Abnormal cardiac Na+ channel properties and QT heart rate adaptation in neonatal ankyrin, knockout mice

The cytoskeleton of the cardiomyocyte has been shown to modulate ion channel function. Cytoskeletal disruption in vitro alters Na+ channel kinetics, producing a late Na+ current that can prolong repolarization. This study describes the properties of the cardiac Na+ channel and cardiac repolarization in neonatal mice lacking ankyrin,, a cytoskeletal "adaptor" protein. Using whole-cell voltage clamp techniques, I-Na density was lower in ankyrin(B)(-/-) ventricular myocytes than in wild-type (WT) myocytes (-307+/-26 versus -444+/-39 pA/pF, P<0.01). Ankyrin(B)(-/-) myocytes exhibited a hyperpolarizing shift in activation and inactivation kinetics compared with WT. Slower recovery from inactivation contributed to the negative shift in steady-state inactivation in ankyrin(B)(-/-). Single Nac channel mean open time was longer in ankyrin(B)(-/-) versus WT at test potentials (V-t) of -40 mV (1.0+/-0.1 versus 0.61+/-0.04 ms, P<0.05) and -50 mV (0.8+/-0.1 versus 0.39+/-0.05 ms, P<0.05). Ankyrin(B)(-/-) exhibited late single-channel openings at V-t -40 and -50 mV, which were not seen in WT. Late I-Na contributed to longer action potential durations measured at 90% repolarization (APD(90)) at 1 Hz stimulation in ankyrin(B)(-/-) compared with WT (354+/-26 versus 274+/-22 ms, P<0.05), From ECG recordings of neonatal mice, heart rates were slower in ankyrin(B)(-/-) than in WT (380+/-14 versus 433+/-13 bpm, P<0.01). Although the QT interval was similar in ankyrin(B)(-/-) and WT at physiological heart rates, QT-interval prolongation in response to heart rate deceleration was greater in ankyrin(B)(-/-). In conclusion, Na+ channels in ankyrin(B)(-/-) display reduced I-Na density and abnormal kinetics at the whole-cell and single-channel level that contribute to prolonged APD(90) and abnormal QT-rate adaptation.