Heteromultimeric Kv1.2-Kv1.5 channels underlie 4-aminopyridine-sensitive delayed rectifier K+ current of rabbit vascular myocytes

The molecular identity of vascular delayed rectifier K+ channels (K-DR) is poorly characterized. Inhibition by 4-aminopyridine (4-AP) of K-DR of rabbi t portal vein (RPV) myocytes was studied by patch clamp and compared with t hat of channels composed of Kv1.5 and/or Kv1.2 subunits cloned from the RPV and expressed in mammalian cells. 4-AP block of K-DR was pulse-frequency d ependent, required channel activation, and was associated with a positive s hift in voltage dependence of activation. 4-AP caused a voltage-dependent r eduction in mean open time of K-DR. Relief of 4-AP block of whole cell curr ents during washout required channel activation and was unaffected by volta ge. Homotetrameric Kv1.5 channels did not exhibit the shift in voltage depe ndence of activation exhibited by the native channels. In contrast, Kv1.2 c hannels displayed a shift in voltage dependence of activation, and this cha racteristic was also evident during 4-AP treatment when Kvl.2 was coexpress ed with Kv1.5 or coupled to Kv1.5 in a tandem construct to produce heterote trameric [Kv1.5/Kv1.2](2) Channels. K-DR currents were not sensitive to cha rybdotoxin, which blocks homotetrameric Kvl.2 channels. The findings of thi s study (1) indicate that vascular K-DR are inhibited by 4-AP via an open-s tate block mechanism and trapping of the drug within the pore on channel cl osure and (2) provide novel evidence based on a comparison of functional ch aracteristics that indicate the dominant form of vascular K-DR channel comp lex in RPV involves the heteromultimeric association of Kv1.2 and Kv1.5 sub units.