Distribution of the muscarinic K+ channel proteins Kir3.1 and Kir3.4 in the ventricle, atrium, and sinoatrial node of heart

The functionally important effects on the heart of ACh released from vagal nerves are principally mediated by the muscarinic K+ channel. The aim of th is study was to determine the abundance and cellular location of the muscar inic K+ channel subunits Kir3.1 and Kir3.4 in different regions of heart. W estern blotting showed a very low abundance of Kir3.1 in rat ventricle, alt hough Kir3.1 was undetectable in guinea pig and ferret ventricle. Although immunofluorescence on tissue sections showed no labeling of Kir3.1 in rat, guinea pig, and ferret ventricle and Kir3.4 in rat ventricle, immunofluores cence on single ventricular cells from rat showed labeling in t-tubules of both Kir3.1 and Kir3.4. Kir3.1 was abundant in the atrium of the three spec ies, as shown by Western blotting and immunofluorescence, and Kir3.4 was ab undant in the atrium of rat, as shown by immunofluorescence. Immunofluoresc ence showed Kir3.1 expression in SA node from the three species and Kir3A e xpression in the SA node from rat. The muscarinic K+ channel is activated b y ACh via the m2 muscarinic receptor and, in atrium and SA node from ferret , Kir3.1 labeling was co-localized with m2 muscarinic receptor labeling thr oughout the outer cell membrane.