from both a more rapid repolarization of the AP and an as yet unidentified AP-independent mechanism.