Changes in ionic currents and reduced conduction velocity in hypertrophied ventricular myocardium of Xin alpha-deficient mice

Abstract

Objective: mXin alpha, a downstream target gene of Nkx2.5 transcription factor, was shown to encode a proline-rich and Xin repeats-containing protein which localizes to the intercalated disc of adult hearts. Our previous voltage-clamp studies have shown that the ventricular myocytes of mXin alpha -deficient mice exhibited a significant reduction in K+ currents (Ito and IK1), L-type Ca2+ currents, and maximum diastolic potential, leading to the development of early afterdepolarization (EAD) and arrhythmias. However, changes in cationic inward currents could also contribute to the genesis of EAD and arrhythmias in mXin alpha -deficient mice.

Methods: The present study aims to characterize changes in Na+ currents on depolarization and transient inward currents (Iti) on repolarization. Conduction velocity (CV) on the frontal surface of ventricles were also measured and compared.

Results: Results of optical mapping on the Langendorff-perfused hearts at 37oC revealed a 36% reduction of CV in mXin alpha -/- ventricle. Pacing (3 Hz)-induced tachyarrhythmias were more frequently found and ventricular fibrillation (VF, 21 Hz for 5 min) occurred in one out of 8 mXin alpha-/- heart. When perfused at 30 degrees C, no VF was observed in both types of preparations. Voltage-clamp study on isolated ventricular myocytes at 37 degrees C shows increase in INa and Iti in mXin alpha -/- cardiomyocytes thus could explain the occurrence of re-entrant triggered arrhythmias.

Conclusion: The present results revealed that the CV was slower, but INa and Iti were increased in mXin alpha -/-cardiomyocytes thus were prone to reentrant triggered arrhythmias. Hypothermia could reduce the occurrence of arrhythmias.

Figure 1

Activation maps of murine ventricles induced by 3 Hz electrical pacing at 37°C. Panel A and panel B show the activation maps of a wild-type (mXinα+/+, CV=88 cm/s) and a mXinα-deficient (mXinα-/-, CV=44 cm/s), respectively, on the frontal surface of Langendorff’s perfused ventricular myocardium. In the diagram underneath panel A, *indicates site of electrical stimulation and oblique arrow indicates direction of activation. Note the slower CV and presence of irregular conduction in the mXinα-/- heart (panel B). CV- conduction velocity

Figure 2

Sodium inward currents (I_Na) in a wild-type (mXinα+/+) (panel A) and a mXinα-deficient (mXinα-/-) (panel B) ventricular myocytes. Clamp protocol is shown at right upper corner. Panel C shows current-voltage relationships in the two groups of myocytes. n, number of myocytes tested. Values are mean±S.E.M. p<0.05 - differences are significant between groups by paired Student’s t test.

Figure 3

Transient inward currents (I_ti) induced on repolarization after depolarizing pulses of increasing durations (50, 550, 1050, 1550, 2050, 2550 and 3050 ms) applied once every 10 s on I_ti (indicated by upward arrows after depolarizing pulse duration of 3050 ms). Superimposed current traces were recorded in wild-type (mXinα+/+, panel A), heterozygous (mXinα+/-, panel B) and homozygous (mXinα-/-, panel C) ventricular myocytes. Clamp protocol is shown at right upper corner.