Differential expression of small-conductance Ca$^2+$-activated K$^+$ channels SK1, SK2, and SK3 in mouse atrial and ventricular myocytes.
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Am. J. Physiol. Heart Circ. Physiol. 289 (6): H2714--H2723 (December 2005)

Small-conductance Ca$^2+$-activated K$^+$ channels (SK channels, KCa channels) have been reported in excitable cells, where they aid in integrating changes in intracellular Ca$^2+$ with membrane potential. We recently reported for the first time the functional existence of SK2 (KCa2.2) channels in human and mouse cardiac myocytes. Here, we report cloning of SK1 (KCa2.1) and SK3 (KCa2.3) channels from mouse atria and ventricles using RT-PCR. Full-length transcripts and their variants were detected for both SK1 and SK3 channels. Variants of mouse SK1 channel (mSK1) differ mainly in the COOH-terminal structure, affecting a portion of the sixth transmembrane segment (S6) and the calmodulin binding domain (CaMBD). Mouse SK3 channel (mSK3) differs not only in the number of polyglutamine repeats in the NH2 terminus but also in the intervening sequences between the polyglutamine repeats. Full-length cardiac mSK1 and mSK3 show 99 and 91\% nucleotide identity with those of mouse colon SK1 and SK3, respectively. Quantification of SK1, SK2, and SK3 transcripts between atria and ventricles was performed using real-time quantitative RT-PCR from single, isolated cardiomyocytes. SK1 transcript was found to be more abundant in atria compared with ventricles, similar to the previously reported finding for SK2 channel. In contrast, SK3 showed similar levels of expression in atria and ventricles. Together, our data are the first to indicate the presence of the three different isoforms of SK channels in heart and the differential expression of SK1 and SK2 in mouse atria and ventricles. Because of the marked differential expression of SK channel isoforms in heart, specific ligands for Ca$^2+$-activated K$^+$ currents may offer a unique therapeutic opportunity to modify atrial cells without interfering with ventricular myocytes.
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