Altered State Dependence of C-Type Inactivation in the Long and Short Forms of Human Kv1.5

Evidence from both human and murine cardiomyocytes suggests that truncated isoforms of Kv1.5 can be expressed in vivo. Using whole-cell patch-clamp recordings, we have characterized the activation and inactivation properties of Kv1.5ΔN209, a naturally occurring short form of human Kv1.5 that lacks roughly 75% of the T1 domain. When expressed in HEK 293 cells, this truncated channel exhibited a V_1/2 of −19.5 ± 0.9 mV for activation and −35.7 ± 0.7 mV for inactivation, compared with a V_1/2 of −11.2 ± 0.3 mV for activation and −0.9 ± 1.6 mV for inactivation in full-length Kv.15. Kv1.5ΔN209 channels exhibited several features rarely observed in voltage-gated K⁺ channels and absent in full-length Kv1.5, including a U-shaped voltage dependence of inactivation and “excessive cumulative inactivation,” in which a train of repetitive depolarizations resulted in greater inactivation than a continuous pulse. Kv1.5ΔN209 also exhibited a stronger voltage dependence to recovery from inactivation, with the time to half-recovery changing e-fold over 30 mV compared with 66 mV in full-length Kv1.5. During trains of human action potential voltage clamps, Kv1.5ΔN209 showed 30–35% greater accumulated inactivation than full-length Kv1.5. These results can be explained with a model based on an allosteric model of inactivation in Kv2.1 (Klemic, K.G., C.-C. Shieh, G.E. Kirsch, and S.W. Jones. 1998. Biophys. J. 74:1779–1789) in which an absence of the NH₂ terminus results in accelerated inactivation from closed states relative to full-length Kv1.5. We suggest that differential expression of isoforms of Kv1.5 may contribute to K⁺ current diversity in human heart and many other tissues.