Comparison of cloned Kir2 channels with native inward rectifier K+ channels from guinea-pig cardiomyocytes

1. The aim of the study was to compare the properties of cloned Kir2 channe ls with the properties of native rectifier channels in guinea-pig (gp) card iac muscle. The cDNAs of gpKir2.1, gpKir2.2, gpKir2.3 and gpKir2.4 were obt ained by screening a cDNA library from guinea-pig cardiac ventricle. 2. A partial genomic structure of all gpKir2 genes was deduced by compariso n of the cDNAs with the nucleotide sequences derived from a guinea-pig geno mic library. 3. The cell-specific expression of Kir2 channel subunits was studied in iso lated cardiomyocytes using a multi-cell RT-PCR approach. It was found that gpKir2.1, gpKir2.2 and gpKir2.3, but not gpKir2.3, are expressed in cardiom yocytes. 4. Immunocytochemical analysis with polyclonal antibodies showed that expre ssion of Kir2.4 is restricted to neuronal cells in the heart. 5. After transfection in human embryonic kidney cells (HEK293) the mean sin gle-channel conductance with symmetrical K+ was found to be 30.6 pS for gpK ir2.1, 40.0 pS for gpKir2.2 and 14.2 pS for Kir2.3. 6. Cell-attached measurements in isolated guinea-pig cardiomyocytes (n = 35 1) revealed three Populations of inwardly rectifying K+ channels with mean conductances of 34.0, 23.8 and 10.7 pS. 7. Expression of the gpKir2 subunits in Xenopus oocytes showed inwardly rec tifying currents. The Ba2+ concentrations required for half-maximum block a t -100 mV were 3.24 muM for gpKir2.1, 0.51 muM for gpKir2.2, 10.26 muM for gpKir2.3 and 235 muM for gpKir2.4. 8. Ba2+ block of inward rectifier channels of cardiomyocytes n as studied i n cell-attached recordings. The concentration and voltage dependence of Ba2 + block of tile large-conductance inward rectifier channels was virtually i dentical to that of gpKir2.2 expressed in Xenopus oocytes. 9. Our results suggest that the large-conductance inward rectifier channels found in guinea-pig cardiomycocytes (34.0 pS) correspond to gpKir2.2. The intermediate-conductance (23.8 pS) and low-conductance (10.7 pS) channels d escribed here may correspond to gpKir2.1 and gpKir2.3, respectively.