Differential contribution of sialic acid to the function of repolarizing K+ currents in ventricular myocytes

We investigated the contribution of sialic acid residues to the K+ currents involved in the repolarization of mouse ventricular myocytes. Ventricular K+ currents had a rapidly inactivating component followed by slowly decayin g and sustained components. This current was produced by the summation of t hree distinct currents: I-to, which contributed to the transient component; I-ss, which contributed to the sustained component; and I-K,I-slow, which contributed to both components. Incubation of ventricular myocytes with the sialidase neuraminidase reduced the amplitude of I-to without altering I-K ,I-slow and I-ss. We found that the reduction in I-to amplitude resulted fr om a depolarizing shift in the voltage of activation and a reduction in the conductance of I-to. Expression of Kv4.3 channels, a major contributor to I-to in the ventricle, in a sialylation-deficient Chinese hamster ovary cel l line (lec2) mimicked the effects of neuraminidase on the ventricular I-to . Furthermore, we showed that sialylated glycolipids have little effect on the voltage dependence of I-to. Finally, consistent with its actions on I-t o, neuraminidase produced an increase in the duration of the action potenti al of ventricular myocytes and the frequency of early afterdepolarizations. We conclude that sialylation of the proteins forming Kv4 channels is impor tant in determining the voltage dependence and conductance of I-to and that incomplete glycosylation of these channels could lead to arrhythmias.