CLONING AND FUNCTIONAL EXPRESSION OF AN INWARDLY RECTIFYING K+ CHANNEL FROM HUMAN ATRIUM

The cardiac inward rectifier current (I-K1) contributes to the shape a nd duration of the cardiac action potential and helps to set the resti ng membrane potential. Although several inwardly rectifying KC channel s (IRKs) from different tissues have been cloned recently, the nature and number of K+ channels contributing to the cardiac I-K1 are present ly unknown. To address this issue in human heart, we have used the rev erse-transcriptase-polymerase chain reaction (PCR) technique with huma n atrial total RNA as a template to identify two sequences expressed i n heart that are homologous to previously cloned IRKs. One of the PCR products we obtained was virtually identical to IRK1 (cloned from a mo use macrophage cell line); the other, which we named hIRK, exhibited < 70% identity to IRK1. A full-length clone encoding hIRK was isolated f rom a human atrial cDNA library and functionally expressed in Xenopus oocytes. This channel, like IRK1, exhibited strong inward rectificatio n and was blocked by divalent cations. However, hIRK differed from IRK 1 at the single-channel level: hIRK had a single-channel conductance o f 36 pS compared with 21 pS for IRK1. We have identified single channe ls of 41, 35, 21, and 9 pS in recordings from dispersed human atrial m yocytes. However, none of these atrial inward rectifiers exhibited sin gle-channel properties exactly like those of cloned hIRK expressed in oocytes. Our findings suggest that the cardiac I-K1 in human atrial my ocytes is composed of multiple inwardly rectifying channels distinguis hable on the basis of single-channel conductance, each of which may be the product of a different gene.