Objective: The Ca2+ independent transient outward K+ current) in the heart is responsible for the initial phase of repolarization. The hKv4.3 K+ channel a-subunit contributes to the I-tol current in many regions of the human heart. Consistently, downregulation of hKv4.3 transcripts in heart failure and atrial fibrillation is linked to reduction in I-tol conductance. The recently cloned KChIP family of calcium sensors has been shown to modulate A-type potassium channels of the Kv4 K+ channel subfamily. Methods and results: We describe the cloning and tissue distribution of hKChIP2, as well as its functional interaction with hKv4.3 after expression in Xenopus oocytes. Furthermore, we isolated a short splice variant of the hKChIP2 gene (hKCNIP2), which represents the major hKChIP2 transcript. Northern blot analyses revealed that hKChIP2 is expressed in the human heart and occurs in the adult atria and ventricles but not in the fetal heart. Upon coexpression with hKv4.3 both hKChIP2 isoforms increased the current amplitude, slowed the inactivation and increased the recovery from inactivation of hKv4.3 currents. For the first time we analyzed the influence of a KChIP protein on the voltage of half-maximal inactivation of Kv4 channels. We demonstrate that the hKChIP2 isoforms shifted the half-maximal inactivation to more positive potentials, but to a different extent. By elucidating the genomic structure, we provide important information for future analysis of the hKCNIP2 gene in candidate disorders. In the course of this work we mapped the hKCNIP2 gene to chromosome 10q24. Conclusions: Heteromeric hKv4.3/hKChIP2 currents more closely resemble native epicardial I,,,, suggesting that hKChIP2 is a true beta-subunit of human cardiac As a result hKChIP2 might play a role in cardiac diseases, where a contribution of 1,,, has been shown. (C) 2001 Elsevier Science B.V. All rights reserved.