Kir2.1. encodes the inward rectifier potassium channel in rat arterial smooth muscle cells

1.The molecular nature of the strong inward rectifier K+ channel in vascular smooth muscle was explored by using isolated cell RT-PCR, cDNA cloning and expression techniques. 2. RT-PCR of RNA from single smooth muscle cells of rat cerebral (basilar), coronary and mesenteric arteries revealed transcripts for K(ir)2.1. Transcripts for K(ir)2.2 and K(ir)2.3 were not found. 3. Quantitative PCR analysis revealed significant differences in transcript levels of K(ir)2.1 between the different vascular preparations (n = 3; P < 0.05). A two-fold difference was detected between K(ir)2.1 mRNA and beta-actin mRNA in coronary arteries when compared with relative levels measured in mesenteric and basilar preparations. 4. K(ir)2.1 was cloned from rat mesenteric vascular smooth muscle cells and expressed in Xenopus oocytes. Currents were strongly inwardly rectifying and selective for K+. 5. The effect of extracellular Ba2+, Ca2+, Mg2+ and Cs2+ ions on cloned K(ir)2.1 channels expressed in Xenopus oocytes was examined. Ba2+ and Cs+ block were steeply voltage dependent, whereas block by external Ca2+ and Mg2+ exhibited little voltage dependence. The apparent half-block constants and voltage dependences for Ba2+, Cs+, Ca2+ and Mg2+ were very similar for inward rectifier K+ currents from native cells and cloned K(ir)2.1 channels ex-pressed in oocytes. 6. Molecular studies demonstrate that K(ir)2.1 is the only member of the K(ir)2 channel subfamily present in vascular arterial smooth muscle cells. Expression of cloned K(ir)2.1 in,Xenopus oocytes resulted in inward rectifier K+ currents that strongly resemble those that are observed in native vascular arterial smooth muscle cells. We conclude that K(ir)2.1 encodes fnr inward rectifier K+ channels in arterial smooth muscle.