Functional expression of a GFP-tagged Kv1.5 alpha-subunit in mouse ventricle

The experiments here were undertaken to determine the feasibility of increa sing the cell surface expression of voltage-gated ion channels in cardiac c ells in vivo and to explore the functional consequences of ectopic channel expression. Transgenic mice expressing a green fluorescent protein (GFP)-ta gged, voltage-gated K+ (Kv) channel alpha -subunit, Kv1.5-GFP, driven by th e cardiac-specific alpha -MHC promoter, were generated. In recent studies, Kv1.5 has been shown to encode the micromolar 4-aminopyridine (4-AP)-sensit ive delayed rectifier K+ current (I-K,I-slow) in mouse myocardium. Unexpect edly, Kv1.5-GFP expression is heterogeneous in the ventricles of these anim als. Although no electrocardiographic abnormalities were evident, expressio n of Kv1.5-GFP results in marked decreases in action potential durations in GFP-positive ventricular myocytes. In voltage-clamp recordings from GFP-po sitive ventricular myocytes, peak outward K+ currents are significantly hig her, and their waveforms are distinct from those recorded from wild-type ce lls. Pharmacological experiments revealed a selective increase in a micromo lar 4-AP-sensitive current, similar to the 4-AP-sensitive component of I-K, I-slow in wild-type cells. The inactivation rate of the "overexpressed" cur rent, however, is significantly slower than the Kv1.5-encoded component of I-K,I-slow in wild-type cells, suggesting differences in association with a ccessory subunits and/or posttranslational processing.