Molecular evidence for a role of Shaw (Kv3) potassium channel subunits in potassium currents of dog atrium

1. We previously described an ultrarapid delayed rectifier current in dog a trial myocytes (I-Kur,I-d) with properties resembling currents reported for Kv3.1, channels in neural tissue; however, there was no direct molecular e vidence for Shaw subfamily (Kv3) subunit expression in the heart. To identi fy the molecular basis of I-Kur,I-d, we cloned a full-length cDNA (dKv3.1) from canine atrium with homology-based reverse transcription (RT)polymerase chain reaction (PCR) cloning techniques. 2. A 1755 bp full-length cDNA (dKv3.1) was obtained, with 94.2% homology to rat brain Kv3.1 (rbKv3.1). The deduced amino acid sequence had 99.3% homol ogy with rbKv3.1. 3. Heterologous expression of dKv3.1 in Xenopus oocytes produced currents w ith activation voltage dependence, rectification, and activation and deacti vation kinetics that strongly resemble native I-Kur,I-d. Like I-Kur,I-d, dK v3.1 was found to be highly sensitive to extracellular C-aminopyridine (4-A P) and tetraethylammonium (TEA). 4. RNase protection assays, Western blots and immunohistochemical studies d emonstrated the presence of dKv3.1 transcripts and proteins in dog atrial p reparations and isolated canine atrial myocytes. Protein corresponding to t he Kv1.5 subunit, which can also carry ultrarapid delayed rectifier current , was absent. Unlike neural tissues, which express two splice variants (Kv3 .1a and Kv3.1b), canine atrium showed only Kv3.1b transcripts. 5. Whole-cell patch-clamp studies showed that I-Kur,I-d is absent in canine ventricular myocytes, and immunohistochemical and Western blot analysis de monstrated the absence of dKv3.1 protein in canine ventricle. 6. We conclude that the Shaw-type channel dKv3.1 is present in dog atrium, but not ventricle, and is the likely molecular basis of canine atrial I-Kur ,I-d.