Contribution of KChIP2 to the developmental increase in transient outward current of rat cardiomyocytes

The Ca2+-independent, voltage-gated transient outward current (Ito) displays a marked increase during development of cardiomyocytes. However, the molecular mechanism remained unclear. In rat adult ventricular myocytes, Ito can be divided into a fast (Ito,f) and a slow (Ito,s) component by recovery process from inactivation. Voltage-gated K+ channel-interacting proteins 2 (KChIP2) has recently been shown to modify membrane expressions and current densities of Ito,f. Here we examined the developmental change of Ito and the putative molecular correlates of Ito,f (Kv4.2 and Kv4.3) and KChIP2 in rat ventricular myocytes. Even in rat embryonic day 12 (E12) myocytes, we detected Ito. However, Ito in E12 was solely composed of Ito,s. In postnatal day 10 (P10), we recorded much increased Ito composed of two components (Ito,f and Ito,s), and Ito,f was dominant. Thus, the developmental increase of Ito from E12 to P10 can be explained by the dramatic appearance of Ito,f. Real-time RT-PCR revealed that Kv4.2 and Kv4.3 mRNA levels were slightly changed. By contrast, KChIP2 mRNA level increased from E12 to P10 by 731-fold. Therefore, the huge increase of KChIP2 expression was likely to be the cause of the great increase of Ito,f. In order to confirm that KChIP2 is crucial to induce Ito,f, we used adenoviral gene transfer technique. When KChIP2 was over-expressed in E12 myocytes, a great amplitude of Ito,f appeared. Immunocytochemical experiments also demonstrated that KChIP2 enhanced the trafficking of Kv4.2 channels to cell surface. These results indicate that KChIP2 plays an important role in the generation of functional Ito,f channels during development.